HbA1c (Glycated Hemoglobin)

HbA1c measures the percentage of hemoglobin that has glucose permanently attached, reflecting average blood sugar over the preceding 2-3 months, the lifespan of a red blood cell. The clinical claim: HbA1c above 6.5% defines diabetes, and even moderate elevations predict long-term complications and death, making it the standard tool for both diagnosing diabetes and tracking glycemic control over time rather than the single-moment snapshot a fasting glucose test gives.

A cohort study of 13,334 men and 21,927 women with type 2 diabetes followed for a mean of 8.7 years found a J-shaped relationship between HbA1c and all-cause mortality: hazard ratios relative to the 6.0-6.9% reference band rose to 1.31 in men and 1.74 in women at HbA1c 11.0% or higher, but very low HbA1c (below 6.0%) also carried modestly elevated risk (HR 1.06 in men, 1.21 in women), a pattern that held after adjusting for insulin use and smoking status [1]. This J-shape means both under-treatment and over-aggressive glucose lowering carry mortality signal, an important nuance beyond assuming lower is always better.

How to measure it: a standard blood draw analyzed by high-performance liquid chromatography or immunoassay, reported as a percentage (US convention) or mmol/mol (IFCC/international convention). No fasting is required, which is part of why it displaced fasting glucose for routine diabetes screening [6]. Results can be skewed by conditions altering red blood cell turnover: anemia, recent blood transfusion, hemoglobinopathies, and chronic kidney disease can all produce misleadingly low or high readings unrelated to actual glucose control [2].

How to intervene on it: metformin, GLP-1 receptor agonists, SGLT2 inhibitors, and insulin all lower HbA1c in large randomized trials, with the choice of agent depending on additional cardiovascular and renal benefits documented in trials like EMPA-REG and LEADER. Weight loss and dietary carbohydrate reduction lower it in non-drug studies as well, generally by 0.5-1.5 percentage points depending on baseline severity and adherence [5].

Critics note HbA1c targets have been walked back for elderly or multimorbid patients after trials like ACCORD showed that very tight glycemic control (targeting HbA1c near 6.0%) increased mortality in some populations, reinforcing the J-shaped cohort data above [3]. HbA1c also does not capture glucose variability: two people with the same HbA1c can have very different swings between highs and lows, a gap that HbA1c-variability studies link to added mortality risk independent of the mean value [4].

The plain takeaway: HbA1c is the standard, well-validated tool for diagnosing and tracking diabetes risk over months, but the goal is a target range, not the lowest possible number, and results can be distorted by anything that changes how long red blood cells live.