Factors Affecting HbA1c Test Results

Conditions Affecting HbA1c Measurements

• Any condition that shortens RBC survival or decreases mean RBC age produces falsely low HbA1c results regardless of the assay method used 1, 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 3
• Chronic kidney disease can shorten erythrocyte lifespan, resulting in falsely low values 4
• Iron deficiency anemia prolongs erythrocyte lifespan and exposes cells to glucose for longer periods, causing falsely elevated HbA1c levels 4, 5
• Occult malignancies affecting bone marrow function can alter red blood cell production and lifespan, leading to falsely elevated HbA1c 6
• Chronic infections may cause inflammatory responses affecting red blood cell turnover, potentially resulting in higher HbA1c values than expected based on blood glucose levels 6

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, 2
• Hemoglobin variants (HbS, HbC) can result in falsely elevated or falsely lowered HbA1c levels depending on the specific assay method used 4, 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 6

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, 2
• 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 4, 5

Medications and Nutritional Factors

• Vitamins C and E are reported to falsely lower HbA1c results, possibly by inhibiting glycation of hemoglobin 1, 2
• Oral hypoglycemic agents (metformin or sulfonylureas) may alter the relationship between blood glucose and HbA1c levels, although clinical significance remains unclear 4
• Hypertriglyceridemia, hyperbilirubinemia, chronic alcoholism, chronic ingestion of salicylates, and opiate addiction are reported to interfere with some assay methods, falsely increasing results 1, 2

Laboratory and Technical Factors

• 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% 4, 5
• 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 4, 5
• Comparing HbA1c tests performed in different clinical laboratories introduces additional error 4, 5
• 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

• HbA1c should be interpreted with caution when there is a discrepancy with self-monitored blood glucose readings, especially in patients with suspected occult disease 6
• When HbA1c values seem discordant with clinical presentation or glucose monitoring results, consider conditions affecting red blood cell lifespan as potential confounding factors 6
• 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, 2
• Consider continuous glucose monitoring (CGM) to better understand glycemic patterns when HbA1c reliability is questioned 7
• 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 6
• 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 4, 5
• Laboratories should be aware of potential interferences, including hemoglobin variants, and consider these in their particular patient population when selecting assay methods 1, 2
• 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, 2

HbA1c Measurement and Diabetes Management

Introduction to HbA1c Methods

• The American Diabetes Association recommends using boronate affinity methods for HbA1c measurement when hemoglobin variants are suspected, as they are less susceptible to interference and structural abnormalities 8

Diagnostic Criteria and HbA1c Measurement

• A fasting glucose of 250 mg/dL indicates severe hyperglycemia, and an HbA1c of 13.6% corresponds to a mean plasma glucose of approximately 326 mg/dL, which aligns with the observed fasting glucose, according to the American Diabetes Association 8
• The American Diabetes Association recommends confirming diabetes diagnosis with a repeat fasting glucose test, and the HbA1c discrepancy does not affect the diabetes diagnosis but is important for establishing baseline glycemic control severity and monitoring treatment response accurately 9

Recommended Diagnostic Algorithm and Ongoing Management

• The American College of Clinical Endocrinologists recommends repeating HbA1c using boronate affinity method to confirm the result, and ordering hemoglobin electrophoresis or variant analysis to identify any structural hemoglobin abnormalities that could explain the discordance 8
• The Endocrine Society recommends using boronate affinity HbA1c exclusively for ongoing monitoring if hemoglobin variants are confirmed, and supplementing HbA1c monitoring with self-monitoring of blood glucose and continuous glucose monitoring to capture glycemic patterns and variability 8, 10, 11

Critical Pitfalls to Avoid in HbA1c Measurement and Diabetes Management

• The American Diabetes Association recommends avoiding averaging discordant HbA1c results, as this provides no clinically useful information and may lead to inappropriate treatment decisions, and not dismissing the higher value as laboratory error without investigation 12
• The American Association of Clinical Endocrinologists recommends avoiding switching between different HbA1c methods for serial monitoring once a reliable method is established, as this introduces additional measurement variability 13, 8