Why Autoimmune Diseases Can Distort Your A1C Test Results

Autoimmune conditions—like rheumatoid arthritis, lupus, type 1 diabetes, and celiac disease—occur when the immune system attacks healthy tissues. These disorders affect virtually every organ system, including how the body handles blood sugar. The A1C test (hemoglobin A1c) is a cornerstone of diabetes diagnosis and management, measuring average blood glucose over the past two to three months. But in people with autoimmune diseases, the test often delivers misleading numbers. An A1C can read falsely high or falsely low because of anemia, inflammation, altered hemoglobin, or medications. Misinterpreting those results can delay diagnosis, prompt unnecessary treatments, or mask dangerous glucose swings. Understanding this interplay is essential for patients, primary care clinicians, rheumatologists, and endocrinologists alike.

The A1C test works by measuring what percentage of hemoglobin—the protein in red blood cells that carries oxygen—has glucose attached to it. Since red blood cells live roughly 120 days, the test reflects average glucose over the prior three months. It’s convenient—no fasting required—and widely used for screening, diagnosis, and monitoring. The American Diabetes Association (ADA) considers an A1C below 5.7% normal, 5.7%–6.4% prediabetes, and 6.5% or higher diabetes. But the test is an indirect measure. Its accuracy depends on normal red blood cell lifespan, normal hemoglobin structure, and no interfering factors. Many conditions—not just autoimmune ones—can distort the result, including kidney failure, hemoglobin variants like sickle cell trait, pregnancy, recent transfusions, and any condition altering red cell turnover.

How Autoimmune Conditions Interfere With A1C

Autoimmune diseases disrupt A1C accuracy through several distinct biological mechanisms. The most common are anemia (with shortened or altered red cell life), hemoglobin modifications, chronic inflammation, and medication side effects. Each can push the result in a different direction, making interpretation a real challenge.

Anemia and Changes in Red Blood Cell Lifespan

Many autoimmune conditions—especially those with systemic inflammation—produce anemia of chronic disease (ACD). Inflammatory cytokines like interleukin‑6 and tumor necrosis factor‑α block iron utilization, suppress erythropoietin, and shorten red cell survival. When red cells die early, they have less time to accumulate glucose, so the A1C reads falsely low. Conversely, iron-deficiency anemia (common in autoimmune gastritis or celiac disease) can raise A1C, possibly because iron-deficient hemoglobin glycates more readily. The net effect depends on the type and severity of anemia, making A1C an unreliable gauge for patients with an active autoimmune disease and any degree of anemia.

Beyond anemia of chronic disease, autoimmune hemolytic anemia (AIHA) causes even more dramatic distortion. In AIHA, antibodies destroy red blood cells rapidly. A patient with normal glucose may have an A1C as low as 3–4%, falsely suggesting chronic hypoglycemia. Meanwhile, a diabetic patient with AIHA could show an A1C of 6.0% when their true average glucose is dangerously high. This is one of the most extreme examples of A1C misinterpretation in autoimmune medicine.

Altered Hemoglobin Structure and Variants

Autoimmune diseases can directly modify hemoglobin. Antibodies attacking red cells may change the hemoglobin molecule’s shape or its susceptibility to glycation. Additionally, many people carry inherited hemoglobin variants such as HbS (sickle cell trait), HbC, or HbE, which are more common in certain ethnic groups. These variants can cause falsely high or low A1C readings depending on the assay method. Because some autoimmune conditions are more prevalent in African‑American, Hispanic, or Mediterranean populations, the combination of a variant and an autoimmune process compounds the risk. Clinicians should always check for hemoglobinopathies when A1C and other glucose measures disagree.

Chronic Inflammation and Glycation Rate

Systemic inflammation—marked by elevated cytokines, oxidative stress, and immune activation—can also change how fast hemoglobin glycates. Some research suggests inflammatory mediators increase glycation independently of glucose, pushing A1C up. Other studies show inflammation shortens red cell lifespan, pushing A1C down. The net effect varies by disease activity. In rheumatoid arthritis patients with high disease activity, for example, A1C often does not correlate well with fasting glucose or continuous glucose monitoring (CGM) readings. This makes the test particularly unreliable during flares.

Medication Effects That Skew Results

Treatments for autoimmune diseases also influence A1C. Glucocorticoids like prednisone are potent insulin resistance inducers and raise blood glucose, increasing A1C over time. But some disease-modifying antirheumatic drugs (DMARDs) such as methotrexate or TNF‑α inhibitors may improve insulin sensitivity and lower glucose. The net change in A1C from therapy can be difficult to separate from the underlying autoimmune effect. Furthermore, medications like dapsone (sometimes used for certain autoimmune conditions) accelerate red cell destruction and can drive A1C artificially low. Clinicians must account for the patient’s full medication list when interpreting A1C.

Specific Autoimmune Conditions and Their Impact on A1C

Rheumatoid Arthritis (RA)

RA is a chronic inflammatory arthritis associated with systemic inflammation and a high rate of anemia. Multiple studies have shown that A1C levels in RA patients often diverge from other glycemic measures. In one study of non‑diabetic RA patients, A1C was significantly higher than predicted from glucometer readings, likely due to inflammation‑driven glycation. In anemic RA patients, A1C can be falsely low. For RA patients with diabetes, relying on A1C alone may lead to overtreatment or undertreatment. Clinicians should consider using fructosamine, glycated albumin, or CGM for these patients.

Systemic Lupus Erythematosus (SLE)

SLE is a prototypic autoimmune disease with multiorgan involvement. Corticosteroid use is common, which raises blood glucose. At the same time, lupus nephritis and chronic anemia complicate A1C interpretation. A 2020 review found that A1C underestimated blood glucose in SLE patients by an average of 0.5% compared to CGM. Antiphospholipid antibodies may also interfere with certain lab assays. Relying solely on A1C for diabetes screening in SLE can miss prediabetes or underestimate glycemic burden during flare-ups.

Type 1 Diabetes (T1D) – The Autoimmune Overlap

Type 1 diabetes is itself an autoimmune condition. Patients with T1D often develop other autoimmune diseases—such as autoimmune thyroiditis, celiac disease, or Addison’s disease—that further disturb A1C measurement. Untreated celiac disease causes malabsorption and iron-deficiency anemia, which can elevate A1C spuriously. Thyroid dysfunction alters red cell turnover and glucose metabolism. In these patients, A1C may not accurately summarize glycemic control, and episodes of hypoglycemia or hyperglycemia can be masked. For people with T1D and celiac disease, correcting the anemia with iron therapy can bring A1C back into alignment with glucose logs.

Celiac Disease

Celiac disease is an autoimmune enteropathy triggered by gluten. It commonly co‑occurs with type 1 diabetes but can also appear alone. Chronic inflammation and villous atrophy lead to iron-deficiency anemia, which can raise A1C. Nutritional deficiencies may also alter hemoglobin glycation kinetics. In children with celiac disease and T1D, A1C levels often appear higher than glucose logs would predict. Clinicians should screen for celiac disease in any T1D patient whose A1C seems out of proportion to fingerstick or CGM data.

Autoimmune Hemolytic Anemia (AIHA)

As mentioned, AIHA causes rapid red blood cell destruction, drastically lowering A1C regardless of actual glucose levels. This effect can be so profound that a patient with diabetes and AIHA may show an A1C in the normal range despite chronic hyperglycemia. In such cases, A1C is essentially useless and should not be ordered. Instead, rely on fasting glucose, OGTT, or CGM for diagnosis and monitoring.

Implications for Diagnosis and Management

Misinterpreting A1C in autoimmune patients carries serious consequences. A falsely low A1C may lead a clinician to underestimate glycemic burden, miss a diabetes diagnosis, or reduce diabetes medications inappropriately, risking hyperglycemic emergencies. A falsely high A1C can prompt overtreatment with insulin or oral agents, increasing the risk of severe hypoglycemia. In screening, an erroneous A1C may incorrectly label someone as having prediabetes, causing unnecessary anxiety and further testing.

In research settings, using A1C as a primary outcome in trials involving autoimmune populations can bias results. For example, a drug that reduces inflammation might lower A1C independently of glucose, making it appear more effective at controlling diabetes than it actually is. Conversely, a drug that causes hemolytic anemia would lower A1C and could mask a detrimental effect on glucose metabolism.

Case in point: A 55‑year‑old woman with seropositive rheumatoid arthritis and no prior history of diabetes presents with an A1C of 6.10. Fasting glucose is 98 mg/dL. Concerned about prediabetes, her doctor initiates metformin. However, her fructosamine level is normal, and a subsequent oral glucose tolerance test shows no impairment. The elevated A1C is likely due to inflammation‑induced glycation. The patient experiences gastrointestinal side effects from metformin unnecessarily. A simple awareness of the autoimmune confound could have prevented this.

Strategies for Accurate Glucose Assessment in Autoimmune Patients

To avoid the pitfalls of A1C misinterpretation, clinicians should adopt a multimodal testing strategy. No single test is perfect, but using complementary measures provides a clearer picture.

Combine Alternative Blood Tests

  • Fasting plasma glucose (FPG) and the oral glucose tolerance test (OGTT) remain valid because they directly measure glucose, not hemoglobin glycation. They are unaffected by red cell lifespan or anemia.
  • Fructosamine measures glycated serum proteins (mainly albumin) and reflects glycemic control over the preceding two to three weeks. Because albumin turnover is independent of red cell lifespan, fructosamine is useful when A1C is unreliable. It is especially helpful in conditions with altered red cell turnover, like hemolytic anemia or recent transfusion.
  • Glycated albumin (GA) is similar to fructosamine but more standardized. Some studies suggest GA correlates better with glycemic status in patients with anemia or kidney disease.
  • Continuous glucose monitoring (CGM) provides real‑time glucose data over 7–14 days. CGM is increasingly affordable and gives a direct view of glucose patterns, including postprandial spikes and hypoglycemia. For patients with complex autoimmune conditions, short-term CGM can effectively validate or replace A1C.

Account for the Patient’s Autoimmune Status

  • Screen for anemia using a complete blood count and iron studies when A1C and other glucose measures disagree.
  • Check for hemoglobin variants with hemoglobin electrophoresis or HPLC if the patient is from a high-risk ethnic group or if A1C is unexpectedly high or low.
  • Measure inflammatory markers like C‑reactive protein (CRP) and erythrocyte sedimentation rate (ESR) to gauge potential confounding from active inflammation.
  • Establish a baseline A1C early in the disease course, along with FPG and perhaps a one‑time OGTT. This baseline helps define the patient’s personal “normal” A1C range.

Recognize When to Avoid A1C Entirely

  • In patients with autoimmune hemolytic anemia, recent blood transfusion, or known hemoglobin variants that affect the assay, avoid A1C altogether. Use FPG, OGTT, or CGM for diagnosis and monitoring.
  • For diabetes monitoring, consider using the “estimated average glucose” (eAG) formula only after verifying A1C validity. Some labs now report whether hemoglobin variants are present.

Monitor Medication Effects Closely

  • When adjusting immunosuppressive or biologic therapy, monitor glucose with CGM or frequent fingersticks for a few weeks after a dose change to see the true effect on glycemia.
  • For patients on high‑dose corticosteroids, recognize that A1C may lag behind acute glucose elevations. Rely more on CGM or fasting glucose during steroid tapers.

The Role of Healthcare Providers and Patient Education

Patient education is critical. People with autoimmune conditions should understand that their A1C result might not tell the whole story. They should keep detailed glucose logs if diabetic, and report any symptoms of anemia (fatigue, pallor, shortness of breath) or signs of a flare (joint pain, rash, fever) to their care team. Healthcare providers must maintain a high index of suspicion. A simple rule: if the A1C does not match the clinical picture—for example, a low A1C in a patient with many hyperglycemic readings, or a high A1C in a patient with consistently normal glucose—investigate further before changing therapy.

Coordination between rheumatologists, endocrinologists, and primary care providers is essential. When a patient with rheumatoid arthritis or lupus develops diabetes, all specialists should be aware of the potential confounders. Similarly, primary care providers overseeing diabetes management in a patient with an autoimmune disease should routinely order a complete blood count and consider inflammatory markers when interpreting A1C. Multidisciplinary communication prevents missteps.

For additional resources, see the 2020 review on A1C limitations in autoimmune disease, the ADA’s position on hemoglobin variants and A1C, and the Arthritis Foundation’s guide to diabetes in arthritis patients. Also consider the American Diabetes Association’s advice on A1C accuracy for a broader overview of factors that can interfere.

Conclusion

Autoimmune conditions present unique and often overlooked challenges for A1C interpretation. Anemia, altered red blood cell lifespan, hemoglobin variants, chronic inflammation, and medications can all distort the relationship between A1C and true glycemic status. These distortions can lead to misdiagnosis, inappropriate treatment adjustments, and inaccurate monitoring. By understanding the mechanisms at play and employing complementary testing strategies such as fructosamine, continuous glucose monitoring, and traditional glucose measurements, clinicians can overcome the limitations of A1C in this special population. As autoimmune diseases become more common and the diabetes epidemic continues, the need for a nuanced, multi-test approach has never been greater. Patients and providers alike must remain vigilant, questioning any A1C result that doesn’t fit the clinical picture and using alternative tools to guide care.