Both tests diagnose diabetes in Canada, and neither is better in every situation. Fasting plasma glucose is a snapshot of your blood sugar after an 8-hour fast; HbA1c is an average over about three months and needs no fasting. Diabetes Canada accepts either, or a two-hour glucose tolerance test, and a diagnosis usually requires a confirmatory repeat. Your provider chooses based on convenience, your medical history and what the result will be used for; both are OHIP-covered and can be drawn at any Bio-Test centre in Ottawa without an appointment.
What each test measures
Fasting plasma glucose (FPG) measures the glucose in your blood at one moment, after no food for at least eight hours. HbA1c (glycated hemoglobin) measures how much glucose has attached to the hemoglobin in your red blood cells, which reflects average blood sugar over roughly the previous two to three months because red cells live about 120 days.
Diabetes Canada thresholds
- Fasting plasma glucose: 7.0 mmol/L or higher means diabetes; 6.1 to 6.9 is impaired fasting glucose (prediabetes).
- HbA1c: 6.5% or higher means diabetes; 6.0 to 6.4% is prediabetes.
- Two-hour plasma glucose after a 75 g drink: 11.1 mmol/L or higher means diabetes; 7.8 to 11.0 is impaired glucose tolerance.
- Without classic symptoms, a diagnosis is confirmed by a second test on another day.
When HbA1c is preferred
HbA1c is convenient: no fasting, any time of day, and it is not thrown off by a stressful morning or a skipped breakfast. It is the standard test for monitoring known diabetes every three to six months. It is the usual screening choice for adults who cannot easily fast.
When fasting glucose is preferred
HbA1c can be misleading when red blood cells are not typical: in pregnancy, with anemia or recent blood loss, with hemoglobin variants such as sickle trait, in kidney failure, or after a transfusion. Fasting glucose, or a glucose tolerance test in pregnancy, is used instead. Fasting glucose is also cheaper and faster, and your provider may prefer it when an earlier result needs confirming. See the A1c screening guide for who should be screened and how often.
Preparation
For fasting glucose: nothing but water for 8 to 12 hours, morning draw, medications as directed. For HbA1c: no preparation. If both are on one requisition, fast so the glucose is valid. Our fasting guide has the details.
Coverage and where to get tested
Both tests are insured by OHIP when ordered by an authorized provider with a valid health card. Walk in to any of Bio-Test's 11 centres in Ottawa and Eastern Ontario; results reach your provider within 24 to 48 hours and appear in the patient portal.