What factors can affect Hemoglobin A1c (HbA1c) test results?

Factors Affecting HbA1c Test Results

Multiple patient-specific, biological, and technical factors can significantly alter HbA1c measurements independent of actual glycemic control, making it essential to assess these confounding variables before making treatment decisions based on HbA1c values alone. 1

Red Blood Cell Lifespan Alterations

Conditions That Falsely Lower HbA1c

• Any condition that shortens RBC survival or decreases mean RBC age produces falsely low HbA1c results regardless of the assay method used. 1
• Hemolytic anemia causes decreased HbA1c values that do not reflect true glycemic control, while glycated albumin and 1,5-AG remain accurate. 2
• Recovery from acute blood loss falsely lowers HbA1c by reducing mean RBC age. 1
• Pregnancy alters RBC turnover, lowering usual HbA1c concentrations in women without diabetes. 1
• Chronic kidney disease can shorten erythrocyte lifespan, resulting in falsely low values. 1

Conditions That Falsely Elevate HbA1c

• Iron deficiency anemia prolongs erythrocyte lifespan and exposes cells to glucose for longer periods, causing falsely elevated HbA1c levels. 1
• Iron deficiency with or without anemia leads to increased HbA1c values compared with controls, with no concomitant rise in glucose indices. 3
• Occult malignancies affecting bone marrow function can alter red blood cell production and lifespan, leading to falsely elevated HbA1c. 4
• Chronic infections may cause inflammatory responses affecting red blood cell turnover, potentially resulting in higher HbA1c values than expected based on blood glucose levels. 4

Clinical Pitfall: Differences in mean red cell half-life (ranging from approximately 48 to 68 days) may explain some interindividual variability in the relationship between measured average glucose and HbA1c levels. 1

Hemoglobin Variants and Structural Abnormalities

• HbA1c cannot be measured and should not be reported in individuals with homozygous hemoglobin variants such as HbSS or HbEE; glycated proteins like fructosamine or glycated albumin must be used instead. 1
• Hemoglobin variants (HbS, HbC) can result in falsely elevated or falsely lowered HbA1c levels depending on the specific assay method used. 1
• Sickling hemoglobinopathies are well-represented in African Americans and significantly affect HbA1c measurement accuracy. 5
• A hemoglobin variant or interference should be suspected if the HbA1c reading is surprisingly high or low, or significantly changed coincident with a change in laboratory method. 4

Age and Race-Related Variations

• Age-related increases in mean HbA1c occur in people without diabetes at approximately 0.1% per decade after age 30 years. 1
• African Americans with prediabetes have HbA1c values that are 0.4% higher than white persons for any given level of glycemia, unexplained by measured differences in glycemia, clinical factors, or sociodemographic factors. 1
• A confirmatory fasting blood glucose level of at least 7.0 mmol/L (≥126 mg/dL) should be obtained if the initial HbA1c value is 6.5% to 6.9% in African Americans. 1

Medications and Nutritional Factors

• Vitamins C and E are reported to falsely lower HbA1c results, possibly by inhibiting glycation of hemoglobin. 1
• Oral hypoglycemic agents (metformin or sulfonylureas) may alter the relationship between blood glucose and HbA1c levels, although clinical significance remains unclear. 1
• Hypertriglyceridemia, hyperbilirubinemia, chronic alcoholism, chronic ingestion of salicylates, and opiate addiction are reported to interfere with some assay methods, falsely increasing results (though these studies are old and may not pertain to modern methods). 1

Laboratory and Technical Factors

Intralaboratory and Interlaboratory Variation

• Sequential HbA1c values within 0.5% do not statistically differ from one another unless the assay coefficient of variation is less than 3%, ideally less than 2%. 1
• A single HbA1c measurement, even from a high-quality laboratory, has a margin of error such that the true value falls within a range defined by the coefficient of variation. 1
• Comparing HbA1c tests performed in different clinical laboratories introduces additional error. 1
• Point-of-care HbA1c testing is not subject to systematic quality oversight and may be less reliable than central laboratory testing. 4

Sample Collection and Handling

• Improper handling of specimens, such as storage at high temperatures, can introduce large artifacts that may not be detectable depending on the assay method. 1
• Whole blood samples are generally stable for up to 1 week at 4°C. 1
• Samples stored at -70°C or colder are stable long term (at least 1 year), but specimens are not as stable at -20°C. 1

Clinical Management Recommendations

When to Suspect Unreliable HbA1c

• HbA1c should be interpreted with caution when there is a discrepancy with self-monitored blood glucose readings, especially in patients with suspected occult disease. 4
• When HbA1c values seem discordant with clinical presentation or glucose monitoring results, consider conditions affecting red blood cell lifespan as potential confounding factors. 4

Alternative Testing Strategies

• In clinical settings where abnormalities in red blood cell turnover, hemoglobin variants, or other interfering factors compromise interpretation, assays of other glycated proteins such as fructosamine or glycated albumin may be used, although they reflect a shorter period of average glycemia than HbA1c. 1
• Consider continuous glucose monitoring (CGM) to better understand glycemic patterns when HbA1c reliability is questioned. 6
• Evaluation for conditions affecting red blood cell lifespan should include complete blood count to assess for anemia and iron studies to evaluate for iron deficiency. 4

Critical Decision-Making Points

• Treatment decisions based solely on a single HbA1c measurement without consideration of other clinical data, such as glucose monitoring results, may lead to unnecessary initiation or intensification of therapy. 1
• Laboratories should be aware of potential interferences, including hemoglobin variants, and consider these in their particular patient population when selecting assay methods. 1
• Glucose testing will be necessary for screening, diagnosis, and management in individuals with disorders that affect red blood cell turnover, as HbA1c may provide spurious (generally falsely low) results regardless of the method used. 1