Three windows onto the same blood sugar
Diabetes mellitus is defined by chronically high blood glucose, so its diagnosis rests on measuring sugar directly. But sugar is a moving target — it rises after meals and falls when fasting — so we use three different tests, each a window onto a different time scale. Fasting glucose is a snapshot after an overnight fast. The oral glucose tolerance test is a stress test of how the body clears a sugar load over two hours. And HbA1c is a memory of the average over the past two to three months.
HbA1c: sugar's fingerprint on blood
When glucose floats in blood, a little of it sticks permanently to hemoglobin inside red blood cells. The higher the average sugar, the more hemoglobin gets coated. Because red cells live about three months, the percentage of coated hemoglobin — HbA1c — reads out the average blood glucose over that window. Its great strength is that it needs no fasting and no special timing, and it cannot be faked by skipping breakfast before the appointment.
Rough HbA1c -> average glucose (estimated average glucose, eAG) eAG (mg/dL) = 28.7 x HbA1c(%) - 46.7 HbA1c 5% -> ~ 97 mg/dL (5.4 mmol/L) normal HbA1c 6% -> ~126 mg/dL (7.0 mmol/L) prediabetes range HbA1c 7% -> ~154 mg/dL (8.6 mmol/L) common treatment target HbA1c 9% -> ~212 mg/dL (11.8 mmol/L) poorly controlled Thresholds (typical): normal < 5.7% | prediabetes 5.7-6.4% | diabetes >= 6.5%
The tolerance test and the gray zone
The oral glucose tolerance test (OGTT) is a controlled challenge: after fasting, you drink a measured 75-gram sugar solution and blood is checked at two hours. A healthy body, with intact glucose homeostasis, clears the load and the two-hour value lands low. A struggling body leaves sugar lingering. The OGTT is the most sensitive of the three and is the standard for screening gestational diabetes, where catching mild hyperglycemia early protects the pregnancy.
Between fully normal and diabetes lies a gray zone. Prediabetes describes values that are higher than healthy but below the diabetes line. When the OGTT specifically shows a sluggish two-hour clearance, it is called impaired glucose tolerance. These are not minor labels — they are an early warning that glucose homeostasis is fraying, and the stage where lifestyle change can still turn the story around.