Which Test to Ask For

Three different blood tests can identify prediabetes, and they measure genuinely different things — which is why one can be normal while another is not.

Fasting plasma glucose measures your blood sugar after at least eight hours without food. It is cheap, quick and widely available, and it is the most commonly done. Its weakness is that it only sees your baseline; it misses people whose fasting level is fine but who handle a glucose load poorly.

HbA1c estimates your average blood sugar over the past two to three months. It needs no fasting, which makes it convenient, and it smooths out day-to-day variation.

The oral glucose tolerance test gives you a 75 gram glucose drink and measures your blood two hours later. It is the most sensitive of the three because it directly tests how your body handles a load — but it takes two hours and costs more.

For most people, asking for a fasting glucose plus an HbA1c is a reasonable and affordable combination.

If you have strong risk factors and both come back at the upper end of normal, the glucose tolerance test is worth the extra time — it is the one that finds impaired glucose tolerance the others can miss.

Whichever you have, keep the actual numbers, not just the word "normal".