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Diabetes mellitus is diagnosed by identifying chronic hyperglycaemia. The World Health Organisation (WHO) and the American Diabetes Association (ADA) use a fasting plasma glucose (FPG) of 7 mmol/L or higher to define diabetes (Table 3.1). This originates from epidemiological studies in the 1990s which appeared to show that the risk of microvascular complications (e.g retinopathy) increases sharply at a FPG threshold of 7 mmol/L (Figure 3.1). Lately, the notion of a clear glycaemic threshold separating people at high and low risk of diabetic microvascular complications has been called into question. The relationship between plasma glucose and microangiopathy is likely to be continuous, thus a FPG of 7 mmol/L is an arbitrary cut‐off for defining diabetes which may be lowered in the future.

There are currently 34.2 million people in the USA with diabetes (10.5% of the population). Approximately 7 million of these are not yet aware that they have diabetes. The total number of people with diabetes worldwide is projected to increase from 171 million in 2000 to 366 million in 2030. A key demographic change to the rising prevalence of diabetes worldwide is an increasing proportion of people >65 years of age.

Table 3.1 Diagnostic criteria for Type 2 diabetes.

Diabetes may be diagnosed based on plasma glucose criteria, either the fasting plasma glucose (FPG) or the 2‐h plasma glucose (2‐h PG) value after a 75‐g oral glucose tolerance test (OGTT) or A1C criteria.

Criteria for the diagnosis of diabetes.

FPG ≥126 mg/dL (7.0 mmol/L). Fasting is defined as no caloric intake for at least 8 h.*

2‐h PG ≥200 mg/dL (11.1 mmol/L) during an OGTT. The test should be performed as described by the WHO, using a glucose load containing the equivalent of 75 g anhydrous glucose dissolved in water.*

A1C ≥6.5% (48 mmol/mol). The test should be performed in a laboratory using a method that is NGSP certified and standardized to the DCCT assay.*

In a patient with classic symptoms of hyperglycemia or hyperglycemic crisis, a random plasma glucose ≥200 mg/dL (11.1 mmol/L).

^* In the absence of unequivocal hyperglycemia, results should be confirmed by repeat testing.

FPG, 2‐h PG after 75‐g OGTT, and A1C are equally appropriate for diagnostic testing. It should be noted that the tests do not necessarily detect diabetes in the same individuals. The concordance between the FPG and 2‐h PG tests is imperfect. The overlap depends on the ethnic and geographical population, and on other characteristics such as age and body mass index. Some individuals have asymptomatic, isolated post‐challenge hyperglycaemia, while others have fasting hyperglycaemia but normal post‐load glycaemic responses. The fasting criteria for diabetes tend to pick out younger and more obese subjects.

Table 3.2 Use of HbA1c >6.5% (48 mmol/mol) as a cut‐off for making the diagnosis of diabetes offers some advantages but there are several disadvantages.

Advantages	Disadvantages
Avoids the need for a fasting blood sample, and the pre‐analytical instability of glucose measurements.HbA1c reflects glycaemia over several weeks.Lower biological variability of HbA1c compared with FPG or 2 h glucose.Virtual absence of significant retinopathy among people with HbA1c < 6.5%.	HbA1c measurements can give spurious results in:anaemia (Fe‐deficiency)haemoglobinopathiesrenal failuredifferent ethnic groups.Diagnosis by HbA1c will identify a different population to that diagnosed by FPG.Distribution of HbA1c values varies in different ethnic groups.HbA1c increases with ageSome patients and ethnic groups may be diagnosed with diabetes by some criteria but not others.

Figure 3.1 Prevalence of diabetes‐specific retinopathy (moderate or more severe retinopathy) by vigintiles of the distribution of FPG, 2‐h PG, and A1C.

Adapted from Colaguiri et al. Diabetes Care 2011; 34: 145‐150.

Numerous studies have confirmed that, compared with FPG and A1C cut points, the 2‐h PG value diagnoses more people with diabetes. When using A1C to diagnose diabetes, it is important to recognize that A1C is an indirect measure of average blood glucose levels and to take other factors into consideration that may impact haemoglobin glycation independently of glycaemia including age, race/ethnicity, and anaemia/haemoglobinopathies.