There is no single test that can fully diagnose fibromyalgia and there is debate over what should be considered essential diagnostic criteria and whether an objective diagnosis is possible. In most cases, people with fibromyalgia symptoms may also have laboratory test results that appear normal and many of their symptoms may mimic those of other rheumatic conditions such as arthritis or osteoporosis. The most widely accepted set of classification criteria for research purposes was elaborated in 1990 by the Multicenter Criteria Committee of the American College of Rheumatology. These criteria, which are known informally as "the ACR 1990", define fibromyalgia according to the presence of the following criteria:

- A history of widespread pain lasting more than three months – affecting all four quadrants of the body, i.e., both sides, and above and below the waist.

- Tender points – there are 18 designated possible tender points (although a person with the disorder may feel pain in other areas as well). Diagnosis is no longer based on the number of tender points.

The ACR criteria for the classification of patients were originally established as inclusion criteria for research purposes and were not intended for clinical diagnosis but have now become the "de facto" diagnostic criteria in the clinical setting. It should be noted that the number of tender points that may be active at any one time may vary with time and circumstance. A controversial study was done by a legal team looking to prove their client's disability based primarily on tender points and their widespread presence in non-litigious communities prompted the lead author of the ACR criteria to question now the useful validity of tender points in diagnosis. Use of control points has been used to cast doubt on whether a person has fibromyalgia, and to claim the person is malingering; however, no research has been done for the use of control points to diagnose fibromyalgia, and such diagnostic tests have been advised against, and people complaining of pain all over should still have fibromyalgia considered as a diagnosis.

In 2010, the American College of Rheumatology approved provisional revised diagnostic criteria for fibromyalgia that eliminated the 1990 criteria's reliance on tender point testing. The revised criteria use a widespread pain index (WPI) and symptom severity scale (SS) in place of tender point testing under the 1990 criteria. The WPI counts up to 19 general body areas in which the person has experienced pain in the preceding two weeks. The SS rates the severity of the person's fatigue, unrefreshed waking, cognitive symptoms, and general somatic symptoms, each on a scale from 0 to 3, for a composite score ranging from 0 to 12. The revised criteria for diagnosis are:

- WPI ≥ 7 and SS ≥ 5 "OR" WPI 3–6 and SS ≥ 9,

- Symptoms have been present at a similar level for at least three months, "and"

- No other diagnosable disorder otherwise explains the pain.

There are no characteristic laboratory abnormalities to diagnose CFS; testing is used to rule out other conditions which could be responsible for the symptoms. When symptoms are attributable to certain other conditions, the diagnosis of CFS is excluded. As such, a diagnosis of CFS/ME is generally one of exclusion (of alternative diagnoses).

Notable definitions include:

- Centers for Disease Control and Prevention (CDC) definition (1994), the most widely used clinical and research description of CFS, is also called the Fukuda definition and is a revision of the "Holmes" or "CDC 1988" scoring system. The 1994 criteria require the presence of four or more symptoms beyond fatigue, while the 1988 criteria require six to eight.

- The ME/CFS 2003 Canadian Clinical working definition states: "A patient with ME/CFS will meet the criteria for fatigue, post-exertional malaise and/or fatigue, sleep dysfunction, and pain; have two or more neurological/cognitive manifestations and one or more symptoms from two of the categories of autonomic, neuroendocrine, and immune manifestations; and the illness persists for at least 6 months".

Clinical practice guidelines are generally based on case descriptions, with the aim of improving diagnosis, management and treatment. An example is the CFS/ME guideline for the National Health Services in England and Wales, produced in 2007 by the National Institute for Health and Clinical Excellence (NICE).

The International Statistical Classification of Diseases and Related Health Problems (ICD), maintained by the World Health Organization, does not recognize multiple chemical sensitivity or environmental sensitivity as a valid diagnosis. The Australian Department of Health recognizes that sometimes debilitating symptoms are attributed to MCS but notes that diagnosis, treatment, and any underlying mechanism remain uncertain. The German Institute for Medical Documentation and Information recognizes MCS as a physical disease and is subsequentially recognized in Austria. The American Medical Association does not recognize MCS as an organic disease because of the lack of scientific evidence supporting a cause-and-effect relationship between very low level exposure and the symptoms of MCS. The American Academy of Allergy, Asthma, and Immunology, the California Medical Association, the American College of Physicians, and the International Society of Regulatory Toxicology and Pharmacology also do not recognize MCS. The US Occupational Safety and Health Administration (OSHA) indicates that MCS is highly controversial and that there is insufficient scientific evidence to explain the relationship between the suggested causes of MCS and its symptoms. OSHA recommends evaluation by a physician knowledgeable of the symptoms presented.

In various studies, about one half of the patients who seek medical treatment for symptoms of MCS meet the criteria for depressive and anxiety disorders. Because many people eliminate whole categories of food in an effort to reduce symptoms, a complete review of the patient's diet may be needed to avoid nutritional deficiencies.

A pain scale measures a patient's pain intensity and other features. Pain scales are based on self-report, observational (behavioral), or physiological data. Self-report is considered primary and should be obtained if possible. Pain measurements help determine the severity, type, and duration of the pain. They are also used to construct an accurate diagnosis, determine a treatment plan, and evaluate the effectiveness of treatment. Pain scales are available for neonates, infants, children, adolescents, adults, seniors, and persons with impaired communication. Pain assessments are often regarded as "the 5th Vital Sign".

Sudden cessation of high-dose corticosteroids, opioids, barbiturates, benzodiazepines, caffeine or alcohol can induce myalgia in many respects.

Treatment may involve investigation, reassurance and explanation, and possibly specialist treatment such as antidepressants or cognitive behavioral therapy.

Although pain is subjective and is considered to exist as a spectrum rather than an exact determination, different assessment tools compare pain levels over time. This kind of assessment incorporates pain scales and requires a high enough developmental level so that the child can respond to the question(s). A verbal response is not always necessary to quantify the pain.

RLS symptoms may gradually worsen with age, though more slowly for those with the idiopathic form of RLS than for patients who also have associated medical condition. Nevertheless, current therapies can control the disorder, minimizing symptoms and increasing periods of restful sleep. In addition, some patients have remissions, periods in which symptoms decrease or disappear for days, weeks, or months, although symptoms usually eventually reappear. Being diagnosed with RLS does not indicate or foreshadow another neurological disease.

Functional magnetic resonance imaging brain scanning has been used to measure pain, and correlates well with self-reported pain.

Functional somatic syndromes may occur in 6 to 36% of the population.

At the time of first PANS presentation, note any family history of pharyngitis, impetigo, perianal dermatitis, and GAS infections. If possible, family members should have a throat swab cultured for GAS. Ongoing vigilance against GAS infections in any of the patient’s close contacts is important, as symptom exacerbations have been reported following exposure to a sibling with GAS (even when the PANDAS patient had no evidence of infection). Prompt medical attention to symptoms suggestive of streptococcal infection is important not only to protect the patient, but also siblings, who may be at an increased genetic risk for PANS.

Below is a list of criteria for diagnosing TMS, according to Schechter and Sarno:

- "Lack of known physical cause:" Schechter and Sarno state that a physical examination, tests and imaging studies is needed to rule out serious conditions, such as tumors. Sarno considers spinal disc herniations to generally be harmless, because he says the symptom location does not even correlate to the herniation location.

- "Tender points:" While medical doctors use eleven of eighteen tender points as a diagnostic criteria for fibromyalgia, Sarno states that he uses six main tender points to diagnose TMS: two tender points in the upper trapezius muscles, two in the lumbar paraspinal muscles and two in the lateral upper buttocks. He states that these are found in 99% of TMS patients.

- "History of other psychosomatic disorders:" Schechter and Sarno consider a prior history of other psychosomatic disorders an indication that the patient may have TMS. They list irritable bowel syndrome and tension headache as examples of psychosomatic disorders.

Schechter and Sarno state that if a patient is unable to visit a medical doctor who is trained in TMS, then the patient should see a traditional medical doctor to rule out serious disorders, such as fractures, tumors and infections.

Mainstays of treatment include psychoeducational, psychotherapeutic, behavioral, family, school-based and pharmacological interventions, which reduce suffering and improve functioning until the immunologic and infectious processes are addressed.

A diagnosis of a functional neurological disorder is dependent on positive features from the history and examination.

Patients with functional movement disorders and limb weakness may experience symptom onset triggered by an episode of acute pain, a physical injury or physical trauma. They may also experience symptoms when faced with a psychological stressor, but this isn't the case for most patients. Patients with functional neurological disorders are more likely to have a history of another illness such as irritable bowel syndrome, chronic pelvic pain or fibromyalgia but this cannot be used to make a diagnosis.

FND does not show up on blood tests or structural brain imaging such as MRI or CT scanning. However, this is also the case for many other neurological conditions so negative investigations should not be used alone to make the diagnosis. FND can, however, occur alongside other neurological diseases and tests may show non-specific abnormalities which cause confusion for doctors and patients.

There are no specific tests for RLS, but non-specific laboratory tests are used to rule out other causes such as vitamin deficiencies. According to the National Institutes of Health's National Institute of Neurological Disorders and Stroke, four symptoms are used to confirm the diagnosis:

- The symptoms are more severe at night and do not occur, or are negligible, in the morning (although in extreme cases, symptoms may occur in the daytime)

- An irresistible urge to move the legs and/or arms, often associated with a sensation of pain, burning, pricking, tingling, numbness, or other unpleasant or unusual sensations

- The sensations begin following relaxation or a period of staying still, and during sleep

- Temporary relief from these sensations during movement of the affected legs and/or arms

Historically, misdiagnosis rates have been high due to the complex nature of the disorder although some research now indicates that misdiagnosis may be reducing.

The nerve conduction study usually provides useful information for making diagnosis. A CT scan is sometimes used to rule out some causes from the central nervous system.

A functional disorder is a medical condition that impairs the normal function of a bodily process, but where every part of the body looks completely normal under examination, dissection or even under a microscope. This stands in contrast to a structural disorder (in which some part of the body can be seen to be abnormal) or a psychosomatic disorder (in which symptoms are caused by psychological or psychiatric illness). Definitions vary somewhat between fields of medicine.

Generally, the mechanism that causes a functional disorder is unknown, poorly understood, or occasionally unimportant for treatment purposes. The brain or nerves are often believed to be involved. It is common that a person with one functional disorder will have others.

The diagnosis of dysautonomia depends on the overall function of three autonomic functions – cardiovagal, adrenergic, and sudomotor. A diagnosis should, at a bare minimum, include measurements of blood pressure and heart rate while lying flat, and after at least 3 minutes of standing. The best way to achieve a diagnosis includes a range of testing, notably an autonomic reflex screen, tilt table test, and testing of the sudomotor response (QSART or thermoregulatory sweat test).

Additional tests and examinations to determine a diagnosis of dysautonomia include

Inadequate treatment of pain is widespread throughout surgical wards, intensive care units, and accident and emergency departments In general practice, the management of all forms of chronic pain including cancer pain, and in end of life care. This neglect extends to all ages, from newborns to medically frail elderly. African and Hispanic Americans are more likely than others to suffer unnecessarily while in the care of a physician; and women's pain is more likely to be undertreated than men's.

The International Association for the Study of Pain advocates that the relief of pain should be recognized as a human right, that chronic pain should be considered a disease in its own right, and that pain medicine should have the full status of a medical specialty. It is a specialty only in China and Australia at this time. Elsewhere, pain medicine is a subspecialty under disciplines such as anesthesiology, physiatry, neurology, palliative medicine and psychiatry. In 2011, Human Rights Watch alerted that tens of millions of people worldwide are still denied access to inexpensive medications for severe pain.

Breastfeeding may decrease pain when babies are immunized.

Multiple sclerosis (neurologic pain interpreted as muscular), Myalgic Encephalomyelitis (chronic fatigue syndrome), Myositis, Mixed connective tissue disease, Lupus erythematosus, Fibromyalgia syndrome, Familial Mediterranean fever, Polyarteritis nodosa, Devic's disease, Morphea, Sarcoidosis

Imaging is indicated when there are red flags, ongoing neurological symptoms that do not resolve, or ongoing or worsening pain. In particular, early use of imaging (either MRI or CT) is recommended for suspected cancer, infection, or cauda equina syndrome. MRI is slightly better than CT for identifying disc disease; the two technologies are equally useful for diagnosing spinal stenosis. Only a few physical diagnostic tests are helpful. The straight leg raise test is almost always positive in those with disc herniation. Lumbar provocative discography may be useful to identify a specific disc causing pain in those with chronic high levels of low back pain. Similarly, therapeutic procedures such as nerve blocks can be used to determine a specific source of pain. Some evidence supports the use of facet joint injections, transforminal epidural injections and sacroilliac injections as diagnostic tests. Most other physical tests, such as evaluating for scoliosis, muscle weakness or wasting, and impaired reflexes, are of little use.

Complaints of low back pain are one of the most common reasons people visit doctors. For pain that has lasted only a few weeks, the pain is likely to subside on its own. Thus, if a person's medical history and physical examination do not suggest a specific disease as the cause, medical societies advise against imaging tests such as X-rays, CT scans, and MRIs. Individuals may want such tests but, unless red flags are present, they are unnecessary health care. Routine imaging increases costs, is associated with higher rates of surgery with no overall benefit, and the radiation used may be harmful to one's health. Fewer than 1% of imaging tests identify the cause of the problem. Imaging may also detect harmless abnormalities, encouraging people to request further unnecessary testing or to worry. Even so, MRI scans of the lumbar region increased by more than 300% among United States Medicare beneficiaries from 1994 to 2006.