When an A1C result and blood-glucose readings disagree, neither should be dismissed automatically. A1C and pregnancy, anemia, recent blood loss, kidney disease and some haemoglobin variants can make the result harder to interpret. The next step is usually to consider what each test measures and whether a different laboratory test can clarify the picture.
Why A1C Can Differ From Glucose Tests
A1C measures the proportion of haemoglobin with glucose attached to it. It gives an estimate of average blood sugar over roughly the previous two to three months, while a fasting plasma-glucose test or an oral glucose tolerance test measures glucose at a particular time. A finger-stick meter or continuous glucose monitor can show patterns that a single A1C result does not capture.
Because A1C depends on red blood cells as well as glucose, a condition that changes how long those cells live—or how the laboratory assay detects haemoglobin—can shift the result. The direction and size of the effect depend on the cause, so a mismatch does not by itself prove that one test is wrong.
A1C And Pregnancy, Anaemia And Blood Loss
Iron-deficiency anaemia can make A1C read higher than expected in some people. The size of the effect is not predictable from the A1C alone. If you have anaemia or low iron, mention it when discussing the result; your clinician may interpret the number alongside blood counts and iron tests. Frequent blood donation can deplete iron in some people; learn more about iron levels and blood donation.
Recent significant blood loss can have the opposite effect: the body makes new red blood cells, which have had less time to accumulate glucose, and A1C may be lower than expected. A transfusion can also make the result difficult to interpret because donor red cells affect the mix. Its effect is not reliably in one direction.
During pregnancy, changes in red-cell turnover can affect A1C, and A1C is not a substitute for the glucose-based screening or diagnostic tests used for gestational diabetes. A result that appears reassuring does not rule out gestational diabetes. Follow the testing pathway recommended by your maternity or diabetes care team; screening schedules and protocols can differ by country.
Kidney Disease And Haemoglobin Variants
Advanced kidney disease may involve anaemia, shortened red-cell survival, dialysis or treatment that changes red-cell production. These factors can make A1C less reliable, and the effect may vary. Tell your clinician about kidney disease and relevant treatments if the A1C does not fit your glucose readings.
Some inherited haemoglobin variants interfere with particular A1C laboratory methods. The effect depends on both the variant and the assay; some variants also affect red-cell lifespan. A haemoglobin variant does not automatically make every A1C result unusable. The laboratory or clinician can check whether the method used is affected and consider a glucose-based test if needed.
When Plasma-Glucose Testing May Be More Useful
Clinicians may rely on plasma-glucose criteria when red-cell conditions or a haemoglobin variant make A1C difficult to interpret, or when results conflict with the clinical picture. Options include fasting plasma glucose and the two-hour value during a 75-gram oral glucose tolerance test. In a person with classic symptoms of high blood sugar, a random plasma-glucose result may also be used for diagnosis.
For nonpregnant adults, the American Diabetes Association’s 2025 Standards of Care list diabetes thresholds of A1C 6.5% (48 mmol/mol), fasting plasma glucose 126 mg/dL (7.0 mmol/L), or a two-hour oral glucose tolerance result of 200 mg/dL (11.1 mmol/L). These thresholds are not a guide to interpreting a result during pregnancy, where separate protocols apply. Local guidance may also differ.
Why Confirmation Matters
Unless high blood sugar is unequivocal, the ADA recommends confirming a diabetes diagnosis with two abnormal results. These can be two different tests, such as A1C and fasting glucose, or the same test repeated at another time. If results disagree, the clinician can consider whether A1C may be affected and choose an appropriate repeat or plasma-glucose test rather than treating one number as the whole story.
If your results do not match, bring the dates and values of your laboratory tests and glucose readings, and mention pregnancy, anaemia, recent bleeding or transfusion, kidney disease, and any known haemoglobin variant. A clinician can help decide which result is most interpretable and whether follow-up testing is needed. Seek clinical advice rather than changing treatment based on an unexplained mismatch.
