The A1C test is accurate for most people, but it can be meaningfully off in certain situations. For the average person with no blood disorders or chronic illness, the test reliably reflects blood sugar control over the previous two to three months. Modern lab instruments perform well, with between-laboratory variation consistently below 2%. The catch is that A1C doesn’t directly measure blood sugar. It measures how much sugar has attached to your red blood cells, and anything that changes those blood cells can throw the number off.
How the Test Actually Works
When glucose circulates in your blood, some of it sticks to hemoglobin, the protein inside red blood cells that carries oxygen. The more glucose in your blood over time, the more hemoglobin gets coated. Since red blood cells live about 120 days, the A1C result captures a rolling average of your blood sugar over roughly two to three months.
The standard formula for converting A1C into an estimated average glucose is: average glucose (mg/dL) = 28.7 × A1C − 46.7. So an A1C of 7% translates to an average glucose of about 154 mg/dL. But “average” comes with a wide band. At an A1C of 7%, the 95% confidence interval spans from 123 to 185 mg/dL. Two people with the same A1C can have meaningfully different day-to-day glucose patterns.
Lab Accuracy Is Tighter Than It Used to Be
Certified labs must produce results where 36 out of 40 test samples fall within 5% of the reference value. In practical terms, if your true A1C is 7.0%, the lab might report anything from about 6.65% to 7.35%. Between-laboratory variation has dropped below 1.6% in recent years, which is a significant improvement over earlier decades. For routine monitoring, this level of precision is enough to track trends over time, though a small change between two visits (say, 6.8% to 7.1%) could reflect lab variation rather than a real shift in blood sugar control.
Red Blood Cell Lifespan Is the Biggest Variable
Anything that shortens or lengthens how long your red blood cells survive will push your A1C in a misleading direction. Shorter-lived red blood cells spend less time in the bloodstream collecting glucose, so your A1C reads lower than your actual average sugar. Longer-lived cells accumulate more glucose, pushing the number higher.
Conditions that shorten red blood cell lifespan and falsely lower A1C include:
- Hemolytic anemias, where red blood cells break down prematurely
- Chronic kidney disease, especially advanced stages
- Recent significant blood loss or blood transfusions
- Spleen enlargement, which filters out red blood cells faster
Iron deficiency anemia works in the opposite direction. When your body can’t produce enough new red blood cells, the existing ones circulate longer and accumulate more glucose, making your A1C read higher than expected. Untreated iron deficiency can raise A1C by 0.1 to 0.2 percentage points independent of any change in blood sugar.
Kidney Disease Makes A1C Unreliable
Advanced kidney disease creates a perfect storm of factors that distort A1C. Anemia is common because the kidneys produce less of the hormone that stimulates red blood cell production. Circulating toxins shorten red blood cell lifespan. And when anemia is treated with medications that boost red blood cell production, the flood of young cells further skews the reading downward.
Research quantifies just how much accuracy drops. For people with moderately reduced kidney function (filtration rate of 30 to 44 mL/min), the correlation between average glucose and A1C remains strong at 0.91. But once kidney function falls below 30 mL/min, that correlation drops to 0.61, meaning A1C explains only about a third of the variation in actual glucose levels. Clinical guidelines explicitly flag A1C as unreliable for people with advanced kidney disease or those on dialysis.
Hemoglobin Variants Affect Some Test Methods
Millions of people carry genetic hemoglobin variants, most commonly sickle cell trait (HbS), HbC, HbE, and HbD. Whether these variants interfere with your A1C result depends on which lab method is used.
Immunoassay methods, used in many point-of-care devices and some hospital labs, tend to falsely raise A1C in people with sickle cell trait or HbC. Ion-exchange chromatography, another common method, can be thrown off by HbE or HbD depending on the specific instrument. One testing approach, boronate affinity chromatography, is generally unaffected by any of the major hemoglobin variants because it separates sugar-coated hemoglobin from uncoated hemoglobin regardless of the hemoglobin type.
Sickle cell trait is present in about 7.3% of African Americans compared to 0.3% of White Americans, which contributes to population-level differences in A1C accuracy. But the issue goes beyond hemoglobin variants alone.
Race and Ethnicity Create a Measurable Gap
At the same average blood sugar level, African American individuals have A1C values roughly 0.33 percentage points higher than White individuals. That gap persists even after excluding people with hemoglobin variants, anemia, kidney problems, and other known confounders. The biological explanation isn’t fully understood, but it likely involves differences in the rate at which glucose attaches to hemoglobin or subtle variations in red blood cell characteristics.
A third of a percentage point might sound small, but it can matter at diagnostic thresholds. An A1C of 6.5% is the cutoff for a diabetes diagnosis. If your biology naturally runs 0.3 points higher at the same glucose level, you could be diagnosed with diabetes or placed in a higher risk category based on a number that doesn’t accurately reflect your blood sugar.
Pregnancy Shifts A1C in Both Directions
During pregnancy, A1C becomes a moving target. In the first and second trimesters, the body ramps up red blood cell production, flooding the bloodstream with young cells that haven’t had time to accumulate much glucose. At the same time, the growing placenta consumes more glucose, pulling sugar out of the mother’s blood. Both factors push A1C lower than expected.
By the third trimester, the pattern reverses. Hemoglobin levels drop (a normal pregnancy change), and the remaining red blood cells end up with relatively more glucose attached. Insulin resistance climbs as well, with peripheral tissue sensitivity falling by about 50% by late pregnancy. The net result is an A1C that trends upward in the final months, partly reflecting real metabolic changes and partly reflecting shifts in red blood cell dynamics that have nothing to do with glucose control.
How CGM Compares to A1C
Continuous glucose monitors measure actual glucose levels every few minutes, and they calculate a metric called the Glucose Management Indicator (GMI) that’s meant to parallel A1C. Comparing the two reveals how much individual biology affects the A1C number.
In one study of 26 patients, the lab A1C was on average 0.34 percentage points higher than the GMI derived from continuous glucose data. Only about 4% of patients had a discrepancy smaller than 0.1 points. Half had a gap between 0.5 and 1.0 points, and nearly 1 in 5 had a discrepancy greater than a full percentage point. That means if your CGM data suggests an estimated A1C of 7.0%, your lab result might come back at 8.0% or higher.
This doesn’t necessarily mean the CGM is right and the lab is wrong, or vice versa. They’re measuring different things. But the gap highlights that A1C is a proxy for average glucose, not a direct measurement, and the translation between the two varies from person to person.
Alternatives When A1C Isn’t Trustworthy
When conditions make A1C unreliable, two alternative blood tests can step in. Fructosamine measures how much sugar has attached to proteins in your blood (mainly albumin) rather than to hemoglobin. Because these proteins turn over in two to three weeks, fructosamine reflects a shorter window of glucose control and is completely unaffected by red blood cell disorders, hemoglobin variants, or anemia from iron, B12, or folate deficiency.
Glycated albumin works on a similar principle and has proven particularly useful for people with chronic kidney disease, including those on dialysis. Both tests are also valuable during pregnancy or after a recent medication change, when you need to assess glucose control over weeks rather than months. These tests aren’t routinely ordered for everyone, but they fill an important gap for the significant number of people whose A1C doesn’t tell the whole story.