Diabetes Mellitus Diagnosis and Classification
On this page
Direct answer
Diabetes mellitus is diagnosed at fasting plasma glucose of 126 mg/dL or more, 2-hour glucose of 200 mg/dL or more on a 75 gram oral glucose tolerance test, or HbA1c of 6.5 per cent or more, confirmed on repeat testing unless hyperglycaemia is unequivocal. It is classified into type 1, from autoimmune beta-cell destruction, type 2, from insulin resistance with relative insulin deficiency, gestational diabetes and specific types.
What you must remember
- Fasting criterion: fasting plasma glucose of 126 mg/dL or more; impaired fasting glucose is 100 to 125 mg/dL by ADA criteria.
- OGTT criterion: 2-hour glucose of 200 mg/dL or more; 140 to 199 mg/dL is impaired glucose tolerance.
- HbA1c criterion: 6.5 per cent or more; prediabetes is 5.7 to 6.4 per cent.
- HbA1c pitfalls: iron deficiency tends to raise it, while haemolysis, blood loss, transfusion and haemoglobinopathies make it unreliable, a real concern given thalassaemia trait.
- Type 1 antibodies: GAD65 is the most common, followed by IA-2, ZnT8 and insulin autoantibodies.
- Type 2 phenotype: Indian patients often develop type 2 diabetes at a lower BMI with central adiposity.
- Secondary causes: chronic pancreatitis, haemochromatosis, Cushing syndrome, acromegaly, phaeochromocytoma and glucocorticoids.
A 45-year-old whose tests seem to disagree
A 45-year-old man has a fasting plasma glucose of 132 mg/dL on one morning.
- Confirm with a repeat: A repeat fasting sample is 128 mg/dL and his HbA1c is 6.8 per cent. Two concordant abnormal results on separate days confirm diabetes, and the HbA1c adds weight because it reflects three months of glycaemia.
- Beware HbA1c in iron deficiency: A 30-year-old woman with iron deficiency anaemia has an HbA1c of 6.6 per cent, yet her fasting glucose is 118 mg/dL. Iron deficiency tends to raise HbA1c, so the result may overstate her glycaemia, and the clinician should use glucose-based testing and recheck after iron repletion.
- Route a lean patient with ketosis: A thin 22-year-old with ketosis and weight loss needs a different branch of the algorithm. Islet autoantibodies and C-peptide help separate type 1 diabetes and latent autoimmune disease from type 2, which matters because insulin is life-saving in type 1.
- Respect the random glucose rule: Random glucose of 200 mg/dL or more counts only when classic symptoms are present.
Where the thresholds trip candidates
The thresholds and the choice of test are where candidates most often slip.
- Trap: applying one system's impaired fasting glucose cut-off to the other. Correct: the ADA uses 100 mg/dL, whereas the WHO 1999 criteria used 110 mg/dL, so 105 mg/dL is impaired under one system and normal under the other.
- Trap: using HbA1c alone in thalassaemia or after transfusion. Correct: abnormal red cell turnover distorts the reading.
- Trap: labelling every adult with diabetes as type 2. Correct: an adult with slowly progressive insulin deficiency may have latent autoimmune diabetes, and a young lean patient with ketosis is type 1 until proven otherwise.
Frequently asked questions
What HbA1c value diagnoses diabetes mellitus?
An HbA1c of 6.5 per cent or more diagnoses diabetes mellitus under ADA criteria, provided the result is confirmed on repeat testing unless hyperglycaemia is clear. HbA1c between 5.7 and 6.4 per cent indicates prediabetes. Its reliability is reduced in anaemia, haemoglobinopathies and recent blood loss.
Which antibodies support a diagnosis of type 1 diabetes?
GAD65 antibodies are the most common islet autoantibodies in type 1 diabetes, followed by IA-2, ZnT8 and insulin autoantibodies. Their presence supports autoimmune beta-cell destruction. Multiple positive antibodies increase the risk of progression to insulin dependence.
Why can HbA1c be misleading in thalassaemia?
HbA1c is misleading in thalassaemia because altered red cell lifespan and abnormal haemoglobin variants change how glycation is measured. Results can be falsely low or high. In such patients, fructosamine or glucose-based monitoring is often preferred.
What is the difference between the ADA and WHO impaired fasting glucose cut-offs?
The ADA defines impaired fasting glucose as 100 to 125 mg/dL, whereas the WHO 1999 criteria used 110 to 125 mg/dL. Values between 100 and 109 mg/dL are therefore classified differently. Examiners test this difference because it changes the diagnosis.
Which drugs can cause secondary diabetes mellitus?
Glucocorticoids, thiazide diuretics, pentamidine and some antipsychotics can cause secondary diabetes mellitus. Calcineurin inhibitors such as tacrolimus are also implicated. Withdrawing or changing the drug is considered when clinically appropriate.