HbA1c Reliability Interpreter: Falsely High or Low HbA1c?
HbA1c Reliability Interpreter: Falsely High or Low HbA1c?
Check whether an HbA1c can be trusted before acting on it. Enter the result and what you know about the patient — haemoglobin variants and whether your method is affected, haemolysis, blood loss or transfusion, splenectomy, iron deficiency and its treatment, kidney disease and ESA use, pregnancy, and recent onset — and the page says whether the HbA1c is likely to be falsely high, falsely low or unreliable in either direction, and what to use instead.
Can this HbA1c be trusted?
HbA1c + variant, red cells, iron, kidney, pregnancy → falsely high, low or unreliableA 58-year-old on haemodialysis, treated with erythropoietin, has an HbA1c of 6.4% while home glucose readings are mostly 10–14 mmol/L.
How the verdict is reached
Any unpredictable factor, or factors in both directions → unreliable
Only lowering factors → likely falsely low · only raising factors → likely falsely high
IFCC (mmol/mol) = (NGSP % − 2.15) × 10.929
- Raise
- untreated iron, B12 or folate deficiency; splenectomy; alcohol dependence, chronic opiate use (WHO 2011)
- Lower
- haemolysis, blood loss, transfusion, ESA, replacement therapy, dialysis, pregnancy after the first trimester, G6PD deficiency, dapsone, ribavirin, antiretrovirals, liver disease, recent onset
- Unpredictable
- variants on an affected method, HbSS/HbCC/HbSC, raised HbF, CKD G4–G5, aspirin and vitamins C and E
Worked example
A 58-year-old on haemodialysis, treated with erythropoietin, has an HbA1c of 6.4% while home glucose readings are mostly 10–14 mmol/L.
HbA1c 6.4% = (6.4 − 2.15) × 10.929 = 46 mmol/mol
Dialysis → lowers (NGSP); erythropoietin → lowers (ADA, WHO)
Two factors, both in the same direction, nothing unpredictable → likely falsely low
Use glucose monitoring or glycated albumin rather than titrating to this HbA1c
Factors that influence HbA1c (WHO 2011, Annex 1)
| Mechanism | Raises HbA1c | Lowers HbA1c | Variable |
|---|---|---|---|
| Erythropoiesis | Iron or B12 deficiency, decreased erythropoiesis | Erythropoietin, iron, B12, reticulocytosis, chronic liver disease | |
| Altered haemoglobin | Haemoglobinopathies, HbF, methaemoglobin | ||
| Glycation | Alcoholism, chronic renal failure, lower intra-erythrocyte pH | Aspirin, vitamins C and E, some haemoglobinopathies, higher intra-erythrocyte pH | Genetic determinants |
| Red cell destruction | Longer life span: splenectomy | Shorter life span: haemoglobinopathies, splenomegaly, rheumatoid arthritis, antiretrovirals, ribavirin, dapsone | |
| Assay | Hyperbilirubinaemia, carbamylated Hb, alcoholism, large doses of aspirin, chronic opiate use | Hypertriglyceridaemia | Haemoglobinopathies |
Why an HbA1c can be wrong while the laboratory is right
HbA1c measures the fraction of haemoglobin that has had glucose attached to it, and it tracks average glucose only because red cells live about 120 days. Anything that changes how long red cells live — haemolysis, bleeding, transfusion, an enlarged or absent spleen, a new burst of red cells after iron or erythropoietin — changes the HbA1c without any change in glucose. The assay can be perfectly accurate and the result still misleading.
The ADA Standards of Care (section 2, carried unchanged into the 2026 edition) list the conditions in which the relationship between HbA1c and glycaemia is altered and say that plasma glucose criteria should be used to diagnose diabetes in them: some haemoglobin variants, pregnancy in the second and third trimesters and postpartum, G6PD deficiency, HIV, haemodialysis, recent blood loss or transfusion, and erythropoietin therapy. The WHO’s 2011 report adds the direction of each effect, and NGSP maintains the method-by-method list of which assays are affected by HbS, HbC, HbE and HbD traits and by raised HbF.
Two traps deserve emphasis. A haemoglobin trait is not a problem on a method that is not affected by it, because red cell survival is normal — the ADA Standards ask for a non-interfering assay rather than abandoning HbA1c. But in HbSS, HbCC and HbSC the problem is the disease, not the assay, and NGSP warns against relying on HbA1c whatever the method. And iron deficiency raises HbA1c while its treatment lowers it: an HbA1c that falls after iron is started may be the iron, not the diabetes treatment.
The alternatives have their own limits. Fructosamine (see the fructosamine to HbA1c calculator) reflects two to three weeks and follows albumin turnover; glucose monitoring measures glucose directly. When HbA1c and glucose disagree markedly with no explanation, ask the laboratory to review the chromatogram. The adult DKA and HHS interpreter covers the crises. This interpreter supports, and does not replace, clinical judgement.
Frequently asked questions
What causes a falsely low HbA1c?
Anything that shortens red cell survival or makes the red cell population younger: haemolysis, recovery from blood loss, transfusion, erythropoietin, iron or B12 replacement, dialysis, pregnancy after the first trimester, G6PD deficiency and drugs such as dapsone and ribavirin. A rapid recent rise in glucose also leaves the HbA1c behind.
What causes a falsely high HbA1c?
Older red cells: untreated iron, B12 or folate deficiency, and splenectomy. The WHO also lists alcohol dependence and, on some assays, chronic opiate use, hyperbilirubinaemia and carbamylated haemoglobin.
Can HbA1c be used in sickle cell trait?
Yes, on a method that sickle trait does not interfere with, because red cell survival is normal in trait; NGSP lists which methods are affected. In sickle cell disease (HbSS) and HbSC, HbA1c should not be relied on whatever the method.
What should be used instead of HbA1c?
For diagnosis, plasma glucose criteria (the ADA Standards). For monitoring, glucose monitoring, or fructosamine or glycated albumin — remembering that these follow albumin rather than red cells, and fructosamine is also raised in iron deficiency.
Related calculators
References
- American Diabetes Association Professional Practice Committee. 2. Diagnosis and classification of diabetes: Standards of Care in Diabetes—2026. Diabetes Care. 2026;49(Suppl 1):S27. The A1C limitations text is carried unchanged from the 2024 and 2025 editions.
- National Glycohemoglobin Standardization Program. HbA1c assay interferences (ngsp.org/interf.asp, updated June 2026) and Factors that interfere with HbA1c test results (ngsp.org/factors.asp). Clinically significant interference is defined there as more than ±6% at 6% and/or 9% HbA1c.
- World Health Organization. Use of glycated haemoglobin (HbA1c) in the diagnosis of diabetes mellitus: abbreviated report of a WHO consultation. Geneva: WHO; 2011. Annex 1: factors that influence HbA1c and its measurement.
- John WG; UK Department of Health Advisory Committee on Diabetes. Use of HbA1c in the diagnosis of diabetes mellitus in the UK. The implementation of World Health Organization guidance 2011. Diabet Med. 2012;29(11):1350–1357.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.