Your Diagnosis Tests, Explained
Reviewed by CDE Mehandi Sharma · Updated
Three tests are commonly used to diagnose diabetes, and they measure different things.
A fasting plasma glucose test measures your blood sugar after at least eight hours without food. An oral glucose tolerance test measures it two hours after you drink a sugary solution, which shows how well your body clears a load of glucose. HbA1c is different again — it estimates your average blood sugar over the past two to three months, so it does not need fasting.
| Test | Normal | Prediabetes | Diabetes |
|---|---|---|---|
| HbA1c | Below 5.7% | 5.7% to 6.4% | 6.5% or above |
| Fasting glucose | Below 100 mg/dL | 100 to 125 mg/dL | 126 mg/dL or above |
| 2-hour glucose test | Below 140 mg/dL | 140 to 199 mg/dL | 200 mg/dL or above |
| Random glucose | — | — | 200 mg/dL or above, with symptoms |
One result above the line usually needs confirming with a repeat test before a diagnosis is made, unless your blood sugar is very high and you clearly have symptoms.
This is also why one number rarely tells the whole story. A fasting value can look acceptable while your after-meal readings are running high, which is why your doctor may ask for more than one test.