What are the causes of an uncalculated or unreliable hemoglobin A1c (HbA1c) result?

Causes of Uncalculated or Unreliable HbA1c

HbA1c cannot be measured in individuals with homozygous hemoglobin variants (HbSS, HbEE) who lack HbA, and must be replaced with fructosamine or glycated albumin testing. 1, 2

Primary Causes of Undetectable/Unmeasurable HbA1c

Homozygous Hemoglobin Variants

• HbA1c cannot be measured and should not be reported in individuals with homozygous hemoglobin variants such as HbSS (sickle cell disease) or HbEE, as these patients lack HbA entirely. 1, 2
• Glycated proteins such as fructosamine or glycated albumin must be used as alternatives in these patients. 1, 2
• Southeast Asian populations have higher prevalence of HbE variants, and Mediterranean populations have higher prevalence of beta-thalassemia, making these groups particularly vulnerable. 2

Heterozygous Hemoglobin Variants (Method-Dependent Interference)

• Hemoglobin C, D Punjab, E, or S trait can interfere with HbA1c measurement depending on the specific assay method used. 1, 3
• Certain methods show clinically significant interference: Tosoh G11 variant mode (HbAD), Roche b 101 (HbAC and HbAE), and Siemens DCA Vantage (HbAE and HbAS). 3
• Boronate affinity methods are less susceptible to interference from hemoglobin variants and should be used when variants are suspected. 4

Conditions Causing Falsely Low HbA1c (Spurious Results)

Shortened Red Blood Cell Lifespan

• Any condition that shortens RBC survival or decreases mean RBC age produces falsely low HbA1c results regardless of the assay method used. 1, 2, 4
• Hemolytic anemia reduces the time red blood cells are exposed to glucose, resulting in falsely low glycation. 2
• Recovery from acute blood loss falsely lowers HbA1c by reducing mean RBC age. 1, 4
• Chronic kidney disease can shorten erythrocyte lifespan, producing falsely low values. 2, 4
• Pregnancy (second and third trimesters) alters RBC turnover, lowering usual HbA1c concentrations even in women without diabetes. 1, 2, 4

Normal Variation in Red Cell Half-Life

• Differences in mean red cell half-life range from 48 to 68 days (mean 58 days), which may explain some interindividual variability in the relationship between measured average glucose and HbA1c levels. 1, 5

Conditions Causing Falsely Elevated HbA1c

Prolonged Red Blood Cell Lifespan

• Iron deficiency anemia prolongs erythrocyte lifespan, exposing red blood cells to glucose for extended periods, resulting in falsely elevated HbA1c levels. 4, 5
• Iron deficiency anemia is one of the most common causes of falsely elevated HbA1c encountered in clinical practice. 5

Other Interfering Substances (Method-Dependent)

• Vitamins C and E may falsely lower HbA1c results by inhibiting glycation of hemoglobin. 1, 4
• Hypertriglyceridemia, hyperbilirubinemia, chronic alcoholism, chronic ingestion of salicylates, and opiate addiction may interfere with some older assay methods, falsely increasing results. 1, 4
• Carbamylation of hemoglobin in uremic patients can cause falsely increased HbA1c values, though this effect is method-dependent. 5

Diagnostic Algorithm When HbA1c Is Unreliable

Initial Laboratory Evaluation

1. Order hemoglobin electrophoresis or variant analysis to identify structural hemoglobin abnormalities. 2, 4
2. Obtain complete blood count to assess for anemia and hemolysis. 2, 5
3. Check reticulocyte count to evaluate red blood cell turnover. 2
4. Order iron studies to evaluate for iron deficiency if anemia is present. 5

Alternative Glycemic Monitoring Strategies

• Use fructosamine or glycated albumin as alternative markers in clinical settings where abnormalities in red blood cell turnover, hemoglobin variants, or other interfering factors compromise HbA1c interpretation, though these reflect a shorter period of average glycemia (2-3 weeks vs 2-3 months). 1, 2, 4, 5
• Implement continuous glucose monitoring (CGM) to better understand glycemic patterns when HbA1c reliability is questioned. 2, 4
• Use plasma glucose criteria exclusively for screening, diagnosis, and management in patients with disorders affecting red blood cell turnover. 1, 2, 4

Method Selection for Ongoing Monitoring

• Use boronate affinity HbA1c methods exclusively for ongoing monitoring if hemoglobin variants are confirmed, as they are less susceptible to interference. 4, 5
• Do not switch between different HbA1c methods for serial monitoring once a reliable method is established, as this introduces additional measurement variability. 4

Critical Clinical Pitfalls to Avoid

Interpretation Errors

• Never base treatment decisions solely on a single HbA1c measurement without consideration of glucose monitoring results, as this may lead to unnecessary initiation or intensification of therapy. 2, 4, 5
• Suspect hemoglobin variant or interference if the HbA1c reading is surprisingly high or low, or significantly changed coincident with a change in laboratory method. 4
• HbA1c should be interpreted with caution when there is a discrepancy with self-monitored blood glucose readings, especially in patients with suspected hemoglobinopathy. 2, 4
• Marked discrepancies between measured HbA1c and plasma glucose levels should prompt consideration that the HbA1c assay may not be reliable for that individual. 2

Management Errors

• Do not average discordant HbA1c results, as this provides no clinically useful information and may lead to inappropriate treatment decisions. 2, 4, 5
• Never dismiss undetectable values as laboratory error without investigation. 2
• Laboratories should be aware of potential interferences, including hemoglobin variants, and consider these in their particular patient population when selecting assay methods. 1, 4

Age and Race-Related Variations (Not Causes of Unreliability)

• Age-related increases in mean HbA1c occur at approximately 0.1% per decade after age 30 years in people without diabetes. 1, 4, 5
• African Americans have mean HbA1c values 0.4% higher than White individuals for any given level of glycemia, though race does not modify the association between HbA1c and adverse cardiovascular outcomes or death. 1, 4, 5