When to Order an A1C Test
A physician should order an A1C test for two primary purposes: (1) screening and diagnosing diabetes or prediabetes in at-risk adults, and (2) monitoring glycemic control every 3-6 months in patients with established diabetes. 1
Screening and Diagnosis Criteria
Who Should Be Screened
Order A1C testing for diabetes screening in:
- Adults with risk factors for diabetes (obesity, family history, sedentary lifestyle, hypertension, dyslipidemia, history of gestational diabetes, PCOS)
- Adults ≥45 years of age as part of routine screening
- Younger adults with established risk factors
- Patients with symptoms suggestive of hyperglycemia (polyuria, polydipsia, unexplained weight loss)
Diagnostic Thresholds
Use laboratory-based A1C testing with NGSP-certified methods performed in accredited laboratories 1:
- A1C ≥6.5% (≥48 mmol/mol) = Diabetes diagnosis
- A1C 5.7-6.4% (39-46 mmol/mol) = Prediabetes (high risk for future diabetes)
- A1C <5.7% = Normal
Important caveat: Confirm any diagnostic A1C result with either a repeat A1C test on a different sample or a glucose-based test (fasting glucose or OGTT) 1. This reduces the risk of systematic error inherent in A1C testing alone 2.
When NOT to Use A1C for Diagnosis
Do not order A1C for gestational diabetes screening or diagnosis 1. Use glucose-based testing instead.
Monitoring Criteria for Established Diabetes
Frequency of Testing
- Every 3 months until individualized glycemic targets are achieved 1
- Every 6 months minimum once targets are stable and maintained 1
- Every 3 months after therapy modifications or when targets are not met 1
The A1C reflects average glucose over the preceding 60-90 days due to the 120-day lifespan of red blood cells 1.
Technical Requirements
Laboratory Standards
- Use only NGSP-certified methods 1
- Perform testing in accredited laboratories 1
- Avoid point-of-care (POC) devices for diagnosis 1. POC testing is CLIA-waived in the U.S. without required proficiency testing, making it unreliable for diagnostic purposes. POC devices may be used for monitoring only if FDA-approved and performed at CLIA-certified laboratories with moderate complexity or higher 1.
Critical Pitfalls and When Results May Be Unreliable
Conditions That Falsely Lower A1C
- Shortened red blood cell lifespan: hemolytic anemia, recent blood loss, hemoglobinopathies, chronic kidney disease with erythropoietin therapy 3, 4
- Pregnancy (increased RBC turnover)
- Recent transfusion
Conditions That Falsely Elevate A1C
- Iron deficiency anemia (prolonged RBC lifespan)
- Vitamin B12 or folate deficiency
- Splenectomy
Analytical Interference
- Hemoglobin variants (HbS, HbC, HbE) can interfere depending on the assay method 3
- The laboratory may detect and warn of suspected interference, but if clinical presentation and glucose measures are discordant, measure fasting glucose instead 3
When to Use Alternative Testing
If A1C results are discordant with clinical picture or equivocal, perform plasma glucose testing 2. Consider measuring fructosamine or 1,5-anhydroglucitol in patients with conditions affecting RBC turnover 5.
Combining A1C with plasma glucose measurements reduces systematic bias and offers the benefits of each test 2. This is particularly important because A1C ≥6.5% as a sole criterion misses a substantial number of people with type 2 diabetes, including some with fasting hyperglycemia 2.
Advantages of A1C Testing
The clinical convenience of A1C makes it increasingly preferred for diagnosis 1:
- No fasting required
- Preanalytic stability (sample stable for days)
- Decreased biological variability compared to glucose
- Reflects longer-term glycemia, less affected by recent stress or illness 6