Psychosis is first and foremost a diagnosis of exclusion. So a new-onset episode of psychosis "cannot" be considered a symptom of a psychiatric disorder until other relevant and known causes of psychosis are properly excluded, or ruled out. Many clinicians improperly perform, or entirely miss this step, introducing avoidable diagnostic error and misdiagnosis.

An initial assessment includes a comprehensive history and physical examination by a physician, psychiatrist, psychiatric nurse practitioner or psychiatric physician assistant. Biological tests should be performed to exclude psychosis associated with or caused by substance use, medication, toxins, surgical complications, or other medical illnesses.

Delirium should be ruled out, which can be distinguished by visual hallucinations, acute onset and fluctuating level of consciousness, indicating other underlying factors, including medical illnesses. Excluding medical illnesses associated with psychosis is performed by using blood tests to measure:

- Thyroid-stimulating hormone to exclude hypo- or hyperthyroidism,

- Basic electrolytes and serum calcium to rule out a metabolic disturbance,

- Full blood count including ESR to rule out a systemic infection or chronic disease, and

- Serology to exclude syphilis or HIV infection.

Other investigations include:

- EEG to exclude epilepsy, and an

- MRI or CT scan of the head to exclude brain lesions.

Because psychosis may be precipitated or exacerbated by common classes of medications, medication-induced psychosis should be ruled out, particularly for first-episode psychosis. Both substance- and medication-induced psychosis can be excluded to a high level of certainty, using a

- Urinalysis and a

- Full serum toxicology screening.

Because some dietary supplements may also induce psychosis or mania, but cannot be ruled out with laboratory tests, a psychotic individual's family, partner, or friends should be asked whether the patient is currently taking any dietary supplements.

Common mistakes made when diagnosing people who are psychotic include:

- Not properly excluding delirium,

- Not appreciating medical abnormalities (e.g., vital signs),

- Not obtaining a medical history and family history,

- Indiscriminate screening without an organizing framework,

- Missing a toxic psychosis by not screening for substances "and" medications

- Not asking family or others about dietary supplements,

- Premature diagnostic closure, and

- Not revisiting or questioning the initial diagnostic impression of primary psychiatric disorder.

Only after relevant and known causes of psychosis are excluded, a mental health clinician may make a psychiatric differential diagnosis using a person's family history, incorporating information from the person with psychosis, and information from family, friends, or significant others.

Types of psychosis in psychiatric disorders may be established by formal rating scales. The Brief Psychiatric Rating Scale (BPRS) assesses the level of 18 symptom constructs of psychosis such as hostility, suspicion, hallucination, and grandiosity. It is based on the clinician's interview with the patient and observations of the patient's behavior over the previous 2–3 days. The patient's family can also answer questions on the behavior report. During the initial assessment and the follow-up, both positive and negative symptoms of psychosis can be assessed using the 30 item Positive and Negative Symptom Scale (PANSS).

Psychosis as a symptom of a psychiatric disorder is first and foremost a diagnosis of exclusion. So a new-onset episode of psychosis "cannot" be considered to be a symptom of a psychiatric disorder until other relevant and known medical causes of psychosis are excluded, or ruled out. Many clinicians improperly perform, or entirely miss this step, introducing avoidable diagnostic error and misdiagnosis.

An initial assessment includes a comprehensive history and physical examination. Although no biological laboratory tests exist which confirm schizoaffective disorder, biological tests should be performed to exclude psychosis associated with or caused by substance use, medications, toxins or poisons, surgical complications, or other medical illnesses. Since non-medical mental health practitioners are not trained to exclude medical causes of psychosis, people experiencing psychosis should be referred to an emergency department or hospital.

Delirium should be ruled out, which can be distinguished by visual hallucinations, acute onset and fluctuating level of consciousness, indicating other underlying factors which includes medical illnesses. Excluding medical illnesses associated with psychosis is performed by using blood tests to measure:

- Thyroid-stimulating hormone to exclude hypo- or hyperthyroidism,

- Basic electrolytes and serum calcium to rule out a metabolic disturbance,

- Full blood count including ESR to rule out a systemic infection or chronic disease, and

- Serology to exclude syphilis or HIV infection.

Other investigations which may be performed include:

- EEG to exclude epilepsy, and an

- MRI or CT scan of the head to exclude brain lesions.

Blood tests are not usually repeated for relapse in people with an established diagnosis of schizoaffective disorder, unless there is a specific "medical" indication. These may include serum BSL if olanzapine has previously been prescribed, thyroid function if lithium has previously been taken to rule out hypothyroidism, liver function tests if chlorpromazine has been prescribed, CPK levels to exclude neuroleptic malignant syndrome, and a urinalysis and serum toxicology screening if substance use is suspected. Assessment and treatment may be done on an outpatient basis; admission to an inpatient facility is considered if there is a risk to self or others.

Because psychosis may be precipitated or exacerbated by common classes of psychiatric medications, such as antidepressants, ADHD stimulant medications, and sleep medications, prescribed medication-induced psychosis should be ruled out, particularly for first-episode psychosis. This is an essential step to reduce diagnostic error and to evaluate potential medication sources of further patient harm. Regarding prescribed medication sources of patient harm, Yale School of Medicine Professor of Psychiatry Malcolm B. Bowers, Jr, MD wrote:

Illicit drugs aren't the only ones that precipitate psychosis or mania—prescribed drugs can too, and in particular, some psychiatric drugs. We investigated this and found that about 1 in 12 psychotic or manic patients in an inpatient psychiatric facility are there due to antidepressant-induced psychosis or mania. That's unfortunate for the field [of psychiatry] and disastrous for some of our patients.

Substance-induced psychosis should also be ruled out. Both substance- and medication-induced psychosis can be excluded to a high level of certainty while the person is psychotic, typically in an emergency department, using both a

- Broad spectrum urine toxicology screening, and a

- Full serum toxicology screening (of the blood).

Some dietary supplements may also induce psychosis or mania, but cannot be ruled out with laboratory tests. So a psychotic person's family, partner, or friends should be asked whether he or she is currently taking any dietary supplements.

Common mistakes made when diagnosing psychotic patients include:

- Not properly excluding delirium,

- Missing a toxic psychosis by not screening for substances "and" medications,

- Not appreciating medical abnormalities (e.g., vital signs),

- Not obtaining a medical history and family history,

- Indiscriminate screening without an organizing framework,

- Not asking family or others about dietary supplements,

- Premature diagnostic closure, and

- Not revisiting or questioning the initial diagnostic impression of primary psychiatric disorder.

Only after these relevant and known causes of psychosis have been ruled out can a psychiatric differential diagnosis be made. A mental health clinician will incorporate family history, observation of a psychotic person's behavior while the person is experiencing active symptoms, to begin a psychiatric differential diagnosis. Diagnosis also includes self-reported experiences, as well as behavioral abnormalities reported by family members, friends, or significant others. Mistakes in this stage include:

- Not screening for dissociative disorders. Dissociative identity disorder and psychotic symptoms in schizoaffective disorder have considerable overlap, yet a different overall treatment approach.

Bipolar disorder is commonly diagnosed during adolescence or early adulthood, but onset can occur throughout the life cycle. The disorder can be difficult to distinguish from unipolar depression and the average delay in diagnosis is 5–10 years after symptoms begin. Diagnosis of bipolar disorder takes several factors into account and considers the self-reported experiences of the symptomatic individual, abnormal behavior reported by family members, friends or co-workers, observable signs of illness as assessed by a clinician, and often a medical work-up to rule-out medical causes. In diagnosis, caregiver-scored rating scales, specifically the mother, has been found to be more accurate than teacher and youth report in predicting identifying youths with bipolar disorder. Assessment is usually done on an outpatient basis; admission to an inpatient facility is considered if there is a risk to oneself or others. The most widely used criteria for diagnosing bipolar disorder are from the American Psychiatric Association's (APA) "Diagnostic and Statistical Manual of Mental Disorders", Fifth Edition (DSM-5) and the World Health Organization's (WHO) "International Statistical Classification of Diseases and Related Health Problems", 10th Edition (ICD-10). The ICD-10 criteria are used more often in clinical settings outside of the U.S. while the DSM criteria are used clinically within the U.S. and are the prevailing criteria used internationally in research studies. The DSM-5, published in 2013, included further and more accurate specifiers compared to its predecessor, the DSM-IV-TR. Semi structured interviews such as the Kiddie Schedule for Affective Disorders and Schizophrenia (KSADS) and the Structured Clinical Interview for DSM-IV (SCID) are used for diagnostic confirmation of bipolar disorder.

Several rating scales for the screening and evaluation of bipolar disorder exist, including the Bipolar spectrum diagnostic scale, Mood Disorder Questionnaire, the General Behavior Inventory and the Hypomania Checklist. The use of evaluation scales cannot substitute a full clinical interview but they serve to systematize the recollection of symptoms. On the other hand, instruments for screening bipolar disorder tend to have lower sensitivity.

In the ICD-10 there are several disorders with the manic syndrome: organic manic disorder (), mania without psychotic symptoms (), mania with psychotic symptoms (), other manic episodes (), unspecified manic episode (), manic type of schizoaffective disorder (), bipolar affective disorder, current episode manic without psychotic symptoms (), bipolar affective disorder, current episode manic with psychotic symptoms ().

There are a number of pharmacological and psychotherapeutic techniques used to treat bipolar disorder. Individuals may use self-help and pursue recovery.

Hospitalization may be required especially with the manic episodes present in bipolar I. This can be voluntary or (if mental health legislation allows and varying state-to-state regulations in the USA) involuntary (called civil or involuntary commitment). Long-term inpatient stays are now less common due to deinstitutionalization, although these can still occur. Following (or in lieu of) a hospital admission, support services available can include drop-in centers, visits from members of a community mental health team or an Assertive Community Treatment team, supported employment and patient-led support groups, intensive outpatient programs. These are sometimes referred to as partial-inpatient programs.

Schizophrenia is diagnosed based on criteria in either the American Psychiatric Association's (APA) fifth edition of the "Diagnostic and Statistical Manual of Mental Disorders" (DSM 5), or the World Health Organization's International Statistical Classification of Diseases and Related Health Problems (ICD-10). These criteria use the self-reported experiences of the person and reported abnormalities in behavior, followed by a clinical assessment by a mental health professional. Symptoms associated with schizophrenia occur along a continuum in the population and must reach a certain severity and level of impairment, before a diagnosis is made. As of 2013 there is no objective test.

In 2013, the American Psychiatric Association released the fifth edition of the DSM (DSM-5). To be diagnosed with schizophrenia, two diagnostic criteria have to be met over much of the time of a period of at least one month, with a significant impact on social or occupational functioning for at least six months. The person had to be suffering from delusions, hallucinations, or disorganized speech. A second symptom could be negative symptoms, or severely disorganized or catatonic behaviour. The definition of schizophrenia remained essentially the same as that specified by the 2000 version of DSM (DSM-IV-TR), but DSM-5 makes a number of changes.

- Subtype classifications – such as catatonic and paranoid schizophrenia – are removed. These were retained in previous revisions largely for reasons of tradition, but had subsequently proved to be of little worth.

- Catatonia is no longer so strongly associated with schizophrenia.

- In describing a person's schizophrenia, it is recommended that a better distinction be made between the current state of the condition and its historical progress, to achieve a clearer overall characterization.

- Special treatment of Schneider's first-rank symptoms is no longer recommended.

- Schizoaffective disorder is better defined to demarcate it more cleanly from schizophrenia.

- An assessment covering eight domains of psychopathology – such as whether hallucination or mania is experienced – is recommended to help clinical decision-making.

The ICD-10 criteria are typically used in European countries, while the DSM criteria are used in the United States and to varying degrees around the world, and are prevailing in research studies. The ICD-10 criteria put more emphasis on Schneiderian first-rank symptoms. In practice, agreement between the two systems is high. The current proposal for the ICD-11 criteria for schizophrenia recommends adding self-disorder as a symptom.

If signs of disturbance are present for more than a month but less than six months, the diagnosis of schizophreniform disorder is applied. Psychotic symptoms lasting less than a month may be diagnosed as brief psychotic disorder, and various conditions may be classed as psychotic disorder not otherwise specified, while schizoaffective disorder is diagnosed if symptoms of mood disorder are substantially present alongside psychotic symptoms. If the psychotic symptoms are the direct physiological result of a general medical condition or a substance, then the diagnosis is one of a psychosis secondary to that condition. Schizophrenia is not diagnosed if symptoms of pervasive developmental disorder are present unless prominent delusions or hallucinations are also present.

The most widely used criteria for diagnosing schizoaffective disorder are from the American Psychiatric Association's "Diagnostic and Statistical Manual of Mental Disorders-5".

The DSM-IV schizoaffective disorder definition was plagued by problems of being inconsistently (or unreliably) used on patients; when the diagnosis is made, it doesn't stay with most patients over time; and it has questionable diagnostic validity (that is, it doesn't describe a distinct disorder, nor predict any particular outcome). These problems have been slightly reduced (or "modestly improved") in the DSM-5 according to Carpenter.

When psychotic symptoms are confined to an episode of mania or depression (with or without mixed features), the diagnosis is that of a “psychotic” mood disorder, namely either bipolar disorder or major depression). Only when psychotic states persist in a sustained fashion for two weeks or longer without concurrent affective symptoms is the diagnosis schizoaffective disorder or schizophrenia.

The second cardinal guideline in the DSM-5 diagnosis of schizoaffective disorder is one of timeframe.

These two changes are intended by the DSM-5 workgroup to accomplish two goals:

- Increase the diagnosis' consistency (or reliability) when it is used;

- Significantly decrease the overall use of the schizoaffective disorder diagnosis.

If the schizoaffective diagnosis is used less often, other diagnoses (like psychotic mood disorders and schizophrenia) are likely to be used more often; but this is hypothetical until real-world data arrive. Validity problems with the diagnosis remain and await further work in the fields of psychiatric genetics, neuroimaging, and cognitive science that includes the overlapping fields of cognitive, affective, and social neuroscience, which may change the way schizoaffective disorder is conceptualized and defined in future versions of the DSM and ICD.

Routine medical assessments are often prescribed to rule-out or identify a somatic cause for bipolar I symptoms. These tests can include ultrasounds of the head, x-ray computed tomography (CAT scan), electroencephalogram, HIV test, full blood count, thyroid function test, liver function test, urea and creatinine levels and if patient is on lithium, lithium levels are taken. Drug screening includes recreational drugs, particularly synthetic cannabinoids, and exposure to toxins.

The evidence for the effectiveness of early interventions to prevent psychosis appeared inconclusive. Whilst early intervention in those with a psychotic episode might improve short term outcomes, little benefit was seen from these measures after five years. However, there is evidence that cognitive behavioral therapy (CBT) may reduce the risk of becoming psychotic in those at high risk, and in 2014 the UK National Institute for Health and Care Excellence (NICE) recommended preventive CBT for people at risk of psychosis.

The long-term outcome for psychotic depression is generally poorer than for non-psychotic depression.

For women taking psychiatric medication, the decision as to whether continue during pregnancy and whether to take them while breast feeding is difficult in any case; there is no data to guide this decision with respect to preventing postpartum psychosis. There is no data to guide a decision as to whether women at high risk for postpartum psychosis should take antipsychotic medicine to prevent it. For women at risk of postpartum psychosis, informing medical care-givers, and monitoring by a psychiatrist during pregnancy, in the perinatal period, and for a few weeks following delivery, is recommended.

For women with known bipolar disorder, taking medication during pregnancy roughly halves the risk of a severe postpartum episode, as does starting to take medication immediately after the birth.

Questionnaires and checklists such as the Beck Depression Inventory or the Children's Depression Inventory can be used by a mental health provider to help detect, and assess the severity of depression. The Seasonal Pattern Assessment Questionnaire can be used to screen for seasonal affective disorder. Semi structured interviews such as the Kiddie Schedule for Affective Disorders and Schizophrenia (KSADS) and the Structured Clinical Interview for DSM-IV (SCID) are used for diagnostic confirmation of depression.

Before beginning treatment for mania, careful differential diagnosis must be performed to rule out secondary causes.

The acute treatment of a manic episode of bipolar disorder involves the utilization of either a mood stabilizer (valproate, lithium, or carbamazepine) or an atypical antipsychotic (olanzapine, quetiapine, risperidone, or aripiprazole). Although hypomanic episodes may respond to a mood stabilizer alone, full-blown episodes are treated with an atypical antipsychotic (often in conjunction with a mood stabilizer, as these tend to produce the most rapid improvement).

When the manic behaviours have gone, long-term treatment then focuses on prophylactic treatment to try to stabilize the patient's mood, typically through a combination of pharmacotherapy and psychotherapy. The likelihood of having a relapse is very high for those who have experienced two or more episodes of mania or depression. While medication for bipolar disorder is important to manage symptoms of mania and depression, studies show relying on medications alone is not the most effective method of treatment. Medication is most effective when used in combination with other bipolar disorder treatments, including psychotherapy, self-help coping strategies, and healthy lifestyle choices.

Lithium is the classic mood stabilizer to prevent further manic and depressive episodes. A systematic review found that long term lithium treatment substantially reduces the risk of bipolar manic relapse, by 42%. Anticonvulsants such as valproate, oxcarbazepine and carbamazepine are also used for prophylaxis. More recent drug solutions include lamotrigine, which is another anticonvulsant. Clonazepam (Klonopin) is also used. Sometimes atypical antipsychotics are used in combination with the previous mentioned medications as well, including olanzapine (Zyprexa) which helps treat hallucinations or delusions, Asenapine (Saphris, Sycrest), aripiprazole (Abilify), risperidone, ziprasidone, and clozapine which is often used for people who do not respond to lithium or anticonvulsants.

Verapamil, a calcium-channel blocker, is useful in the treatment of hypomania and in those cases where lithium and mood stabilizers are contraindicated or ineffective. Verapamil is effective for both short-term and long-term treatment.

Antidepressant monotherapy is not recommended for the treatment of depression in patients with bipolar disorders I or II, and no benefit has been demonstrated by combining antidepressants with mood stabilizers in these patients.

The "Diagnostic and Statistical Manual of Mental Disorders" (DSM-IV) recognizes two types of bipolar disorders—bipolar I and bipolar II. People with bipolar I disorder suffer from at least one manic or mixed episode, and may experience depressive episodes. On the contrary, as noted above, people with bipolar II disorder experience a milder form of a manic episode, known as a hypomanic episode as well as major depressive episodes. Although bipolar II is thought to be less severe than bipolar I in regards to symptom intensity, it is actually more severe and distressing with respect to episode frequency and overall course. Those with bipolar II often experience more frequent bouts of depressive episodes. Specific criteria defined by the DSM-IV for a bipolar II diagnosis is as follows:

- The presence of a hypomanic or major depressive episode.

- If currently in major depressive episode, history of a hypomanic episode. If currently in a hypomanic episode, history of a major depressive episode. No history of a manic episode.

- Significant stress or impairment in social, occupational, or other important areas of functioning.

Studies have identified major differences between bipolar I and bipolar II in regards to their clinical features, comorbidity rates and family histories. According to Baek et al. (2011), during depressive episodes, bipolar II patients tend to show higher rates of psychomotor agitation, guilt, shame, suicidal ideation, and suicide attempts. Bipolar II patients have shown higher lifetime comorbidity rates of DSM axis I diagnoses such as phobias, anxiety disorders, substance & alcohol abuse, and eating disorders and there is a higher correlation between bipolar II patients and family history of psychiatric illness, including major depression and substance-related disorders. The occurrence rate of psychiatric illness in first degree relatives of bipolar II patients was 26.5%, versus 15.4% in bipolar I patients.

Screening instruments like the Mood Disorders Questionnaire (MDQ) are helpful tools in determining a patient's status on the bipolar spectrum and getting families involved can also improve chances of an accurate diagnosis and acknowledgment of hypomanic episodes. In addition, there are certain features that have been shown to increase the chances that depressed patients are suffering from a bipolar disorder including atypical symptoms of depression like hypersomnia and hyperphagia, a family history of bipolar disorder, medication-induced hypomania, recurrent or psychotic depression, antidepressant refractory depression, and early or postpartum depression.

Diagnosis of cyclothymia is difficult for a number of reasons. The depressive-dysthymic episode of cyclothymia is also a diagnostic feature of many disorders, including adjustment disorders, personality disorders, psychotic disorders, and other mood disorders. Since depression can be triggered or exacerbated by life events and circumstances, the diagnosing clinician must determine when it is an acceptable response and when it is pathological.

Symptoms described in the hypomanic episode are also commonly associated with ADHD, such as increased energy, distractibility and impulsive or risk-seeking behavior. This is of particular concern in child psychiatry because symptoms, especially hyperactivity, may be counted twice toward both disorders or may inflate the prevalence of ADHD.

While childhood ADHD often presents with hyperactivity, adult ADHD often does not. The unstable lifestyle often found both in people with ADHD and in those with cyclothymia can cause problems for differential diagnosis. Important distinguishing factors include that ADHD is characterized mainly by problems with concentration and memory, while cyclothymia mainly by periods of elevated self-confidence and elation.

Whether subtypes of bipolar disorder, such as cyclothymia truly represent separate disorders or are part of a unique bipolar spectrum is still debated in research. Cyclothymia is typically not described in research studies or diagnosed in clinical settings, making it less recognizable and less understood by professionals. This absence of cyclothymia in research and clinical settings suggests that cyclothymia is either being diagnosed as another mood disorder or as a non-affective psychiatric disorder or not coming to scientific or clinical attention due to a lack of diagnostic clarity or because the nature of cyclothymia is still highly contested. Additionally, the current diagnostic criterion for cyclothymia emphasizes that symptoms are persistent, which suggests that they are enduring traits rather than a psychological state, thus, it has been argued that it should be diagnosed as a personality disorder. Since the symptoms tend to overlap with personality disorders, the validity and distinction between these two diagnostic categories has been debated.

Lastly, the tendency of cyclothymia to be comorbid with other mental disorders makes diagnosis difficult. These issues prevent consensus on the definition of cyclothymia and its relationship with other mental disorders among researchers and clinicians. This lack of consensus on an operational definition and symptom presentation is especially pronounced with children and adolescents because the diagnostic criteria have not been adequately adapted to take into account their developmental level. However, there has been a shift from categorical models of bipolar related disorders toward a dimensional model, which is intended to address some of these issues.

This disorder is common in the relatives of patients with bipolar disorder, and some individuals with cyclothymia eventually develop bipolar disorder themselves. It may persist throughout adult life, cease temporarily or permanently, or develop into more severe mood swings, meeting the criteria for bipolar disorder or recurrent depressive disorder in some cases.

The lack of a formal diagnosis in the DSM and ICD has hindered research. The causes of postpartum depression are unknown and are under investigation.

There is a need to better understand whether taking medication for prevention during pregnancy or immediately following birth, is useful.

In 2016, the United States Preventive Services Task Force (USPSTF) recommended screening in the adult populations with evidence that it increases the detection of people with depression and with proper treatment improves outcomes. They recommend screening in those between the age of 12 to 18 as well.

A Cochrane review from 2005 found screening programs do not significantly improve detection rates, treatment, or outcome.

According to a substantial amount of epidemiology studies conducted, women are twice as likely to develop certain mood disorders, such as major depression. Although there is an equal number of men and women diagnosed with bipolar II disorder, women have a slightly higher frequency of the disorder.

In 2011, mood disorders were the most common reason for hospitalization among children aged 1–17 years in the United States, with approximately 112,000 stays. Mood disorders were top principal diagnosis for Medicaid super-utilizers in the United States in 2012. Further, a study of 18 States found that mood disorders accounted for the highest number of hospital readmissions among Medicaid patients and the uninsured, with 41,600 Medicaid patients and 12,200 uninsured patients being readmitted within 30 days of their index stay—a readmission rate of 19.8 per 100 admissions and 12.7 per 100 admissions, respectively. In 2012, mood and other behavioral health disorders were the most common diagnoses for Medicaid-covered and uninsured hospital stays in the United States (6.1% of Medicaid stays and 5.2% of uninsured stays).

A study conducted in 1988 to 1994 amongst young American adults involved a selection of demographic and health characteristics. A population-based sample of 8,602 men and women ages 17–39 years participated. Lifetime prevalence were estimated based on six mood measures:

1. major depressive episode (MDE) 8.6%,

2. major depressive disorder with severity (MDE-s) 7.7%,

3. dysthymia 6.2%,

4. MDE-s with dysthymia 3.4%,

5. any bipolar disorder 1.6%, and

6. any mood disorder 11.5%.

In the US, the American College of Obstetricians and Gynecologists suggests healthcare providers consider depression screening for perinatal women. Additionally, the American Academy of Pediatrics recommends pediatricians screen mothers for PPD at 1-month, 2-month and 4-month visits. However, many providers do not consistently provide screening and appropriate follow-up. For example, in Canada, Alberta is the only province with universal PPD screening. This screening is carried out by Public Health nurses with the baby's immunization schedule.

The Edinburgh Postnatal Depression Scale, a standardized self-reported questionnaire, may be used to identify women who have postpartum depression. If the new mother scores 13 or more, she likely has PPD and further assessment should follow.

A diagnostic assessment may be conducted by a suitably trained general practitioner, or by a psychiatrist or psychologist, who records the person's current circumstances, biographical history, current symptoms, and family history. The broad clinical aim is to formulate the relevant biological, psychological, and social factors that may be impacting on the individual's mood. The assessor may also discuss the person's current ways of regulating mood (healthy or otherwise) such as alcohol and drug use. The assessment also includes a mental state examination, which is an assessment of the person's current mood and thought content, in particular the presence of themes of hopelessness or pessimism, self-harm or suicide, and an absence of positive thoughts or plans. Specialist mental health services are rare in rural areas, and thus diagnosis and management is left largely to primary-care clinicians. This issue is even more marked in developing countries. The mental health examination may include the use of a rating scale such as the Hamilton Rating Scale for Depression or the Beck Depression Inventory or the Suicide Behaviors Questionnaire-Revised. The score on a rating scale alone is insufficient to diagnose depression to the satisfaction of the DSM or ICD, but it provides an indication of the severity of symptoms for a time period, so a person who scores above a given cut-off point can be more thoroughly evaluated for a depressive disorder diagnosis. Several rating scales are used for this purpose.

Primary-care physicians and other non-psychiatrist physicians have more difficulty with underrecognition and undertreatment of depression compared to psychiatric physicians, in part because of the physical symptoms that often accompany depression, in addition to the many potential patient, provider, and system barriers that the authors describe. A review found that non-psychiatrist physicians miss about two-thirds of cases, though this has improved somewhat in more recent studies.

Before diagnosing a major depressive disorder, in general a doctor performs a medical examination and selected investigations to rule out other causes of symptoms. These include blood tests measuring TSH and thyroxine to exclude hypothyroidism; basic electrolytes and serum calcium to rule out a metabolic disturbance; and a full blood count including ESR to rule out a systemic infection or chronic disease. Adverse affective reactions to medications or alcohol misuse are often ruled out, as well. Testosterone levels may be evaluated to diagnose hypogonadism, a cause of depression in men. Vitamin D levels might be evaluated, as low levels of vitamin D have been associated with greater risk for depression.

Subjective cognitive complaints appear in older depressed people, but they can also be indicative of the onset of a dementing disorder, such as Alzheimer's disease. Cognitive testing and brain imaging can help distinguish depression from dementia. A CT scan can exclude brain pathology in those with psychotic, rapid-onset or otherwise unusual symptoms. In general, investigations are not repeated for a subsequent episode unless there is a medical indication.

No biological tests confirm major depression. Biomarkers of depression have been sought to provide an objective method of diagnosis. There are several potential biomarkers, including Brain-Derived Neurotrophic Factor and various functional MRI techniques. One study developed a decision tree model of interpreting a series of fMRI scans taken during various activities. In their subjects, the authors of that study were able to achieve a sensitivity of 80% and a specificity of 87%, corresponding to a negative predictive value of 98% and a positive predictive value of 32% (positive and negative likelihood ratios were 6.15, 0.23, respectively). However, much more research is needed before these tests could be used clinically.

Several studies have shown that the risk of suicide is higher in patients who suffer from Bipolar II than those who suffer from Bipolar I, and especially higher than patients who suffer from major depressive disorder.

In results of a summary of several lifetime study experiments, it was found that 24% of Bipolar II patients experienced suicidal ideation or suicide attempts compared to 17% in Bipolar I patients and 12% in major depressive patients. Bipolar disorders, in general, are the third leading cause of death in 15- to 24-year-olds. Bipolar II patients were also found to employ more lethal means and have more complete suicides overall.

Bipolar II patients have several risk factors that increase their risk of suicide. The illness is very recurrent and results in severe disabilities, interpersonal relationship problems, barriers to academic, financial, and vocational goals, and a loss of social standing in their community, all of which increase the likelihood of suicide. Mixed symptoms and rapid-cycling, both very common in Bipolar II, are also associated with an increased risk of suicide. The tendency for Bipolar II to be misdiagnosed and treated ineffectively, or not at all in some cases, leads to an increased risk.

As a result of the high suicide risk for this group, reducing the risk and preventing attempts remains a main part of the treatment; a combination of self-monitoring, close supervision by a therapist, and faithful adherence to their medication regimen will help to reduce the risk and prevent the likelihood of a completed suicide.

Treatment of mixed states is typically based upon administration of mood stabilizing medication, which may include anticonvulsants such as valproic acid; atypical antipsychotics such as olanzapine, aripiprazole, and ziprasidone; or first-generation antipsychotics such as haloperidol. There is question of lithium's efficacy for treatment of mixed states due to conflicting conclusions drawn from various trials and research. Mood stabilizers work to reduce the manic symptoms associated with the mixed state, but they are not considered particularly effective for improving concurrent depressive symptoms.

Schizophreniform disorder is equally prevalent among men and women. The most common ages of onset are 18–24 for men and 18–35 for women. While the symptoms of schizophrenia often develop gradually over a period of years, the diagnostic criteria for schizophreniform disorder require a much more rapid onset.

Available evidence suggests variations in incidence across sociocultural settings. In the United States and other developed countries, the incidence is low, possibly fivefold less than that of schizophrenia. In developing countries, the incidence is substantially higher, especially for the subtype "With Good Prognostic Features". In some of these settings schizophreniform disorder may be as common as schizophrenia.

Cyclothymia, a condition of continuous mood fluctuations, is characterized by oscillating experiences of hypomania and depression that fail to meet the diagnostic criteria for either manic or major depressive episodes. These periods are often interspersed with periods of relatively normal (euthymic) functioning.

When a patient presents with a history of at least one episode of both hypomania and major depression, each of which meet the diagnostic criteria, bipolar II disorder is diagnosed. In some cases, depressive episodes routinely occur during the fall or winter and hypomanic ones in the spring or summer. In such cases, one speaks of a "seasonal pattern".

If left untreated, and in those so predisposed, hypomania may transition into mania, which may be psychotic, in which case bipolar I disorder is the correct diagnosis.