Measured against an oral glucose tolerance test, HbA1c at the 6.5% threshold has a pooled sensitivity of 50%. It almost never calls someone diabetic in error, and it misses roughly half the people who already meet the criteria. That is its performance on the thing it is designed to detect. Insulin resistance sits earlier in the sequence than anything HbA1c measures, and in Indian bodies a second problem stacks on top: anaemia and inherited haemoglobin variants are common enough here to distort the result itself.

• Against an oral glucose tolerance test, HbA1c at 6.5% has a pooled sensitivity of 50% and a specificity of 97.3%.
• In the Whitehall II cohort, South Asian participants had measurably lower insulin sensitivity than white participants as far back as 20 years before diabetes diagnosis.
• Anaemia affected around 57% of Indian women aged 15 to 49 in NFHS-5, and iron deficiency tends to push HbA1c falsely upward.
• RSSDI's consensus recommends that HbA1c should not be relied on as the only measure when assessing diabetes risk in Indian settings.
• No randomised trial was located, Indian or global, showing that detecting and treating insulin resistance in a normoglycaemic adult improves hard clinical outcomes.

HbA1c quantifies the fraction of haemoglobin that has become irreversibly glycated over the roughly 90 to 120 day lifespan of a red blood cell. It is a retrospective average of blood glucose exposure. It is not an assay of insulin action, insulin levels, or insulin resistance, and glycaemia is the last variable in the chain to move.

The sequence runs in one direction. Insulin resistance develops in muscle, liver and fat tissue. The pancreas compensates by raising insulin output, which holds blood glucose inside the normal range. Only when beta-cell output can no longer keep pace does glucose rise, and that is what eventually moves HbA1c. During this compensatory phase, changes in insulin sensitivity may develop before they are reflected in commonly used glycaemic markers. The mechanism itself is set out in what a blood panel finds that a diet plan cannot.

How long that phase runs is the part that matters. The Whitehall II occupational cohort in the UK found insulin sensitivity and secretion diverging from normal as early as 13 years before diabetes diagnosis. A sub-analysis by ethnicity, covering 120 South Asian and 867 white participants who developed diabetes between 1991 and 2013, found South Asians had measurably lower insulin sensitivity than white participants as far back as 20 years before diagnosis.

Treat the 20-year figure as indicative rather than a population estimate: it is one cohort's ethnic sub-analysis of 120 people, describing UK-resident South Asians rather than a domestic Indian cohort. No prospective, multi-year, India-based study tracing this trajectory was located, which is a significant gap in the evidence. Older evidence points the same direction: UKPDS found beta-cell function already reduced by roughly 50% at the point of clinical diagnosis.

The euglycaemic clamp is the true reference standard for insulin resistance, and it is essentially absent from HbA1c validation work in Indian populations.

What the Studies Show About HbA1c Accuracy

Every figure below uses HOMA-IR, fasting glucose or an oral glucose tolerance test instead, one step removed from the gold standard.

Pooled meta-analysis of 37 studies: Among undiagnosed adults across multiple countries, an HbA1c cut-off of 6.5%, compared with an OGTT, had 50% sensitivity and 97.3% specificity. At the optimal cut-off of 6.03%, sensitivity increased to 73.9%, while specificity fell to 87.2%.

Meta-analysis of 49 studies on prediabetes: Among adults aged 18 and over, using OGTT or FPG as the reference standard, an HbA1c cut-off of 5.7% had 49% sensitivity and 79% specificity.

Rural Indian community study: Among 363 moderate-to-high-risk adults in rural India, an HbA1c cut-off of 6.34% for diabetes, compared with fasting plasma glucose plus 2-hour post-load glucose, had 74% sensitivity and 94% specificity. For prediabetes in the same cohort, a cut-off of 5.65% had 80% sensitivity and 75% specificity.

Systematic review across 7 African countries: Among 12,925 participants, an HbA1c cut-off of 6.5%, compared with an OGTT, had 57.7% sensitivity and 92.3% specificity.

The pattern is consistent: high specificity with modest sensitivity. A clearly raised HbA1c is rarely a false alarm, and a normal one is a much weaker reassurance than it appears. If HbA1c misses a large share of people already dysglycaemic on an OGTT, it is even less able to flag people who are insulin resistant but not yet dysglycaemic at all, since that is an earlier and subtler stage.

How many normoglycaemic Indian adults are already insulin resistant is not answerable at national scale. ICMR-INDIAB reports glycaemic-category prevalence rather than insulin-resistance prevalence among normoglycaemic adults, so no national figure exists to cite.

Everything above concerns what HbA1c is too late to see. What follows is different: conditions common in India that make the number itself unreliable, in either direction.

Iron-deficiency anaemia

National Family Health Survey data show anaemia affecting a majority of Indian women of reproductive age: around 53% in NFHS-4 (2015 to 2016), rising to 57% in NFHS-5 (2019 to 2021).

The dominant finding across studies, including Indian ones, is that iron deficiency falsely elevates HbA1c. Indian data found HbA1c of 6.8 plus or minus 1.4% in iron-deficient adults, with the elevation more pronounced in women at 7.02 plus or minus 1.58%. Reduced red cell production lengthens the average circulating age of red blood cells and so extends glycation exposure time. The effect reverses after iron repletion.

Inherited haemoglobin variants

Beta-thalassaemia trait carrier frequency runs at 3 to 4% nationally, clustering higher in specific communities including Sindhi, Punjabi, Gujarati, Bengali, Mahar and Koli populations. Sickle-cell trait reaches up to 48% in some tribal populations, HbE carrier frequency as high as 50% in parts of the north-east, and HbD Punjab in roughly 2% of the Punjab population.

Direction of effect is where the literature conflicts, and this article does not resolve it. Sickle-cell disease and beta-thalassaemia major are associated with unreliable or falsely low HbA1c through shortened red cell lifespan and haemolysis. Yet a 2023 comparative study found sickle-cell anaemia associated with significantly higher HbA1c (5.83% against 5.32% in controls), and no significant effect from beta-thalassaemia trait. Direction and magnitude depend on assay method, with ion-exchange HPLC particularly prone to interference from haemoglobin variants.

Conditions that shorten red-cell lifespan

Haemolysis, blood loss and chronic kidney disease can alter red-cell lifespan, which may affect how HbA1c reflects average glucose exposure. In chronic renal failure, HbA1c reads approximately 1.5 percentage points lower than glycated albumin suggests is accurate. A recent transfusion introduces donor red cells of unknown glycation history, making HbA1c unreliable for about three months afterwards.

None of the above is a contrarian reading. It is the position of Indian guidance, stated in its own words.

RSSDI's 2020 consensus suggests HbA1c at or above 6.5% as the optimal level for diagnosis of diabetes in Indian patients, then adds in the same recommendation that HbA1c cannot be used as a sole measurement for diagnosis of diabetes in Indian settings. It goes further: use of HbA1c as a sole diagnostic test for screening of diabetes or prediabetes is not recommended, and the panel considers it would be inappropriate in most settings in India at this time, except where the laboratory has robust external quality assurance. The consensus names iron-deficiency anaemia explicitly, alongside ethnicity and age, as a factor affecting HbA1c's predictive value.

The operative instruction is not "order more tests." It is to use fasting glucose or an OGTT alongside HbA1c rather than HbA1c alone, and to check for haematological confounders before trusting an HbA1c value in isolation.

The argument above is a case against relying on one test. It is not a case for insulin-resistance screening at population scale, and the evidence does not support that.

No randomised controlled trial was located, Indian or global, testing whether identifying and treating insulin resistance in a normoglycaemic adult, before any glycaemic abnormality appears, improves hard clinical outcomes such as cardiovascular events, progression to diabetes or mortality.

HbA1c's own strengths are real: no fasting required, good pre-analytical stability, and an established role in monitoring glycaemic control once diabetes is diagnosed.

A normal HbA1c in an Indian adult carries less reassurance than the same result would in the populations where the threshold was validated, for two independent reasons: the physiology it measures moves late, and anaemia or a haemoglobin variant may be moving the number itself.

What partly fills the gap costs nothing. At any matched BMI, Asian Indian adults carry higher total body fat, greater visceral and truncal adiposity and lower lean mass than white Europeans, alongside higher insulin resistance, which is why visceral fat at normal weight is a recurring Indian pattern rather than an edge case. A tape measure and a waist-to-hip ratio will not diagnose anything, but they give a registered medical practitioner something to weigh a normal HbA1c against.

A normal HbA1c is not the end of the conversation: A Voy clinician can read your HbA1c alongside your haematological picture, your waist measurement, and your history to explain what the result actually means for your metabolic risk. Start with a metabolic assessment.

This article is for general information and education only and is not medical advice, diagnosis, or treatment. GLP-1 and other medications referenced are prescription-only and are appropriate only for certain people under the supervision of a qualified clinician. Do not start, stop, or change any medication based on this article. Please consult a registered medical practitioner about your individual circumstances. Information reflects what was available at the time of review and may change.