What Is Glycated Hemoglobin (HbA1c) and Why It Matters

Glycated hemoglobin, or HbA1c, is a blood test that reflects a person’s average blood glucose levels over the preceding 8 to 12 weeks. The test measures the percentage of hemoglobin that has glucose molecules permanently attached to it—a process called non-enzymatic glycation. Because red blood cells live about 120 days, HbA1c provides a reliable snapshot of long-term glycemic control, which is essential for managing diabetes and assessing the risk of complications such as retinopathy, nephropathy, neuropathy, and cardiovascular disease.

The discovery of HbA1c’s relationship to glucose control in the 1970s revolutionized diabetes management. Large landmark trials such as the Diabetes Control and Complications Trial (DCCT) and the United Kingdom Prospective Diabetes Study (UKPDS) demonstrated that lower HbA1c levels are associated with reduced risk of microvascular and macrovascular complications. The test is now a cornerstone of clinical guidelines worldwide, used both for diagnosing diabetes (with a threshold ≥6.5% or 48 mmol/mol) and for monitoring treatment effectiveness in people with established diabetes. Its convenience—no fasting required, single blood draw, and stability over weeks—has made it the default glycemic metric in primary care, endocrinology, and public health programs.

However, its accuracy and interpretation are not uniform across all populations. Clinicians must understand its limitations to avoid misdiagnosis and inappropriate treatment, especially as global populations become more diverse and diabetes prevalence rises sharply in low- and middle-income countries.

How HbA1c Is Measured and Interpreted

The test works by separating hemoglobin species based on charge or structure, using methods such as high-performance liquid chromatography (HPLC), immunoassay, or capillary electrophoresis. Glucose binds non-enzymatically to the N-terminal valine of the beta chain of hemoglobin, forming a stable ketoamine. The rate of this reaction depends on the ambient glucose concentration over the life of the red blood cell. Modern assays are standardized to the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) reference method, but results are reported as both percentage (DCCT-aligned) and mmol/mol.

Standard reference ranges: A normal HbA1c is below 5.7% (39 mmol/mol). Prediabetes ranges from 5.7% to 6.4% (39–46 mmol/mol). Diabetes is diagnosed at ≥6.5% (48 mmol/mol). For people with diabetes, the general target is <7.0% (53 mmol/mol), though individual targets may vary based on age, life expectancy, comorbidities, and hypoglycemia risk. The American Diabetes Association (ADA) and other organizations stress that targets should be personalized—for example, a less stringent target of <8.0% may be appropriate for older adults with multiple chronic conditions or limited life expectancy.

Importantly, the HbA1c test assumes a normal red blood cell lifespan of about 120 days. Any condition that shortens or lengthens red blood cell survival can produce spurious results, independent of glucose levels.

Factors That Influence HbA1c Beyond Blood Glucose

HbA1c is a powerful tool, but numerous non-glycemic factors can alter its value, leading to discordance between HbA1c and actual average glucose levels. These factors are especially relevant in diverse populations where genetic, physiological, and pathological variations are common. Recognizing these confounders is critical to avoid diagnostic errors and mistreatment.

Ethnicity and Race

Multiple large-scale studies have demonstrated that HbA1c levels differ by ethnicity, independent of fasting glucose or oral glucose tolerance test (OGTT) results. For example, African Americans tend to have HbA1c levels that are 0.2–0.4% higher than non-Hispanic whites at the same blood glucose concentrations. Similar disparities have been found in Hispanic, South Asian, East Asian (e.g., Japanese, Chinese, Korean), and Pacific Islander populations. The reasons are not fully understood but may include genetic differences in hemoglobin glycation rates, red blood cell survival, iron metabolism, or the prevalence of hemoglobin variants.

Clinicians relying solely on HbA1c for diabetes diagnosis may overdiagnose diabetes in some ethnic groups (e.g., African Americans) and underdiagnose in others (e.g., Asians, Hispanics) when compared to OGTT. A 2022 analysis from the Diabetes Care journal highlighted that using an HbA1c threshold of 6.5% misses a significant proportion of diabetes cases among Asian and Hispanic individuals. For instance, among Chinese Americans, the optimal HbA1c cutoff for diagnosing diabetes may be as low as 6.0% based on OGTT correlations. The ADA now acknowledges that HbA1c may require adjustment in African American and Hispanic populations, and some guidelines recommend using alternative diagnostic tests in these groups when the clinical picture is ambiguous.

Age

HbA1c naturally increases with age, even in people without diabetes. For each decade after age 30, HbA1c rises by approximately 0.1–0.15%. This may reflect age-related changes in glucose metabolism (e.g., reduced insulin secretion and action), red blood cell lifespan (slightly longer in older adults), or glycation efficiency. For older adults, especially those with frailty or limited life expectancy, using age-specific reference ranges may prevent overtreatment and hypoglycemia. Some guidelines suggest that an HbA1c < 7.5% (58 mmol/mol) may be acceptable in patients over 75 with comorbidities, while others recommend individualizing targets based on functional status and risk of falls. The UK National Health Service also notes that age-specific interpretation is increasingly advocated, though not yet universally adopted.

Hemoglobin Variants and Hemoglobinopathies

Individuals with hemoglobin variants (e.g., HbS, HbC, HbE, HbD, HbF, HbH) can have inaccurate HbA1c readings depending on the assay method used. Some HPLC methods can detect common variants and report an interference flag, but ion-exchange methods may produce erroneous results. Point-of-care devices are particularly susceptible to interference. For example, patients with sickle cell trait (HbAS) or sickle cell disease (HbSS) have reduced red blood cell lifespan (60–90 days instead of 120), which lowers HbA1c regardless of true glucose control. Similarly, beta-thalassemia trait often leads to falsely low HbA1c due to shortened erythrocyte survival and microcytosis. Conversely, hemoglobin F (fetal hemoglobin) can interfere with some assays, causing falsely low or high results depending on the method.

The Centers for Disease Control and Prevention (CDC) recommends that clinicians be aware of hemoglobin variant prevalence in their patient population and consider alternative methods when interference is suspected. High-performance liquid chromatography with a dedicated hemoglobin variant program can help detect these issues. In regions with high prevalence of hemoglobinopathies (e.g., sub-Saharan Africa, Mediterranean, Southeast Asia), laboratories should use assays that are validated for common variants or rely on glycated albumin or fructosamine for monitoring.

Medical Conditions Affecting Red Blood Cell Turnover

Any condition that alters the lifespan of red blood cells will affect HbA1c. This is a critical point often overlooked in routine clinical practice.

  • Anemia – Iron-deficiency anemia can increase HbA1c because it prolongs red blood cell survival (the cells are smaller and survive longer). Conversely, hemolytic anemias (e.g., autoimmune hemolytic anemia, hereditary spherocytosis, hemoglobinopathies) reduce lifespan and decrease HbA1c, leading to underestimation of glycemia. In populations with high rates of iron deficiency (e.g., in low-resource settings), using HbA1c alone may be misleading.
  • Kidney disease – Chronic kidney disease (CKD) is associated with anemia, uremia-induced carbamylated hemoglobin, and altered erythrocyte life. HbA1c may underestimate glycemia in advanced CKD (stages 4–5). The National Kidney Foundation advises using glycated albumin or fructosamine in patients with eGFR <30 mL/min/1.73m². Additionally, erythropoiesis-stimulating agents (ESAs) used to treat anemia of CKD introduce young red cells with less glycation, lowering HbA1c further.
  • Liver disease – Cirrhosis and other chronic liver conditions can affect red blood cell lifespan (often shortened due to hypersplenism) and alter hemoglobin glycation. Additionally, liver disease may affect the metabolism of fructosamine and other glycated proteins.
  • Blood transfusions and erythropoietin treatment – Recent transfusions introduce donor red cells that may have different glycation status; erythropoiesis-stimulating agents increase production of young erythrocytes, lowering HbA1c. The effect can persist for weeks.
  • Splenectomy – Increased red blood cell lifespan after splenectomy can elevate HbA1c.

Pregnancy

During normal pregnancy, red blood cell mass expands and iron demands increase, leading to dilutional and iron-deficiency anemia. HbA1c typically falls in the second and third trimesters compared to non-pregnant levels. For gestational diabetes screening, oral glucose tolerance tests remain the preferred method, as HbA1c thresholds are not well validated in pregnancy. Some studies suggest that an HbA1c ≥6.5% in early pregnancy may indicate overt diabetes, but this approach still requires caution.

Medications and Supplements

Certain drugs can affect HbA1c independently of glucose. High-dose salicylates, antiretroviral medications (e.g., some protease inhibitors), and ribavirin can interfere with assays. Also, drugs that affect red blood cell survival (e.g., dapsone, which causes hemolysis) can lower HbA1c. Iron supplements (in iron-deficiency anemia) may cause a transient rise in HbA1c as red blood cell turnover normalizes. Clinicians should always consider medication history when interpreting HbA1c.

Limitations of HbA1c Beyond Population Factors

Even in healthy individuals with no confounding conditions, HbA1c has inherent limitations that can lead to mismanagement if not recognized.

Glycation Gap

Some individuals have a systematic difference between HbA1c and measured average glucose—known as the “glycation gap.” This may stem from genetic differences in the rate of hemoglobin glycation (the "glycation index") or in the deglycation process mediated by intracellular deglycating enzymes like fructosamine-3-kinase. People with a high glycation gap may have HbA1c that overestimates their true average glucose, potentially leading to unnecessary intensification of therapy, increased hypoglycemia risk, and weight gain. Conversely, a low glycation gap may lead to underdiagnosis of poor control. The glycation gap is more common in certain ethnic groups and in patients with specific genetic variants.

Inability to Capture Glucose Variability

HbA1c is an average and gives no information about day-to-day fluctuations or hypoglycemic events. Two patients can have the same HbA1c of 7.0%: one may have stable glucose between 120–160 mg/dL, while the other swings from 50 to 300 mg/dL. The latter is at much higher risk for severe hypoglycemia, diabetic ketoacidosis, and long-term complications. Continuous glucose monitoring (CGM) and time-in-range (TIR) metrics provide complementary data that are essential for personalized management. The ADA and International Consensus on Time in Range recommend TIR >70% (glucose 70–180 mg/dL) for many patients, and TIR can be used in place of HbA1c for predictive modeling when CGM is available.

Short-Term Changes Missed

HbA1c does not reflect recent changes in glucose control (e.g., after medication adjustment, dietary changes, or acute illness). A lag of 4–6 weeks is typical before changes appear. For rapid assessment of response to therapy, especially in gestational diabetes or during hospitalization, fructosamine (glycated albumin, reflecting 2–3 weeks) or 1,5-anhydroglucitol (reflecting 1–2 weeks, primarily postprandial hyperglycemia) may be used. In some cases, self-monitoring of blood glucose (SMBG) data can fill the gap, but accuracy and patient adherence are concerns.

Accuracy at the Diagnostic Threshold

Near the diagnostic cutoff of 6.5%, HbA1c has modest sensitivity (around 50–60%) compared to OGTT. This means that many individuals with diabetes defined by OGTT will be missed by HbA1c alone, especially in populations with lower glycation rates. Conversely, specificity is high (>95%). Therefore, a single HbA1c ≥6.5% is confirmatory, but a level between 5.7% and 6.4% should prompt further testing (OGTT or repeat HbA1c) if diabetes is suspected. The World Health Organization (WHO) recommends using HbA1c for diagnosis only in settings where standardized assays are available and in populations where confounding factors are minimal.

Alternative and Complementary Tests for Diverse Populations

When HbA1c is unreliable due to the factors above, clinicians should consider the following alternative or complementary measures:

  • Fructosamine – Measures glycated serum proteins, primarily albumin, reflecting glycemia over 2–3 weeks. Not affected by hemoglobin abnormalities, but can be altered by low albumin (nephrotic syndrome, liver disease, malnutrition) or thyroid disorders (hyperthyroidism lowers fructosamine). It is less standardized than HbA1c and has larger intra-individual variability, but it is useful in pregnancy and hemoglobinopathies.
  • Glycated Albumin (GA) – More specific than fructosamine and less affected by albumin levels. GA is expressed as a percentage of total albumin. Studies show good correlation with average glucose and has been validated in dialysis patients and those with anemia. However, it is still not universally available.
  • 1,5-Anhydroglucitol (GlycoMark) – Reflects postprandial hyperglycemia over 1–2 weeks and is independent of hemoglobin. It decreases in the presence of hyperglycemia because glucose competes for renal reabsorption. Especially useful for detecting postprandial excursions that HbA1c may miss. However, it is influenced by renal function and pregnancy.
  • Continuous Glucose Monitoring (CGM) – Provides real-time glucose readings and metrics like time-in-range (TIR), glucose management indicator (GMI), coefficient of variation (CV) for variability, and hypoglycemia frequency. CGM is increasingly recommended for all patients with diabetes on intensive insulin therapy and can help resolve discrepancies between HbA1c and self-monitored blood glucose. The GMI estimates average glucose from CGM and is less affected by confounders than HbA1c.
  • Oral Glucose Tolerance Test (OGTT) – Remains the gold standard for diagnosing diabetes in pregnancy (gestational diabetes) and in individuals with hemoglobin variants or other confounders. It provides a direct measure of glucose disposal after a glucose load and can detect impaired glucose tolerance (IGT) and impaired fasting glucose (IFG). However, it is more burdensome and less reproducible than HbA1c.

In many clinical scenarios, a combination of HbA1c with CGM or fructosamine can give a more complete picture. For example, if a patient with sickle cell trait has an HbA1c of 5.8% but their glucose log shows consistent values >200 mg/dL, glycated albumin or CGM should be used.

Practical Recommendations for Clinicians

To use HbA1c effectively across diverse populations, healthcare providers should follow these evidence-based steps:

  1. Know the population served. Understand local prevalence of hemoglobin variants, ethnic backgrounds, and common conditions like anemia, thalassemia, or renal disease. In sub-Saharan Africa, for instance, up to 30% of individuals may carry a hemoglobin variant. Review institutional lab methods and inquire about interference flags.
  2. Use ethnicity-specific reference ranges when available. Some laboratories and guidelines now provide adjusted thresholds for certain ethnic groups. The American Diabetes Association acknowledges that HbA1c may require adjustment in African American and Hispanic populations, and the National Institute for Health and Care Excellence (NICE) in the UK suggests caution in interpreting HbA1c in people of African, Mediterranean, or Southeast Asian origin.
  3. Confirm discordant results. If HbA1c is inconsistent with SMBG, CGM, or clinical picture (e.g., symptoms of hyperglycemia or hypoglycemia), check for interfering conditions (e.g., order a hemoglobin variant screen, complete blood count, ferritin, renal function) and confirm with alternative tests (fructosamine, glycated albumin, OGTT, or CGM).
  4. Monitor trends, not absolute values. In individuals with consistent confounders (e.g., chronic hemolysis), serial HbA1c trends can still be useful for assessing direction of control—if the confounder is stable, a rising HbA1c likely indicates worsening glycemia.
  5. Document confounders. When reporting HbA1c in the medical record, note known factors that may affect interpretation (e.g., “HbA1c may be falsely low due to sickle cell trait”; “HbA1c measured HPLC method – no interference detected”). This aids communication among providers.
  6. Educate patients. Explain why HbA1c may not be accurate for them and the rationale for alternative tests. Patients who understand the limitations are more likely to adhere to monitoring plans.

Ongoing Research and Future Directions

Efforts to improve glycemic assessment in diverse populations are ongoing. Researchers are developing non-enzymatic glycation models that account for individual differences in hemoglobin glycation rates. They are also exploring genetic determinants of glycation, such as polymorphisms in the hemoglobin gene and in enzymes like fructosamine-3-kinase. Larger genome-wide association studies (GWAS) are linking specific loci to HbA1c levels independent of glucose, which may eventually allow personalized HbA1c adjustments.

Harmonization of HbA1c assays across methods and laboratories continues to improve, but point-of-care HbA1c testing is expanding rapidly. While convenient, its accuracy in diverse populations requires ongoing validation, especially in low-resource settings. Some point-of-care devices are not reliable in the presence of hemoglobin variants. The National Glycohemoglobin Standardization Program (NGSP) maintains a list of certified methods and interference data.

Additionally, the use of CGM-derived glucose management indicator (GMI) is being studied as an alternative that adjusts for individual differences in glycation. GMI is calculated from average CGM glucose and provides an estimated HbA1c that often differs from lab-measured HbA1c by 0.3–0.5% in either direction. In the future, CGM-based metrics may replace HbA1c for some patients, but cost and access remain barriers.

Large-scale studies such as the ACCORD, ADVANCE, and VADT trials have provided rich data on the relationship between HbA1c and outcomes across ethnic groups, informing updated clinical guidelines. The International Diabetes Federation (IDF) also recommends considering ethnicity and comorbidities when setting HbA1c targets. As precision medicine advances, glycemic monitoring will become more individualized, reducing the risk of misclassification and improving outcomes for all.

Conclusion

HbA1c is an indispensable tool in diabetes care, but it is not infallible. Its interpretation must account for the diverse patient populations, underlying medical conditions, and intrinsic limitations of the test itself. By understanding how ethnicity, age, hemoglobinopathies, anemia, kidney disease, pregnancy, medications, and individual glycation differences influence HbA1c, clinicians can avoid diagnostic errors and tailor treatment plans more effectively. Combining HbA1c with complementary measures—such as fructosamine, glycated albumin, CGM, or OGTT—provides a more complete picture of glycemic status and improves outcomes for all individuals with diabetes. Moving forward, the integration of continuous monitoring and genetic insights will help ensure that no patient is misjudged by a test that, while powerful, does not see the whole story.