Diagnostic criteria for diabetes and prediabetes

What it is

Between normal values and diabetes lie two prediabetes states: impaired fasting glucose (IFG), which is defined on the fasting glucose alone, and impaired glucose tolerance (IGT), which is defined on the oral glucose tolerance test.

How it is diagnosed

A fasting glucose of 6.0 mmol/L or below is normal, and a fasting glucose of 7.0 mmol/L or above meets the threshold for diabetes. A fasting glucose greater than or equal to 6.1 but less than 7.0 mmol/L implies impaired fasting glucose. A fasting blood glucose of 6.7 mmol/L on two occasions is therefore impaired fasting glucose, that is prediabetes, and not diabetes.

Impaired glucose tolerance is defined as a fasting plasma glucose less than 7.0 mmol/L together with an OGTT 2-hour value greater than or equal to 7.8 mmol/L but less than 11.1 mmol/L. People with IFG should then be offered an oral glucose tolerance test to rule out a diagnosis of diabetes: a result below 11.1 mmol/L but above 7.8 mmol/L indicates that the person does not have diabetes but does have IGT.

HbA1c gives a second route to the diagnosis. A value of 5.9% or below is considered normal, while 6.5% (48 mmol/mol) is the diagnostic threshold for diabetes.

Gestational diabetes mellitus is found by screening rather than by these thresholds. The U.S. Preventive Services Task Force recommends screening for gestational diabetes mellitus after 24 weeks gestation, with a fasting blood glucose level, a 50-g oral glucose challenge, or an assessment of risk factors. Screening at an earlier date rates as insufficient evidence, and screening at later dates is not recommended.

The Omani national guideline cuts for pre-diabetes are not the same: impaired fasting glucose is 5.6 to 6.9 mmol/l, impaired glucose tolerance is a 2 hrs PG of 7.8 to 11.0 mmol/l, and HbA1C is 5.7 to 6.4%. So the fasting band opens at 5.6 rather than at 6.1, and a HbA1C of 5.7 to 6.4% is already pre-diabetes rather than normal. Both sets appear in the material and both are worth holding, but where they differ it is the Omani cuts that are worked to. Pre-diabetes is not to be counted a clinical entity by itself, but rather a state of increased risk of developing diabetes and cardiovascular disease.

How many tests are needed is set out with them. A patient with classical symptoms is diagnosed on one unequivocal abnormal blood glucose test. If the test in such a patient does not unequivocally confirm hyperglycemia, the same test is repeated from the same sample or from two separate samples. If the patient is not in a hyperglycemic crisis and has no classical symptoms with an abnormal random blood glucose test, the diagnosis requires two abnormal tests made in a short timeframe, and the repeat may be the same test or a new one. If the results of two different tests are discordant, the test with the abnormal result is repeated and the diagnosis is made on the confirmed test: if the fasting blood glucose is below 7 mmol/l and the HbA1c is above 6.5%, the HbA1c is repeated, and if the repeat is above 6.5% that person is considered to have diabetes.

The risk factors that send a patient for testing at all are a BMI of 25 kg/m2 or above, a first degree relative with diabetes mellitus, a history of gestational diabetes mellitus or of delivering a baby weighing more than 4 kg, hypertension with a BP of 140/90 mmHg or above, dyslipidemia, a history of IFG or IGT on prior testing, acanthosis nigricans, polycystic ovarian syndrome, vascular disease that is coronary, cerebrovascular or peripheral, and certain medications such as glucocorticoids, thiazide diuretics, some HIV medications and atypical antipsychotics.