No major organization recommends universal screening for diabetes as there is no evidence that such a program improve outcomes. Screening is recommended by the United States Preventive Services Task Force (USPSTF) in adults without symptoms whose blood pressure is greater than 135/80 mmHg. For those whose blood pressure is less, the evidence is insufficient to recommend for or against screening. There is no evidence that it changes the risk of death in this group of people. They also recommend screening among those who are overweight and between the ages of 40 and 70.

The World Health Organization recommends testing those groups at high risk and in 2014 the USPSTF is considering a similar recommendation. High-risk groups in the United States include: those over 45 years old; those with a first degree relative with diabetes; some ethnic groups, including Hispanics, African-Americans, and Native-Americans; a history of gestational diabetes; polycystic ovary syndrome; excess weight; and conditions associated with metabolic syndrome. The American Diabetes Association recommends screening those who have a BMI over 25 (in people of Asian descent screening is recommended for a BMI over 23).

The World Health Organization definition of diabetes (both type 1 and type 2) is for a single raised glucose reading with symptoms, otherwise raised values on two occasions, of either:

- fasting plasma glucose ≥ 7.0 mmol/l (126 mg/dl)

- with a glucose tolerance test, two hours after the oral dose a plasma glucose ≥ 11.1 mmol/l (200 mg/dl)

A random blood sugar of greater than 11.1 mmol/l (200 mg/dL) in association with typical symptoms or a glycated hemoglobin (HbA) of ≥ 48 mmol/mol (≥ 6.5 DCCT %) is another method of diagnosing diabetes. In 2009 an International Expert Committee that included representatives of the American Diabetes Association (ADA), the International Diabetes Federation (IDF), and the European Association for the Study of Diabetes (EASD) recommended that a threshold of ≥ 48 mmol/mol (≥ 6.5 DCCT %) should be used to diagnose diabetes. This recommendation was adopted by the American Diabetes Association in 2010. Positive tests should be repeated unless the person presents with typical symptoms and blood sugars >11.1 mmol/l (>200 mg/dl).

Threshold for diagnosis of diabetes is based on the relationship between results of glucose tolerance tests, fasting glucose or HbA and complications such as retinal problems. A fasting or random blood sugar is preferred over the glucose tolerance test, as they are more convenient for people. HbA has the advantages that fasting is not required and results are more stable but has the disadvantage that the test is more costly than measurement of blood glucose. It is estimated that 20% of people with diabetes in the United States do not realize that they have the disease.

Diabetes mellitus type 2 is characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency. This is in contrast to diabetes mellitus type 1 in which there is an absolute insulin deficiency due to destruction of islet cells in the pancreas and gestational diabetes mellitus that is a new onset of high blood sugars associated with pregnancy. Type 1 and type 2 diabetes can typically be distinguished based on the presenting circumstances. If the diagnosis is in doubt antibody testing may be useful to confirm type 1 diabetes and C-peptide levels may be useful to confirm type 2 diabetes, with C-peptide levels normal or high in type 2 diabetes, but low in type 1 diabetes.

There are no known ways of preventing LADA type 1 diabetes, though some researchers believe it could be stopped at a very early stage if a diagnosis is made prior to the body's destruction of its beta cells.

Diabetes mellitus is characterized by recurrent or persistent high blood sugar, and is diagnosed by demonstrating any one of the following:

- Fasting plasma glucose level ≥ 7.0 mmol/l (126 mg/dl)

- Plasma glucose ≥ 11.1 mmol/l (200 mg/dl) two hours after a 75 g oral glucose load as in a glucose tolerance test

- Symptoms of high blood sugar and casual plasma glucose ≥ 11.1 mmol/l (200 mg/dl)

- Glycated hemoglobin (HbA) ≥ 48 mmol/mol (≥ 6.5 DCCT %).

A positive result, in the absence of unequivocal high blood sugar, should be confirmed by a repeat of any of the above methods on a different day. It is preferable to measure a fasting glucose level because of the ease of measurement and the considerable time commitment of formal glucose tolerance testing, which takes two hours to complete and offers no prognostic advantage over the fasting test. According to the current definition, two fasting glucose measurements above 126 mg/dl (7.0 mmol/l) is considered diagnostic for diabetes mellitus.

Per the World Health Organization people with fasting glucose levels from 6.1 to 6.9 mmol/l (110 to 125 mg/dl) are considered to have impaired fasting glucose. people with plasma glucose at or above 7.8 mmol/l (140 mg/dl), but not over 11.1 mmol/l (200 mg/dl), two hours after a 75 g oral glucose load are considered to have impaired glucose tolerance. Of these two prediabetic states, the latter in particular is a major risk factor for progression to full-blown diabetes mellitus, as well as cardiovascular disease. The American Diabetes Association since 2003 uses a slightly different range for impaired fasting glucose of 5.6 to 6.9 mmol/l (100 to 125 mg/dl).

Glycated hemoglobin is better than fasting glucose for determining risks of cardiovascular disease and death from any cause.

Glutamic acid decarboxylase autoantibodies (GADA), islet cell autoantibodies (ICA), insulinoma-associated (IA-2) autoantibodies, and zinc transporter autoantibodies (ZnT8) are all associated with LADA; GADAs are commonly found in cases of diabetes mellitus type 1.

The presence of Islet Cell Complement Fixing Autoantibodies also aids in a differential diagnosis between LADA and type 2 diabetes. Persons with LADA often test positive for ICA, whereas type 2 diabetics only seldom do.

Persons with LADA usually test positive for Glutamic acid decarboxylase antibodies, whereas in type 1 diabetes these antibodies are more commonly seen in adults rather than in children. In addition to being useful in making an early diagnosis for type 1 diabetes mellitus, GAD antibodies tests are used for differential diagnosis between LADA and type 2 diabetes and may also be used for differential diagnosis of gestational diabetes, risk prediction in immediate family members for type 1, as well as a tool to monitor prognosis of the clinical progression of type 1 diabetes.

Diabetes mellitus is characterized by recurrent or persistent hyperglycemia, and is diagnosed by demonstrating any one of the following:

- Fasting plasma glucose level at or above 7.0 mmol/L (126 mg/dL).

- Plasma glucose at or above 11.1 mmol/L (200 mg/dL) two hours after a 75 g oral glucose load as in a glucose tolerance test.

- Symptoms of hyperglycemia and casual plasma glucose at or above 11.1 mmol/L (200 mg/dL).

- Glycated hemoglobin (hemoglobin A1C) at or above 48 mmol/mol (≥ 6.5 DCCT %). (This criterion was recommended by the American Diabetes Association in 2010, although it has yet to be adopted by the WHO.)

About a quarter of people with new type 1 diabetes have developed some degree of diabetic ketoacidosis (a type of metabolic acidosis which is caused by high concentrations of ketone bodies, formed by the breakdown of fatty acids and the deamination of amino acids) by the time the diabetes is recognized. The diagnosis of other types of diabetes is usually made in other ways. These include ordinary health screening, detection of hyperglycemia during other medical investigations, and secondary symptoms such as vision changes or unexplained fatigue. Diabetes is often detected when a person suffers a problem that may be caused by diabetes, such as a heart attack, stroke, neuropathy, poor wound healing or a foot ulcer, certain eye problems, certain fungal infections, or delivering a baby with macrosomia or hypoglycemia (low blood sugar).

A positive result, in the absence of unequivocal hyperglycemia, should be confirmed by a repeat of any of the above-listed methods on a different day. Most physicians prefer to measure a fasting glucose level because of the ease of measurement and the considerable time commitment of formal glucose tolerance testing, which takes two hours to complete and offers no prognostic advantage over the fasting test. According to the current definition, two fasting glucose measurements above 126 mg/dL (7.0 mmol/L) is considered diagnostic for diabetes mellitus.

In type 1, pancreatic beta cells in the islets of Langerhans are destroyed, decreasing endogenous insulin production. This distinguishes type 1's origin from type 2. Type 2 diabetes is characterized by insulin resistance, while type 1 diabetes is characterized by insulin deficiency, generally without insulin resistance. Another hallmark of type 1 diabetes is islet autoreactivity, which is generally measured by the presence of autoantibodies directed towards the beta cells.

Currently, MODY is the final diagnosis in 1%–2% of people initially diagnosed with diabetes. The prevalence is 70–110 per million population. 50% of first-degree relatives will inherit the same mutation, giving them a greater than 95% lifetime risk of developing MODY themselves. For this reason, correct diagnosis of this condition is important. Typically patients present with a strong family history of diabetes (any type) and the onset of symptoms is in the second to fifth decade.

There are two general types of clinical presentation.

- Some forms of MODY produce significant hyperglycemia and the typical signs and symptoms of diabetes: increased thirst and urination (polydipsia and polyuria).

- In contrast, many people with MODY have no signs or symptoms and are diagnosed either by accident, when a high glucose is discovered during testing for other reasons, or screening of relatives of a person discovered to have diabetes. Discovery of mild hyperglycemia during a routine glucose tolerance test for pregnancy is particularly characteristic.

MODY cases may make up as many as 5% of presumed type 1 and type 2 diabetes cases in a large clinic population. While the goals of diabetes management are the same no matter what type, there are two primary advantages of confirming a diagnosis of MODY.

- Insulin may not be necessary and it may be possible to switch a person from insulin injections to oral agents without loss of glycemic control.

- It may prompt screening of relatives and so help identify other cases in family members.

As it occurs infrequently, many cases of MODY are initially assumed to be more common forms of diabetes: type 1 if the patient is young and not overweight, type 2 if the patient is overweight, or gestational diabetes if the patient is pregnant. Standard diabetes treatments (insulin for type 1 and gestational diabetes, and oral hypoglycemic agents for type 2) are often initiated before the doctor suspects a more unusual form of diabetes.

In some forms of MODY, standard treatment is appropriate, though exceptions occur:

- In MODY2, oral agents are relatively ineffective and insulin is unnecessary.

- In MODY1 and MODY3, insulin may be more effective than drugs to increase insulin sensitivity.

- Sulfonylureas are effective in the K channel forms of neonatal-onset diabetes. The mouse model of MODY diabetes suggested that the reduced clearance of sulfonylureas stands behind their therapeutic success in human MODY patients, but Urbanova et al. found that human MODY patients respond differently to the mouse model and that there was no consistent decrease in the clearance of sulfonylureas in randomly selected HNF1A-MODY and HNF4A-MODY patients.

The appearance of diabetes-related autoantibodies has been shown to be able to predict the appearance of diabetes type 1 before any hyperglycemia arises, the main ones being islet cell autoantibodies, insulin autoantibodies, autoantibodies targeting the 65-kDa isoform of glutamic acid decarboxylase (GAD), autoantibodies targeting the phosphatase-related IA-2 molecule, and zinc transporter autoantibodies (ZnT8). By definition, the diagnosis of diabetes type 1 can be made first at the appearance of clinical symptoms and/or signs, but the emergence of autoantibodies may itself be termed "latent autoimmune diabetes". Not everyone with autoantibodies progresses to diabetes type 1, but the risk increases with the number of antibody types, with three to four antibody types giving a risk of progressing to diabetes type 1 of 60%–100%. The time interval from emergence of autoantibodies to clinically diagnosable diabetes can be a few months in infants and young children, but in some people it may take years – in some cases more than 10 years. Islet cell autoantibodies are detected by conventional immunofluorescence, while the rest are measured with specific radiobinding assays.

MODY 1 is a form of maturity onset diabetes of the young.

MODY 1 is due to a loss-of-function mutation in the gene on chromosome 20. This gene codes for HNF4-α protein also known as transcription factor 14 (TCF14). HNF4α controls function of HNF1α (see MODY 3; ) and perhaps HNF1β (MODY 5) as well. This transcription network plays a role in the early development of the pancreas, liver, and intestines. In the pancreas these genes influence expression of, among others, the genes for insulin, the principal glucose transporter (GLUT2), and several proteins involved in glucose and mitochondrial metabolism.

Although pancreatic beta cells produce adequate insulin in infancy, the capacity for insulin production declines thereafter. Diabetes (persistent hyperglycemia) typically develops by early adult years, but may not appear until later decades. The degree of insulin deficiency is slowly progressive. Many patients with MODY 1 are treated with sulfonylureas for years before insulin is required.

Liver effects are subtle and not clinically significant. Many people with this condition have low levels of triglycerides, lipoprotein(a), apolipoproteins AII and CIII.

Mutations in the alternative promoter of HNF4A are linked to development of type 2 diabetes.

MODY 2 is a form of maturity onset diabetes of the young.

MODY 2 is due to any of several mutations in the "GCK" gene on chromosome 7 for glucokinase. Glucokinase serves as the glucose sensor for the pancreatic beta cell. Normal glucokinase triggers insulin secretion as the glucose exceeds about 90 mg/dl (5 mM). These loss-of-function mutations result in a glucokinase molecule that is less sensitive or less responsive to rising levels of glucose. The beta cells in MODY 2 have a normal ability to make and secrete insulin, but do so only above an abnormally high threshold (e.g., 126–144 mg/dl, or 7-8 mM). This produces a chronic, mild increase in blood sugar, which is usually asymptomatic. It is usually detected by accidental discovery of mildly elevated blood sugar (e.g., during pregnancy screening). An oral glucose tolerance test is much less abnormal than would be expected from the impaired (elevated) fasting blood sugar, since insulin secretion is usually normal once the glucose has exceeded the threshold for that specific variant of the glucokinase enzyme.

The degree of blood sugar elevation does not worsen rapidly with age, and long-term diabetic complications are rare. In healthy children and adults, a high blood sugar level can be avoided by a healthy diet and exercise, primarily avoiding large amounts of carbohydrates. However, as people who have MODY2 enter their 50's and 60's, even though they continue to eat a healthy diet and exercise, they sometimes are unable to control a high blood sugar level with these measures. In these cases, many medicines for type II diabetes mellitus are not effective, because MODY2 does not cause insulin resistance. Repaglinide (Prandin) can help the body regulate the amount of glucose in the blood by stimulating the pancreas to release insulin before meals. In some cases, the baseline glucose levels are too high as well and insulin is required.

MODY2 is an autosomal dominant condition. Autosomal dominance refers to a single, abnormal gene on one of the first 22 nonsex chromosomes from either parent which can cause an autosomal disorder. Dominant inheritance means an abnormal gene from one parent is capable of causing disease, even though the matching gene from the other parent is normal. The abnormal gene "dominates" the pair of genes. If just one parent has a dominant gene defect, each child has a 50% chance of inheriting the disorder.

This type of MODY demonstrates the common circulation but complex interplay between maternal and fetal metabolism and hormone signals in the determination of fetal size. A small number of infants will have a new mutation not present in their mothers. If the mother is affected and the fetus is not, the maternal glucose will be somewhat high and the normal pancreas of the fetus will generate more insulin to compensate, resulting in a large infant. If the fetus is affected but mother is not, glucoses will be normal and fetal insulin production will be low, resulting in intrauterine growth retardation. Finally, if both mother and fetus have the disease, the two defects will offset each other and fetal size will be unaffected.

When both "GCK" genes are affected the diabetes appears earlier and the hyperglycemia is more severe. A form of permanent neonatal diabetes has been caused by homozygous mutations in the GCK gene.

There is no known preventive measure for type 1 diabetes. Type 2 diabeteswhich accounts for 85–90% of all casescan often be prevented or delayed by maintaining a normal body weight, engaging in physical activity, and consuming a healthy diet. Higher levels of physical activity (more than 90 minutes per day) reduce the risk of diabetes by 28%. Dietary changes known to be effective in helping to prevent diabetes include maintaining a diet rich in whole grains and fiber, and choosing good fats, such as the polyunsaturated fats found in nuts, vegetable oils, and fish. Limiting sugary beverages and eating less red meat and other sources of saturated fat can also help prevent diabetes. Tobacco smoking is also associated with an increased risk of diabetes and its complications, so smoking cessation can be an important preventive measure as well.

The relationship between type 2 diabetes and the main modifiable risk factors (excess weight, unhealthy diet, physical inactivity and tobacco use) is similar in all regions of the world. There is growing evidence that the underlying determinants of diabetes are a reflection of the major forces driving social, economic and cultural change: globalization, urbanization, population aging, and the general health policy environment.

MODY 3 is a form of maturity onset diabetes of the young.

MODY 3 (also known as HNF1A-MODY) is caused by mutations of the HNF1-alpha; gene, a homeobox gene on chromosome 12. This is the most common type of MODY in populations with European ancestry, accounting for about 70% of all cases in Europe. HNF1α is a transcription factor (also known as transcription factor 1, TCF1) that is thought to control a regulatory network (including, among other genes, HNF1α) important for differentiation of beta cells. Mutations of this gene lead to reduced beta cell mass or impaired function. MODY 1 and MODY 3 diabetes are clinically similar. About 70% of people develop this type of diabetes by age 25 years, but it occurs at much later ages in a few. This type of diabetes can often be treated with sulfonylureas with excellent results for decades. However, the loss of insulin secretory capacity is slowly progressive and most eventually need insulin.

This is the form of MODY which can most resemble ordinary type 1 diabetes, and one of the incentives for diagnosing it is that insulin may be discontinued or deferred in favor of oral sulfonylureas. Some people treated with insulin for years due to a presumption of type 1 diabetes have been able to switch to pills and discontinue injections. Long-term diabetic complications can occur if the glucose is not adequately controlled.

High-sensitivity measurements of CRP may help to distinguish between HNF1A-MODY and other forms of diabetes

MODY 4 is a form of maturity onset diabetes of the young.

MODY 4 arises from mutations of the PDX1 homeobox gene on chromosome 13. Pdx-1 is a transcription factor vital to the development of the embryonic pancreas. Even in adults it continues to play a role in the regulation and expression of genes for insulin, GLUT2, glucokinase, and somatostatin.

MODY 4 is so rare that only a single family has been well-studied. A child born with pancreatic agenesis (absence of the pancreas) was found to be homozygous for Pdx-1 mutations. A number of older relatives who were heterozygous had mild hyperglycemia or diabetes. None were severely insulin-deficient and all were controlled with either diet or oral hypoglycemic agents.

MODY 6 is a form of maturity onset diabetes of the young.

MODY 6 arises from mutations of the gene for the transcription factor referred to as neurogenic differentiation 1. The gene is on chromosome 2 in a region of the p arm known as IDDM7 because it includes genes affecting susceptibility to type 1 diabetes. NeuroD1 promotes transcription of the insulin gene as well as some genes involved in formation of beta cells and parts of the nervous system.

This is also one of the rarer forms of MODY. Only 3 kindreds with mutations causing MODY6 have been identified so far. In both, some of the members had more typical type 2 diabetes rather than MODY, and the reasons for the difference of expression have not been worked out. Most of the family members with diabetes were diagnosed after age 40, but a few required insulin for blood sugar control.

In most regions, galactosemia is diagnosed as a result of newborn screening, most commonly by determining the concentration of galactose in a dried blood spot. Some regions will perform a second-tier test of GALT enzyme activity on samples with elevated galactose, while others perform both GALT and galactose measurements. While awaiting confirmatory testing for classic galactosemia, the infant is typically fed a soy-based formula, as human and cow milk contains galactose as a component of lactose. Confirmatory testing would include measurement of enzyme activity in red blood cells, determination of Gal-1-P levels in the blood, and mutation testing. The differential diagnosis for elevated galactose concentrations in blood on a newborn screening result can include other disorders of galactose metabolism, including galactokinase deficiency and galactose epimerase deficiency. Enzyme assays are commonly done using fluorometric detection or older radioactively labeled substrates.

In terms of diagnosis for this condition, the following methods/tests are available:

- Endoscopic

- CT scan

- Serum endocrine autoantibody screen

- Histologic test

Renal cysts and diabetes syndrome (RCAD), also known as MODY 5, is a form of maturity onset diabetes of the young.

HNF1β-related MODY is one of the less common forms of MODY, with some distinctive clinical features, including atrophy of the pancreas and several forms of renal disease. HNF1β, also known as transcription factor 2 (TCF2), is involved in early stages of embryonic development of several organs, including the pancreas, where it contributes to differentiation of pancreatic endocrine Ngn3 cell progenitors from non-endocrine embryonic duct cells. The gene is on chromosome 17q.

The degree of insulin deficiency is variable. Diabetes can develop from infancy through middle adult life, and some family members who carry the gene remain free of diabetes into later adult life. Most of those who develop diabetes show atrophy of the entire pancreas, with mild or subclincal deficiency of exocrine as well as endocrine function.

The non-pancreatic manifestations are even more variable. Kidney and genitourinary malformation and diseases may occur, but inconsistently even within a family, and the specific conditions include a range of apparently unrelated anomalies and processes. The most common genitourinary condition is cystic kidney disease, but there are many varieties even of this. Renal effects begin with structural alterations (small kidneys, renal cysts, anomalies of the renal pelvis and calices), but a significant number develop slowly progressive renal failure associated with chronic cystic disease of the kidneys. In some cases, renal cysts may be detected in utero. Kidney disease may develop before or after hyperglycemia, and a significant number of people with MODY5 are discovered in renal clinics.

With or without kidney disease, some people with forms of HNF1β have had various minor or major anomalies of the reproductive system. Male defects have included epididymal cysts, agenesis of the vas deferens, or infertility due to abnormal spermatozoa. Affected women have been found to have vaginal agenesis, hypoplastic, or bicornuate uterus.

Liver enzyme elevations are common, but clinically significant liver disease is not. Hyperuricaemia and early onset gout have occurred.

People with Laron syndrome have strikingly low rates of cancer and diabetes, although they appear to be at increased risk of accidental death due to their stature.

There is no cure for GALT deficiency, in the most severely affected patients, treatment involves a galactose free diet for life. Early identification and implementation of a modified diet greatly improves the outcome for patients. The extent of residual GALT enzyme activity determines the degree of dietary restriction. Patients with higher levels of residual enzyme activity can typically tolerate higher levels of galactose in their diets. As patients get older, dietary restriction is often relaxed. With the increased identification of patients and their improving outcomes, the management of patients with galactosemia in adulthood is still being understood.

After diagnosis, patients are often supplemented with calcium and vitamin D3. Long-term manifestations of the disease including ovarian failure in females, ataxia. and growth delays are not fully understood. Routine monitoring of patients with GALT deficiency includes determining metabolite levels (galactose 1-phosphate in red blood cells and galactitol in urine) to measure the effectiveness of and adherence to dietary therapy, ophthalmologic examination for the detection of cataracts and assessment of speech, with the possibility of speech therapy if developmental verbal dyspraxia is evident.

Histidenemia is characterized by increased levels of histidine, histamine and imidazole in blood, urine and cerebrospinal fluid. This also results in decreased levels of the metabolite urocanic acid in blood, urine, and skin cells. In Japan, neonatal screening was previously performed on infants within 1 month of birth; infants demonstrating a blood histidine level of 6 mg/dl or more underwent careful testing as suspected histidinemia cases. A typical characteristic of histidinemia is an increase in the blood histidine levels from normal levels (70-120 μM) to an elevated level (290-1420 μM). Further testing includes: observing histidine as well as imidazolepyruvic acid metabolites in the urine. However, neonatal urine testing has been discontinued in most places, with the exception of Quebec.

Inborn errors of carbohydrate metabolism are inborn error of metabolism that affect the catabolism and anabolism of carbohydrates.

An example is lactose intolerance.

Carbohydrates account for a major portion of the human diet. These carbohydrates are composed of three principal monosaccharides: glucose, fructose and galactose; in addition glycogen is the storage form of carbohydrates in humans. The failure to effectively use these molecules accounts for the majority of the inborn errors of human carbohydrates metabolism.

Administration of GH has no effect on IGF-1 production, therefore treatment is mainly by biosynthetic IGF-1. IGF-1 must be taken before puberty to be effective.

The drug product Increlex (mecasermin), developed by the company Tercica, now Genentech, was approved by the US Food and Drug Administration in August 2005 for replacing IGF-1 in patients who are deficient.

IPLEX (Mecasermin rinfabate) is composed of recombinant human IGF-1 (rhIGF-1) and its binding protein IGFBP-3. It was approved by the U.S. Food and Drug Administration (FDA) in 2005 for treatment of primary IGF-1 deficiency or GH gene deletion. Side effects from IPLEX are hypoglycemia. IPLEX's manufacturing company, Insmed, after selling its protein production facility, can no longer develop proteins, thus can no longer manufacture IPLEX as of a statement released in July 2009.

Genetic testing is necessary to identify the syndrome. The DNA test is necessary sometimes because symptoms may not be sufficient to definitely diagnose this condition.

Autoimmune polyendocrine syndrome type 1 treatment is based on the symptoms that are presented by the affected individual, additionally there is:

- Hormone replacement

- Systemic antifungal treatment

- Immunosuppressive treatment