Quick answer: A fasting blood sugar shows your blood glucose at one moment, after 8 hours without food. HbA1c shows your average blood sugar over the last 2 to 3 months and needs no fasting. Both can screen for diabetes. HbA1c is best for checking long-term control, but it isn't reliable for everyone, including people with sickle cell disease or anaemia.
The cut-offs
| Normal | Prediabetes | Diabetes | |
|---|---|---|---|
| Fasting blood sugar | Below 100 mg/dL (5.6 mmol/L) | 100 to 125 mg/dL | 126 mg/dL (7.0 mmol/L) or more |
| HbA1c | Below 5.7% | 5.7 to 6.4% | 6.5% or more |
Diabetes is usually confirmed with a second test, unless you have clear symptoms such as thirst, passing a lot of urine and weight loss.
Which should I choose?
- Screening for the first time: either works. HbA1c is convenient because you don't need to fast.
- Already diagnosed with diabetes: HbA1c every 3 months shows how well your treatment is working.
- Checking sugar after meals: a 2-hour after-meal test (2HPP) can show problems a fasting test misses.
When HbA1c can mislead
HbA1c relies on your red blood cells, so it can be falsely high or low if you have:
- sickle cell disease (SS) or another haemoglobin variant
- anaemia, recent heavy bleeding or a recent blood transfusion
- pregnancy
- advanced kidney disease
In these cases, blood glucose tests are more reliable. Tell us your genotype if you know it.
Preparing for a fasting test
Don't eat for 8 hours before (water is fine), and come in the morning, before 10 AM. Read more: how to prepare for diabetes screening.
Get tested in Akoka
The HbA1c test and blood sugar tests are walk-in at Mascot Healthcare, 52 Sholanke Street, off Chemist Junction, Akoka, Lagos, Monday to Saturday, 9 AM to 5 PM. Prices are on our price list.
This article is general information, not a diagnosis.