Why A1c Can Be Misleading for Diabetes Management in Sickle Cell Disease

For decades, the hemoglobin A1c test has served as the cornerstone of diabetes monitoring, offering a convenient snapshot of average blood glucose over two to three months. Its reliability, however, depends heavily on normal red blood cell physiology. In patients with sickle cell disease (SCD), several hematologic abnormalities can render the A1c test inaccurate, sometimes dangerously so. Understanding these limitations is not just an academic exercise—it is essential for preventing both undertreatment and overtreatment of diabetes, reducing the risk of long-term complications, and improving outcomes in a vulnerable population where the two diseases frequently intersect.

This article explores how SCD interferes with A1c measurements, reviews alternative monitoring methods, and provides actionable guidance for clinicians and patients navigating diabetes care in the context of sickle cell disease. The growing recognition of this issue has prompted clinical guidelines to shift away from A1c-based monitoring in patients with hemoglobinopathies, but many healthcare providers remain unaware of the magnitude of the problem or the available solutions.

The A1c Test: What It Actually Measures

The A1c test quantifies the percentage of hemoglobin that has glucose irreversibly bound to its beta-chain N-terminal valine—a process called glycation. Because red blood cells (RBCs) normally circulate for approximately 120 days, the A1c reflects the average blood glucose concentration over the preceding 8 to 12 weeks. It correlates with the risk of diabetic complications and is used to diagnose diabetes, set treatment targets, and assess glycemic control over time. The test’s convenience as a single blood draw without requiring fasting has made it a mainstay of outpatient diabetes care for decades.

Standard A1c assays are designed for individuals with normal hemoglobin (HbA). They rely on methods such as high-performance liquid chromatography (HPLC), immunoassay, or enzymatic assays that separate or detect glycated hemoglobin based on charge, structure, or antibody binding. Any variation in hemoglobin type or RBC lifespan can disrupt this measurement, producing results that do not accurately reflect the patient’s true glycemic state.

Sickle Cell Disease: A Hematologic Disruptor

Sickle cell disease is an inherited blood disorder caused by a mutation in the β-globin gene, leading to production of hemoglobin S (HbS). Under low oxygen conditions, HbS polymerizes, causing RBCs to become rigid and sickle-shaped. This leads to chronic hemolytic anemia, vaso-occlusive crises, and a markedly shortened RBC lifespan—typically 10 to 30 days instead of the normal 120 days. Additionally, patients with SCD often have variable degrees of anemia and may receive regular blood transfusions that further alter their hemoglobin composition. The chronic inflammation that characterizes SCD also contributes to a higher prevalence of diabetes in this population, making accurate glycemic assessment even more critical.

These fundamental hematologic changes directly interfere with the A1c test in multiple ways, each compounding the difficulty of obtaining a reliable measurement.

Shortened Red Blood Cell Lifespan

Because the A1c value is a time-integrated measure of glucose exposure, it depends on the RBC being present long enough for glucose to accumulate. In SCD, RBCs are destroyed rapidly due to hemolysis. This reduces the duration available for glycation, leading to a lower A1c than would be expected for a given average glucose level. For example, a patient with a true mean glucose of 200 mg/dL might have an A1c of 6.5% instead of the expected 8.5%, falsely suggesting excellent glucose control. This effect is most pronounced in individuals with high hemolytic rates—those with frequent vaso-occlusive crises or severe anemia. The discrepancy is not linear and cannot be corrected with a simple adjustment factor, making it impossible to reliably “reverse-engineer” the true glucose from the measured A1c.

Interference from Hemoglobin Variants

Many A1c assays, particularly older immunoassays and some HPLC methods, cannot reliably distinguish glycated HbS from glycated HbA. As a result, the measured A1c may be grossly inaccurate. In some cases, the presence of HbS can cause a falsely low or falsely high result depending on the assay and the specific hemoglobin variant involved. The National Glycohemoglobin Standardization Program (NGSP) has certified several methods that are less affected by HbS, but not all laboratories use these. Furthermore, patients with SCD may also carry other hemoglobin variants (e.g., HbC, HbF), adding further complexity. Patients with HbSC disease, for instance, face a double challenge: both HbS and HbC interfere with many assays, and the interaction between the two variants can produce unpredictable results.

Increased Red Blood Cell Turnover and Reticulocytosis

Chronic hemolysis in SCD triggers a compensatory increase in RBC production, resulting in a higher proportion of young RBCs (reticulocytes) in circulation. Reticulocytes are present for only 1–2 days before maturing, and they have lower baseline levels of glycated hemoglobin. This skews the A1c downward because young cells have had less time to accumulate glucose. Even if the total RBC lifespan is considered, the mixture of very young cells and older cells complicates the relationship between A1c and average glucose. The reticulocyte count, which can be elevated to 10–30% in SCD, directly correlates with the degree of A1c underestimation. Clinically, a patient with a high reticulocyte count will have an even more falsely reassuring A1c.

The Impact of Blood Transfusions

Many patients with SCD receive regular simple or exchange transfusions to reduce the proportion of HbS and prevent complications. Transfusions introduce donor RBCs with normal HbA and a normal lifespan, which can transiently alter the A1c. If the transfusion is recent, the A1c may reflect the donor cells’ glucose exposure rather than the patient’s own glycemic state. Conversely, in the weeks after transfusion, the patient’s own HbS-containing cells decline, making interpretation even more challenging. A patient who receives a transfusion just before an A1c test may have a result that is heavily influenced by the donor’s glycemic status, which is unrelated to the patient’s own diabetes control. This temporal variability renders a single A1c measurement nearly meaningless in a patient undergoing chronic transfusion therapy.

Iron Overload and Chelation Therapy

An additional but less commonly discussed factor is iron overload from repeated transfusions. Iron overload can affect hemoglobin structure and may alter the glycation process. Some studies have suggested that iron chelation therapy might interfere with certain A1c assays, although data are limited. Patients on chelation with agents such as deferoxamine or deferasirox should be monitored with alternative methods to avoid any potential confounding.

Clinical Consequences of Relying on A1c in SCD

Misleading A1c results in patients with SCD can have serious downstream effects. A falsely low A1c may lead to clinical inertia—failure to intensify glucose-lowering therapy when it is needed—resulting in sustained hyperglycemia and increased risk of microvascular complications (retinopathy, nephropathy, neuropathy). Over the long term, this can accelerate the progression of diabetic kidney disease or worsen retinopathy, both of which are already common in the SCD population due to vaso-occlusive processes. Conversely, a falsely high A1c (though less common in SCD) could prompt aggressive treatment that risks hypoglycemia, particularly in patients also taking insulin or sulfonylureas. Hypoglycemia episodes in patients with SCD can be especially dangerous, potentially triggering vaso-occlusive crises or falls.

Missed or delayed diagnosis of diabetes is another concern. The American Diabetes Association (ADA) recommends A1c ≥6.5% for diagnosis, but in an SCD patient with true prediabetes or diabetes whose A1c is artifactually lowered, diagnoses can be missed entirely. This is especially problematic because SCD itself is associated with a higher risk of developing diabetes, possibly due to chronic inflammation, iron overload from transfusions, or pancreatic damage from vaso-occlusive events. Studies have shown that the prevalence of diabetes in adults with SCD is 15–20% higher than in the general African American population, yet many cases go undiagnosed due to reliance on A1c.

Furthermore, using A1c alone for monitoring can lead to erroneous therapeutic adjustments. For instance, a patient with well-controlled glucose but a low A1c due to hemolysis might have their medication reduced unnecessarily, while another with poor control but a near-normal A1c might not receive appropriate intensification. This inconsistency undermines the goal of personalized diabetes management and can erode patient trust in the monitoring process.

Alternative Monitoring Strategies for Patients with SCD

Given the unreliability of A1c, clinicians must turn to other metrics that are not confounded by RBC lifespan or hemoglobin variants. Fortunately, several validated options exist, and their use is now recommended by professional societies.

Fructosamine and Glycated Albumin

Fructosamine measures glycated total 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 not affected by RBC abnormalities, making it a useful alternative in hemolytic conditions. Glycated albumin (GA) is a more specific measurement of glycated albumin and offers a similar time window. Both can be used to track short-term changes and correlate reasonably well with mean glucose, though they are less standardized than A1c for predicting long-term complications. Reference ranges are available, and serial measurements can trend therapeutic responses. The ADA now includes fructosamine and GA as acceptable alternatives for monitoring in patients with hemoglobinopathies.

One limitation: fructosamine and GA can be influenced by conditions that alter albumin turnover, such as nephrotic syndrome, liver disease, or protein-losing enteropathy, which may coexist in some SCD patients. Still, they are far more reliable than A1c in this population. For clinical practice, a baseline fructosamine or GA should be obtained and trended over time, aiming for consistent improvement or stability.

Continuous Glucose Monitoring (CGM)

CGM devices measure interstitial glucose levels every few minutes, providing a rich dataset of glucose patterns, including time in range (TIR), glycemia variability, and overnight trends. CGM data is completely independent of hemoglobin and RBC kinetics. For patients with SCD, CGM can offer a true picture of glycemic control without the artifacts that plague A1c. Metrics such as TIR (percentage of readings between 70–180 mg/dL) and glucose management indicator (GMI, formerly estimated A1c) derived from CGM can guide therapy adjustments. CGM also provides real-time alerts for hypo- and hyperglycemia, which can be particularly valuable for patients with SCD who may have difficulty recognizing symptoms of deranged glucose levels due to chronic pain or other comorbidities.

Current ADA standards support the use of CGM for diabetes management, and professional CGMs are increasingly used in research and clinical care for SCD patients. While cost and access remain barriers—especially for patients from underserved communities where SCD is most prevalent—the growing availability of personal CGM systems and insurance coverage expansion are promising developments. For patients with SCD, a 10–14 day CGM wear every 3 months, combined with routine SMBG, provides robust data for clinical decision-making.

Self-Monitoring of Blood Glucose (SMBG)

Regular fingerstick blood glucose testing remains essential for day-to-day management and can be used to calculate mean glucose over shorter intervals. However, SMBG alone does not provide a long-term assessment and is subject to patient adherence, device accuracy, and the pain burden of frequent fingersticks—a particular concern in patients who already deal with recurrent venipunctures for transfusions or other procedures. Combining SMBG with periodic fructosamine or CGM yields a more complete picture. For patients using insulin, SMBG is indispensable for dose adjustments and safety.

Glycomark (1,5-Anhydroglucitol)

1,5-AG is a marker of postprandial hyperglycemia and reflects glucose control over the past 1–2 weeks. It is not affected by RBC lifespan, but its utility in SCD has not been extensively studied, and it is less widely available than fructosamine or GA. It may serve as an adjunct in select cases, particularly when postprandial hyperglycemia is suspected despite relatively normal glucose profiles. However, because of limited evidence in the SCD population, it should not be used as a standalone monitoring tool.

Current Guidelines and Research Recommendations

Recognizing these issues, the ADA recommends that A1c should not be used for diabetes diagnosis or monitoring in patients with hemoglobinopathies such as SCD unless a laboratory method verified to be unaffected by the variant is used. The ADA further advises that alternative measures—fructosamine, glycated albumin, or CGM—be employed instead. The National Glycohemoglobin Standardization Program (NGSP) maintains a list of certified assay methods that are less affected by hemoglobin variants; clinicians should query their laboratory to confirm which method is in use and whether it is validated for HbS.

Several studies have documented the inaccuracy of A1c in SCD. A 2017 study in the Journal of Clinical Endocrinology & Metabolism found that A1c underestimated mean glucose by a mean of 1.6% in SCD patients compared to a matched control group. More recent work using CGM has shown that the discrepancy is correlated with hemolytic parameters such as lactate dehydrogenase, bilirubin, and reticulocyte count. One study published in Diabetes Care in 2021 demonstrated that glycated albumin had a stronger correlation with time in range than A1c in patients with SCD.

The Sickle Cell Disease & Diabetes Consortium has called for standardized protocols for monitoring glycemia in SCD patients, including routine use of CGM and glycated albumin. Clinical trials are exploring whether these alternative metrics can predict complications as effectively as A1c does in the general population, with early results suggesting that fructosamine and GA correlate well with the development of microalbuminuria and retinopathy in SCD patients over 5-year follow-up.

Practical Guidance for Clinicians and Patients

  • Avoid relying solely on A1c. For any patient with known SCD, use a method that is not affected by hemoglobin variants or altered RBC lifespan. Check with your laboratory to determine if the assay used is validated in the presence of HbS. If the laboratory cannot confirm, switch to an alternative monitoring method.
  • Use a combination of tools. A reasonable monitoring plan includes: quarterly fructosamine or glycated albumin, intermittent CGM (e.g., 14-day professional CGM every 3–6 months), daily SMBG as needed for insulin dosing, and clinical correlation with blood glucose logs. Document the chosen method and rationale clearly in the medical record.
  • Set individualized targets. Without a reliable A1c, target ranges for TIR and average glucose must be based on age, comorbidities, hypoglycemia risk, and diabetes duration. For most nonpregnant adults with SCD, a TIR >70% (with <1% time below 70 mg/dL and <25% time above 180 mg/dL) is a reasonable goal. For glycated albumin, aim for a level below the upper limit of the assay’s normal range, typically around 15–16% for nondiabetic individuals.
  • Educate patients. Explain why A1c may be misleading in SCD and involve them in choosing monitoring methods. Encourage consistent use of CGM or SMBG and reinforce the importance of keeping glucose logs. Provide written materials that outline the differences between A1c, fructosamine, and CGM.
  • Consider diabetes screening. Because A1c can give false negatives for diagnosis, screen SCD patients for diabetes using fasting plasma glucose, oral glucose tolerance test (OGTT), or fructosamine—especially those with risk factors (obesity, family history of diabetes, iron overload, or history of recurrent pancreatitis). The OGTT should be interpreted with caution, as glucose-induced hemolysis can affect results in some SCD patients; in such cases, fructosamine may be preferable.
  • Collaborate with hematology. Coordinate care with the patient’s hematologist to account for transfusion schedules, hemolytic crises, and iron chelation therapy, all of which can affect glycemic control and monitoring. A multidisciplinary clinic that includes both an endocrinologist and a hematologist is ideal for managing the complex interplay between diabetes and SCD.
  • Document the approach. Clearly document why you are not using A1c and which alternative method is being used. This protects against liability and ensures continuity of care if the patient sees a different provider.

Conclusion

The limitations of A1c in sickle cell disease are well-documented and clinically significant. Shortened RBC lifespan, hemoglobin variants, increased reticulocytosis, and the effects of transfusions render the test unreliable for both diagnosis and ongoing management of diabetes. Clinicians must proactively adopt alternative monitoring strategies—fructosamine, glycated albumin, and continuous glucose monitoring—to ensure accurate assessment and optimal diabetes care. By moving beyond the A1c-centric model, we can better serve patients who live with the dual burdens of sickle cell disease and diabetes, ultimately reducing the risk of complications and improving quality of life. The evidence is clear: for patients with SCD, relying on A1c is no longer acceptable when better options exist.

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