Diagnosis: HbA1c, Fasting Sugar, and the Glucose Tolerance Test

The clues that send you for testing

When blood sugar runs high, the kidneys can't reabsorb all the glucose, so it spills into the urine and drags water with it. That produces the classic triad: excessive urination (polyuria), constant thirst, and unexplained weight loss. Blurry vision, fatigue, and slow-healing infections are common too. But here's the catch with type 2: it can be completely silent for years, so we don't wait for symptoms — we screen.

Three tests, three time windows

The diagnosis rests on three blood tests, and the trick to understanding them is that each looks at a different stretch of time:

1. Fasting plasma glucose — your blood glucose after at least 8 hours without food. A single snapshot of the “floor” your system holds overnight. Diabetes if ≥126 mg/dL (7.0 mmol/L).
2. Oral glucose tolerance test (OGTT) — drink a standard 75 g glucose solution, then measure blood sugar 2 hours later. This is a stress test for the insulin response: can your beta cells clear a big sugar load? Diabetes if the 2-hour value is ≥200 mg/dL (11.1 mmol/L).
3. HbA1c (glycated hemoglobin) — the long-game test. Diabetes if ≥6.5%. No fasting needed, and it captures the average of the past 2–3 months.

Why HbA1c works — sugar that sticks

HbA1c is clever because of simple chemistry. Glucose in the blood slowly and permanently sticks to hemoglobin inside your red blood cells, with no enzyme needed — the more sugar around, the more gets stuck. Since red cells live about three months, the percentage of “sugar-coated” hemoglobin is a built-in recording of your average glucose over that window. You can't fake it by dieting the day before, which is why it has become the workhorse test for both diagnosis and tracking treatment.

HbA1c  -->  ESTIMATED AVERAGE GLUCOSE
Rule of thumb: eAG (mg/dL) = 28.7 x A1c - 46.7

  A1c 5.0%   ~  97 mg/dL   normal
  A1c 5.7%   ~ 117 mg/dL   prediabetes begins
  A1c 6.5%   ~ 140 mg/dL   diabetes threshold
  A1c 7.0%   ~ 154 mg/dL   common treatment target
  A1c 9.0%   ~ 212 mg/dL   poorly controlled
  A1c 12%    ~ 298 mg/dL   very high

Each 1% rise in A1c ~ 29 mg/dL higher average glucose.
(Caution: anemia or abnormal hemoglobin can distort A1c.)

The warning zone: prediabetes

Between “normal” and “diabetes” lies a grey zone called prediabetes: sugars high enough to worry about but not yet diagnostic. This shows up as a fasting glucose of 100–125 mg/dL, an A1c of 5.7–6.4%, or — caught on the glucose tolerance test — a 2-hour value of 140–199 mg/dL, which is also called impaired glucose tolerance.