Fasting Glucose, HbA1c and the Tolerance Test: What the Tests Show

I once believed I could "feel a sugar spike" after something sweet and that a blood sugar test was something like a test of willpower. Then I sat down to read what the sources say about it and discovered that there are actually several tests, they answer different questions, and a single result on its own decides nothing.

What sugar tests exist

Fasting glucose shows the sugar level after an overnight fast. It is a snapshot of a single moment. Glycated hemoglobin (HbA1c) reflects the average glucose level over the previous roughly three months, because red blood cells live on average 120–125 days, and the last 30 days before the blood draw determine about half of the HbA1c value. The glucose tolerance test measures sugar before and after drinking a glucose solution: checks are usually done at 1 hour, 2 hours and, possibly, 3 hours. A random test measures sugar at the moment of examination and is taken without fasting; for a random test, the CDC defines only the diabetes threshold, without normal and prediabetes zones.

• Fasting glucose — a quick snapshot of the current level
• HbA1c — the average picture over several months
• Tolerance test — the body's response to a glucose load
• Random test — sugar at the moment of examination, without fasting

Reference ranges: where the numbers come from

According to the CDC, for HbA1c the normal range is below 5.7%, prediabetes is 5.7–6.4%, and diabetes is 6.5% and above. For fasting glucose and the tolerance test, the CDC identifies the same three zones: normal, prediabetes and diabetes. The result is compared with the reference on the laboratory's report form. The American Diabetes Association in 2010 added HbA1c of 6.5% and above (48 mmol/mol) as another criterion for diagnosing diabetes. Laboratory results may differ depending on the method, the person's age and biological variation, so you should rely on the report form of the specific laboratory.

What affects the result

For HbA1c, special preparation is usually not required. For fasting glucose, blood is drawn after 6–12 hours without food; the exact rules are given by the laboratory or doctor. Tell the doctor and the laboratory about the medications you take. HbA1c is also affected by conditions unrelated to sugar: falsely low results occur after bleeding, blood transfusion, during pregnancy, liver cirrhosis, sickle cell anemia, living at high altitude, taking iron supplements, and also in chronic renal failure; falsely high results occur in iron deficiency anemia, thalassemia, vitamin B12 deficiency, high triglyceride levels, after organ transplantation, and also when taking immunosuppressants.

Prediabetes is a zone of attention, not a verdict

The CDC states directly: prediabetes and type 2 diabetes often have no symptoms, so people with risk factors are advised to ask their doctor about screening. Early detection of prediabetes helps prevent or delay type 2 diabetes. HbA1c measurement is recommended both for checking glucose control in people who may have prediabetes and for monitoring in those already diagnosed.

Myths about sweets and "sugar spikes"

My favorite myth is that how you feel reliably indicates your sugar level. The sources do not confirm this: prediabetes and type 2 diabetes are often asymptomatic, which is precisely why screening exists. The second myth is that one bad result means a diagnosis. The CDC advises: if you had your sugar measured at a health fair or pharmacy, see a doctor who can recheck the result. A diagnosis is a doctor's decision based on the totality of data, not a number on a report form.

Call an ambulance immediately (103 or 112) if there is loss of consciousness, seizures, difficulty breathing or severe chest pain.

Routinely, test results are discussed with a family doctor; after a diagnosis is established, the CDC mentions referral to an endocrinologist and a dietitian. If you have risk factors, ask about screening rather than waiting for symptoms. Blood for the test is drawn in the doctor's office or at a clinic, and results are usually ready quickly.

This material is for reference: the numbers on a report form do not replace a conversation with a doctor who knows your history.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer