Why Sickle Cell Trait Can Make an A1C Test Read Low

About 1 in 13 African Americans carries sickle cell trait, and about 8 percent of African American babies are born with it, according to the National Institute of Diabetes and Digestive and Kidney Diseases. The A1C test is the most common way diabetes and prediabetes get spotted in the United States. Those two facts collide in a specific, measurable way, and the American Diabetes Association Standards of Care addresses it directly in Recommendation 2.4. This page explains what the collision looks like on a lab report.

Does sickle cell trait change an A1C result?

Yes, downward, by a measured amount. Lacy and colleagues reported in JAMA, pooling the CARDIA and Jackson Heart cohorts, that among 4,620 African American adults at the same fasting glucose, HbA1c was 5.72 percent in those with sickle cell trait and 6.01 percent in those without. The difference was 0.29 points, with a 95 percent confidence interval of minus 0.35 to minus 0.23.

A third of a percentage point sounds like rounding. Look at what it did to the labels. In the same analysis, prediabetes was classified in 29.2 percent of participants with the trait and 48.6 percent of those without.

That second figure is the one that matters. Same people, same blood sugar, and a classification that appears or does not appear depending on which test the lab ran. Nothing about anyone’s metabolism changed between those two numbers. Only the instrument did.

Is sickle cell trait the same as sickle cell disease?

No, and the distinction is not a technicality. Sickle cell trait means carrying one copy of the sickle hemoglobin gene. Sickle cell disease means carrying two, and it is a serious inherited condition with its own care requirements. Most people with the trait live without symptoms from it and many never learn they have it.

That last part is why this page exists. A person with sickle cell disease knows. A person with the trait frequently does not, unless it turned up in newborn screening records their family kept, in a blood donation, or in testing done before a pregnancy.

The NIDDK figures put the scale of it plainly: about 1 in 13 African Americans has sickle cell trait, and about 8 percent of African American babies are born with it. That is not a rare variant. It is a common one interacting with a routine test.

Why do some sources say A1C reads high in Black adults?

Because that is also documented, and both things are true at once. The ADA Standards of Care reports that Black individuals average an A1C about 0.3 points higher than non-Hispanic White or Hispanic people at the same glucose level. That pushes in the opposite direction to sickle cell trait.

A third mechanism sits in the same section. The G6PD G202A variant, carried by 11 percent of Black Americans, lowers A1C by about 0.8 points in homozygous men and about 0.7 points in women. One documented effect raises the number, two lower it, and none can be read off a person’s appearance.

This is why any flat statement of the form “A1C reads high in Black people” or “A1C reads low in Black people” is wrong as written. Both sentences describe something real and neither describes an individual. Flattening the picture into one direction is how a useful caution becomes a new bad assumption, the same failure described in the guide on pulse oximeter accuracy on dark skin.

What does the diabetes guideline actually say?

It resolves the mess with one instruction. The ADA Standards of Care, in Recommendation 2.4, states that where hemoglobin variants are present, plasma glucose criteria should be used rather than A1C to diagnose diabetes. Not an adjusted A1C, not a mental correction, not a race-based offset. A different test.

That is a clean answer to a genuinely messy problem, and it is worth appreciating why it works. Plasma glucose measures sugar in the blood at a moment in time. A1C is an inference drawn from hemoglobin, so anything that changes hemoglobin or how long red cells survive changes the inference. Swap the test and the interference disappears.

One more piece of the same guideline belongs here. The ADA is explicit that without unequivocal hyperglycemia, diagnosis requires two abnormal results. A single A1C is not a diagnosis. That applies to everyone, and it is a reasonable thing to remember before any one number changes how you feel about your health.

Which test thresholds do not depend on hemoglobin?

Two of the three. The ADA sets numeric cut points for prediabetes and diabetes on A1C, on fasting plasma glucose and on the 2-hour oral glucose tolerance test. Only the first is calculated from hemoglobin. The other two measure glucose itself, which is why the guideline can send clinicians to them when a variant is in play.

The ADA also lists a random glucose of 200 mg/dL or higher, with classic symptoms, as a diagnostic criterion. Reading a threshold table is not the same as interpreting your own result, and this page cannot do the second thing. How every figure here was traced back to its primary document is described in how we research.

When does screening for diabetes start?

At 35 for most adults who qualify. The US Preventive Services Task Force gives a Grade B recommendation to screening adults aged 35 to 70 who have overweight or obesity, defined as a BMI of 25 or above, at intervals of about every three years. Grade B means the net benefit is moderate to substantial.

The Task Force adds a line that is easy to skip. It says to consider screening at an earlier age if the patient is from a population with a disproportionately high prevalence of diabetes, and it names Black adults among those populations. So 35 is a floor for many people rather than a starting gun.

The background numbers explain the caution. The CDC’s National Diabetes Statistics Report puts total diabetes at 17.4 percent among non-Hispanic Black adults against 13.6 percent of White adults, and undiagnosed diabetes at 4.7 percent against 2.7 percent. Prediabetes, by contrast, the CDC calls similar across racial and ethnic groups. The gap sits in diagnosis and progression, covered in type 2 diabetes in Black women.

What if you do not know your sickle cell trait status?

Ask, rather than assume. Here is the position this page takes and will defend: if you are African American and an A1C has been used to screen you, whether you carry sickle cell trait is a reasonable thing to raise with your clinician, because the guideline itself makes trait status change which test is appropriate.

Phrase it as a question about the record and the method, not as a demand for a test or an attempt to reinterpret a past number. Something like: is my sickle cell trait status known, and if it is not, does that change which test should be used to screen me? That is a question your clinician is equipped to answer and you are not, because it depends on your history, your other results and what is already in your chart.

What this does not license is deciding that a past normal A1C was secretly abnormal. It was one measurement using one method. The point of Recommendation 2.4 is that the choice of method belongs in a clinical conversation, alongside the rest of your risk picture, including blood pressure, which is why high blood pressure in Black women comes up in the same appointment.

Common questions about A1C and sickle cell trait

Does sickle cell trait make A1C read high or low?

Low. In the JAMA analysis of 4,620 African American adults at the same fasting glucose, HbA1c averaged 5.72 percent with the trait and 6.01 percent without, a difference of 0.29 points.

Could a normal A1C have missed something because of the trait?

The mechanism exists, but no page can tell you what your result meant. The ADA directs clinicians to plasma glucose criteria when hemoglobin variants are present, which is the question to raise at a visit.

Is sickle cell trait a disease?

No. Trait means one copy of the sickle hemoglobin gene; sickle cell disease means two and is a serious inherited condition. About 1 in 13 African Americans carries the trait, and most have no symptoms from it.

Which tests are not affected by hemoglobin variants?

Fasting plasma glucose and the 2-hour oral glucose tolerance test both measure glucose directly, so hemoglobin does not enter the calculation. A1C is inferred from hemoglobin, which is where the interference occurs.

When should screening for type 2 diabetes begin?

The USPSTF recommends screening adults aged 35 to 70 with a BMI of 25 or above, roughly every three years, at Grade B. It also says to consider earlier screening for populations with disproportionately high prevalence, naming Black adults.

Sources

• Lacy et al., JAMA, association of sickle cell trait with hemoglobin A1c in African Americans (CARDIA and Jackson Heart). Data 2000 to 2013. Checked when this page was last reviewed.
• NIDDK, Sickle Cell Trait and Other Hemoglobinopathies and Diabetes. Reviewed 2020. Checked when this page was last reviewed.
• American Diabetes Association, Standards of Care 2026, section 2, Tables 2.1 and 2.2 and Recommendation 2.4. Current. Checked when this page was last reviewed.
• US Preventive Services Task Force, Prediabetes and Type 2 Diabetes: Screening. 2021. Checked when this page was last reviewed.
• CDC National Diabetes Statistics Report, Tables 1a and 4, diagnosed, undiagnosed and prediabetes prevalence by race. NHANES 2017 to March 2020. Checked when this page was last reviewed.