Hemoglobin A1c (HbA1c) remains a cornerstone of diabetes management, offering a convenient estimate of average blood glucose over two to three months. Its routine use in clinical practice is supported by decades of evidence linking HbA1c to microvascular and macrovascular outcomes. However, the test's accuracy depends on normal hemoglobin physiology and a standard red blood cell (RBC) lifespan. For millions of people worldwide who carry inherited hemoglobin disorders—known collectively as hemoglobinopathies—HbA1c results can be misleading. An HbA1c may suggest excellent glucose control when in reality glycemia is poor, or it may alarm providers with falsely high values. Recognizing when and why these interferences occur, and knowing which alternative monitoring strategies to adopt, is essential for safe, evidence‑based diabetes care in this growing population.

Understanding Hemoglobinopathies: Scope and Mechanisms

Hemoglobinopathies are inherited blood disorders that affect the structure or production of hemoglobin. Hemoglobin itself is a tetramer composed of two alpha‑globin and two beta‑globin chains. Mutations in the genes encoding these chains can produce abnormal hemoglobin variants (e.g., HbS in sickle cell disease, HbC, HbE) or reduce the synthesis of globin chains (as in the thalassemias). According to the World Health Organization, approximately 5% of the world's population carries a hemoglobin variant, with the highest prevalence in sub‑Saharan Africa, the Mediterranean basin, Southeast Asia, and the Middle East. As global migration patterns evolve, clinicians in all regions are increasingly likely to encounter patients with these disorders.

These conditions disrupt RBC biology in distinct ways. Sickle cell disease (SCD) causes RBCs to become rigid and hemolyze prematurely, shortening the average RBC lifespan from ~120 days to as little as 10–30 days. Thalassemia syndromes produce hypochromic, microcytic cells that also exhibit reduced survival. Other variants, such as HbC or HbE, may alter RBC deformability or oxygen affinity without dramatically shortening lifespan. Each of these changes can profoundly impact HbA1c measurement through two primary mechanisms: altered RBC survival changes the time window for glucose‑hemoglobin glycation, and abnormal hemoglobin variants can produce direct analytical interference with certain laboratory methods.

How HbA1c Testing Works: The Basis for Interference

The HbA1c test measures the percentage of hemoglobin A that is glycated—having glucose molecules non‑enzymatically attached to the N‑terminal valine of the beta‑globin chain. Because the glycation reaction is continuous over the RBC's lifespan, HbA1c reflects integrated glucose exposure over roughly 8–12 weeks. Standard reference methods include high‑performance liquid chromatography (HPLC) and capillary electrophoresis, which separate hemoglobin species based on charge. Clinical laboratories also widely use immunoassays and newer enzymatic assays. Any condition that changes the structure of hemoglobin or the lifespan of RBCs can disrupt the relationship between HbA1c and actual average glucose. In patients with hemoglobinopathies, interference occurs through two pathways:

  1. Altered RBC survival: A shortened lifespan reduces the time available for glycation, leading to a falsely low HbA1c. Conversely, conditions that prolong RBC survival (rare in hemoglobinopathies but possible after splenectomy) can cause falsely elevated values.
  2. Analytical interference: Abnormal hemoglobin variants may co‑elute with HbA1c in certain separation methods, be misidentified as HbA1c, or alter the antibody‑binding site in immunoassays. The direction and magnitude of bias depend on the specific variant and the test method.

Impact of Specific Hemoglobinopathies on HbA1c

Sickle Cell Disease (HbSS, HbSC, HbSβ⁰ Thalassemia)

Sickle cell disease results from a point mutation in the beta‑globin gene (Glu6Val), producing hemoglobin S. In homozygous HbSS disease, RBCs have a severely shortened lifespan due to recurrent sickling and hemolysis. This reduces the time available for glucose to accumulate on hemoglobin, typically yielding falsely low HbA1c levels. Patients with HbSS often have HbA1c values 1–3% lower than would be predicted from average glucose measured by continuous glucose monitoring (CGM) or self‑monitoring. The effect is even more pronounced in individuals with high frequencies of vaso‑occlusive crises or those on chronic transfusion therapy. In compound heterozygous states such as HbSC or HbSβ⁰ thalassemia, the RBC lifespan is also reduced, though the degree of interference varies. For example, HbSC disease typically shows a moderate false lowering, while HbSβ⁰ thalassemia may approach the severity of HbSS.

Sickle Cell Trait (HbAS)

In sickle cell trait, the RBC lifespan is only mildly reduced, and interference is less dramatic but may still be clinically relevant—especially when HbA1c values are near treatment thresholds. Large cohort studies, including those by the National Institutes of Health, have confirmed that HbA1c underestimates glycemia in HbAS individuals by approximately 0.3–0.5% on average. While this bias may not alter clinical decisions in many cases, it warrants caution when using HbA1c as the sole measure of glucose control in patients with borderline results or in those aiming for very tight targets.

Thalassemia Syndromes

Thalassemia involves reduced synthesis of either alpha‑ or beta‑globin chains. Beta‑thalassemia major (homozygous) presents with severe anemia, reliance on regular transfusions, and markedly shortened RBC survival. The resulting HbA1c is frequently falsely low, often to a degree that makes it completely unreliable. In contrast, beta‑thalassemia minor (trait) may produce mildly shortened RBC survival but also a higher proportion of HbA2 and HbF, which can interfere with HPLC methods. Depending on the laboratory technique, thalassemia trait can cause either falsely low or falsely elevated HbA1c readings. For alpha‑thalassemia, the effects are generally less pronounced but still depend on the number of gene deletions and resulting anemia severity; HbA1c tends to be falsely low in symptomatic forms (e.g., HbH disease).

Other Clinically Significant Variants

Hemoglobin C (HbC): Common in West Africa, HbC results from a mutation (Glu6Lys). In homozygous HbCC disease, RBC lifespan is moderately reduced, producing a mild to moderate false lowering of HbA1c. In HbSC disease, the effect is a combination of the HbS and HbC contributions. Heterozygous HbAC (trait) has minimal impact.

Hemoglobin E (HbE): Prevalent in Southeast Asia, particularly Thailand, Cambodia, and Laos. Homozygous HbEE and HbE/β‑thalassemia cause mildly reduced RBC survival, leading to a small false lowering. HbE trait (HbAE) generally does not affect HbA1c.

Hemoglobin D (HbD) and Hemoglobin G (HbG): These variants can co‑elute with HbA1c in certain HPLC systems, producing a falsely elevated result. The specific interference pattern depends heavily on the analytical method used. Laboratories using ion‑exchange HPLC must be able to identify these variant peaks to avoid reporting a spuriously high HbA1c.

How the Test Method Influences Results

Clinicians should be familiar with the HbA1c assay used in their setting, as the interference profile varies widely.

High‑Performance Liquid Chromatography (HPLC)

Ion‑exchange HPLC is the most common reference method and can often separate common variants like HbS, HbC, HbE, and HbF from HbA1c. However, some variants (e.g., HbD, HbG, Hb Lepore, HbJ) may co‑elute with the HbA1c peak, giving a falsely high result. Many modern HPLC instruments include software that flags abnormal peaks, but the operator must be trained to recognize these patterns. Laboratories should have policies to investigate such flags and, if necessary, report the result with a comment or recommend an alternative method.

Immunoassay

Immunoassays use antibodies that recognize the glycated N‑terminal sequence of the beta‑globin chain. Most common variants do not alter this epitope, so direct analytical interference is rare. However, a few mutations (e.g., Hb Raleigh, Hb Graz) can change the antibody‑binding site and cause falsely low results. Even when the antibody‑based measurement is accurate, the underlying RBC lifespan issue remains: if the RBC lifespan is shortened, the glycated hemoglobin measured by immunoassay is still reduced, so the result remains falsely low. Immunoassays are therefore not a panacea for hemoglobinopathy‑related interference.

Capillary Electrophoresis

Capillary electrophoresis is increasingly adopted in large laboratories. It offers excellent separation of hemoglobin variants and often provides a clear identification of abnormal species. Many systems automatically correct for the presence of variants or flag the result as unreliable. Capillary electrophoresis is generally considered one of the most reliable methods for patients with hemoglobinopathies, but no method is perfect for all variants.

Enzymatic Assays

Enzymatic methods, which use a fructosyl‑amino acid oxidase to measure glycated hemoglobin, are largely unaffected by common hemoglobin variants. However, they remain sensitive to changes in RBC lifespan. These assays are relatively new and not yet universally adopted; they may become more important as the technology matures.

Alternative Measures of Glycemic Control

When HbA1c is unreliable due to a hemoglobinopathy, clinicians must turn to markers that do not depend on hemoglobin structure or RBC lifespan. The American Diabetes Association (ADA) specifically advises using alternative measures in patients with conditions that affect RBC survival.

Fructosamine and Glycated Albumin

Fructosamine measures total glycation of serum proteins, primarily albumin. Because albumin has a half‑life of approximately 14–20 days, fructosamine reflects glucose control over the preceding 2–3 weeks. It is unaffected by hemoglobin variants and can be used in patients with sickle cell disease or thalassemia. However, fructosamine is influenced by albumin concentration—hypoalbuminemia (common in liver disease, nephrotic syndrome, or malnutrition) can falsely lower results. Glycated albumin (GA) normalizes for albumin levels and may be more reliable. Several studies in patients with sickle cell disease have shown good correlation between GA and CGM‑derived mean glucose. GA is now available from many large reference laboratories. Clinicians should note that GA and fructosamine provide a shorter window of assessment than HbA1c (2–3 weeks vs. 2–3 months), so more frequent testing (e.g., every 2–4 weeks) may be needed for optimal monitoring.

Continuous Glucose Monitoring (CGM)

Continuous glucose monitoring provides real‑time glucose data from interstitial fluid. Modern CGM devices (e.g., Dexcom G7, Abbott Freestyle Libre 3) offer accuracy comparable to or better than self‑monitoring and provide rich metrics such as time‑in‑range, glucose variability, and the glucose management indicator (GMI). GMI is an estimate of HbA1c derived from CGM data and can serve as a surrogate when HbA1c is unreliable. The ADA and the Endocrine Society recommend CGM as the preferred alternative for patients with hemoglobinopathies who require frequent glucose monitoring. It is important to note that GMI is not a direct measurement of glycated hemoglobin—it is a mathematical transformation of average glucose. The relationship between average glucose and GMI may differ slightly across populations, and GMI should be interpreted as a separate clinical metric, not as a replacement for HbA1c. Nonetheless, CGM provides a comprehensive picture of glycemic control that is unaffected by RBC disorders.

Self‑Monitoring of Blood Glucose (SMBG)

Traditional finger‑stick monitoring remains essential for day‑to‑day insulin dosing and detecting hypoglycemia. However, SMBG provides only point‑in‑time information and does not give an integrated picture of glycemia. It is best used in combination with either fructosamine, glycated albumin, or CGM. The frequency of SMBG should be individualized based on the patient's treatment regimen and glucose variability. For patients with hemoglobinopathies who use CGM, SMBG is still needed for calibration and to confirm glucose readings during periods of suspected sensor inaccuracy.

Clinical Recommendations: A Practical Approach

Clinicians should screen for hemoglobinopathies when HbA1c does not align with other glucose measures (SMBG, CGM, or clinical history), or when the patient is from a high‑prevalence ethnic background. A complete blood count with red cell indices, hemoglobin electrophoresis or HPLC, and a review of family history can identify at‑risk individuals. The following approach is recommended when a hemoglobinopathy is confirmed or strongly suspected:

  • Discontinue HbA1c as the primary glycemic measure in patients with homozygous or compound heterozygous disorders that shorten RBC lifespan (e.g., HbSS, HbSC, beta‑thalassemia major, HbH disease).
  • Choose an alternative marker: glycated albumin or fructosamine may be used if CGM is not available. For patients on intensive insulin therapy, CGM is strongly preferred.
  • Use the glucose management indicator (GMI) cautiously—it is derived from average glucose, not hemoglobin, and may differ systematically from HbA1c in some populations. Periodically cross‑check with measured average glucose from SMBG or CGM downloads.
  • For patients with trait (e.g., HbAS, beta‑thalassemia minor), clinicians may continue to use HbA1c if they are aware of a possible mild bias (usually 0.3–0.5% lower). However, if treatment decisions hinge on borderline HbA1c values, confirm with an alternative method.
  • Document the interference in the patient’s medical record and communicate with the laboratory to flag future HbA1c orders. Some electronic health records can be configured to alert providers when a patient has a known hemoglobinopathy.

The ADA Standards of Medical Care in Diabetes explicitly recommend using alternative methods in patients with hemoglobinopathies. The Centers for Disease Control and Prevention provides resources for clinicians caring for patients with sickle cell disease and diabetes. For a comprehensive review of specific assay interferences by variant, the NIH‑sponsored review on hemoglobin variants and HbA1c measurement is an excellent reference. Additionally, the World Health Organization fact sheet on sickle cell disease provides global epidemiological context.

Looking Ahead: A Growing Clinical Challenge

The global prevalence of both diabetes and hemoglobinopathies is rising. In the United States alone, an estimated 100,000 people have sickle cell disease, and millions carry thalassemia traits. As diabetes becomes more common in these populations, clinicians will increasingly encounter the limitations of HbA1c testing. Developing institutional protocols for managing HbA1c interference—including standardized alternative monitoring pathways and laboratory communication—will improve patient outcomes and reduce diagnostic errors.

In summary, hemoglobinopathies can skew HbA1c results through both altered RBC lifespan and direct analytical interference. Recognizing these effects is the first step toward accurate diabetes management in affected patients. By adopting alternative monitoring strategies—particularly CGM and glycated albumin—clinicians can avoid the pitfalls of unreliable HbA1c values and provide safe, evidence‑based care. As our understanding of these interactions deepens and new assay technologies emerge, the ability to individualize diabetes monitoring will continue to improve.