You can test your A1c through a standard blood draw at a lab, a fingerstick at your doctor’s office, or an at-home test kit you order online. No fasting is required for any of these methods, which makes A1c one of the most convenient blood tests available. Here’s what each option involves and how to interpret your results.
What A1c Actually Measures
Glucose in your bloodstream naturally sticks to hemoglobin, the protein inside red blood cells that carries oxygen. The more glucose circulating in your blood over time, the more hemoglobin gets coated. Since red blood cells live about three months before your body replaces them, an A1c test captures your average blood sugar control over that entire window. It’s not a snapshot of one moment like a finger-prick glucose reading. It’s a running average.
The result is reported as a percentage. A higher percentage means more of your hemoglobin has glucose attached to it, which signals higher average blood sugar levels.
Three Ways to Get Tested
Lab Blood Draw
This is the gold standard. A phlebotomist draws blood from a vein in your arm, and the sample goes to a certified laboratory for analysis. Results typically come back within a day or two. Your doctor orders this test, and it’s the method used for an official diabetes diagnosis. No fasting, no special preparation needed beforehand.
Point-of-Care Fingerstick
Many doctor’s offices, clinics, and pharmacies use point-of-care devices that analyze a drop of blood from your fingertip and return results in minutes. These are the same types of devices used during routine checkups when your provider wants results before you leave the appointment. Fingerstick A1c results correlate strongly with lab draws, with correlation coefficients above 0.94 in head-to-head comparisons. The best point-of-care devices show 100% concordance with venous lab results for classifying diabetes control, while less precise devices still reach about 81% concordance.
At-Home Test Kits
Home A1c kits fall into two categories. Some give you an instant result using a small device and test strip. Others, called mail-in kits, have you prick your finger and collect blood drops on a filter paper card, then mail it to a lab. The mail-in approach is essentially a lab test with a self-collected sample. FDA-cleared mail-in kits show very high agreement with professional lab draws, with R² values of 0.98 when comparing self-collected capillary samples to venous blood.
These kits are available without a prescription from pharmacies and online retailers. They typically cost between $25 and $50 per test. Common medications like acetaminophen, ibuprofen, aspirin, and metformin do not interfere with the results.
Understanding Your Results
The American Diabetes Association defines three ranges:
- Below 5.7%: Normal
- 5.7% to 6.4%: Prediabetes
- 6.5% or higher: Diabetes
A single A1c of 6.5% or above is enough for a diabetes diagnosis when confirmed by a repeat test. If your result lands in the prediabetes range, it means your blood sugar is elevated but not yet at the diabetes threshold. Roughly every 25 mg/dL increase in average blood sugar raises A1c by about 0.6 percentage points. So an average glucose of 150 mg/dL corresponds to an A1c near 6.9%, while 200 mg/dL corresponds to about 8.1%.
How Often to Test
Most people with diabetes get their A1c checked at least twice a year. If you’ve recently changed medications, started a new exercise routine, or your blood sugar has been difficult to manage, your provider may recommend testing every three months. For people without diabetes who are monitoring prediabetes, annual testing is common.
Testing more frequently than every three months doesn’t add much useful information, because the result reflects a three-month average. A retest at six weeks would still overlap heavily with the previous reading.
Conditions That Skew Results
A1c is reliable for most people, but certain conditions can push results artificially high or low. This matters because a misleading number could lead to a wrong diagnosis or an incorrect treatment adjustment.
Results may read falsely high if you have iron deficiency anemia, vitamin B-12 or folate deficiency, kidney disease (uremia), chronic heavy alcohol use, or if you’ve had your spleen removed. These conditions either extend the lifespan of red blood cells (giving glucose more time to accumulate on hemoglobin) or create compounds that mimic glycated hemoglobin in the test.
Results may read falsely low during pregnancy (especially in the second trimester), with hemolytic anemia or any condition that destroys red blood cells faster than normal, with an enlarged spleen, or after a blood transfusion. Shorter red blood cell lifespans mean less time for glucose to attach to hemoglobin, so the reading underestimates true blood sugar levels. High-dose vitamin E (600 to 1,200 mg daily) can also lower results by reducing the chemical process that attaches glucose to proteins.
People with certain hemoglobin variants, particularly homozygous sickle cell disease (HbSS) or hemoglobin C disease (HbCC), generally cannot rely on A1c testing at all. Those who carry one copy of a variant gene (sickle cell trait, for example) can still use A1c as long as the lab uses an appropriate testing method. If you know you carry a hemoglobin variant, mention it when your test is ordered.
CGM as an A1c Estimate
If you wear a continuous glucose monitor, your device or app likely reports a number called the Glucose Management Indicator, or GMI. This estimates what your A1c would be based on your average sensor glucose readings over the past 14 days or more.
GMI is useful for tracking trends between lab visits, but it’s not a replacement for a lab A1c. The two numbers match exactly only about 19% of the time. More than half the time, they differ by 0.3 percentage points or more, and about 28% of the time the gap reaches 0.5 points or wider. That’s a clinically meaningful difference. If your GMI says 7.0%, your actual lab A1c could reasonably be anywhere from 6.5% to 7.5%. The discrepancy exists because CGMs measure glucose in the fluid between cells, not in the blood directly, and because individual biology affects how much glucose sticks to hemoglobin at any given blood sugar level.
Think of GMI as a useful directional tool. If it’s trending up over several weeks, your lab A1c is likely climbing too. But for any medical decisions, diagnoses, or insurance documentation, you need the actual blood test.