How Does Anemia Affect Blood Glucose Levels?

Anemia does not directly raise or lower the sugar circulating in your blood, but it profoundly distorts the tests clinicians use to measure blood glucose, sometimes enough to trigger a wrong diabetes diagnosis or lead to dangerous medication changes. The most commonly affected test is HbA1c, the gold-standard marker used to diagnose and manage diabetes. Depending on the type of anemia you have, your HbA1c reading can be pushed artificially high or artificially low, even when your actual blood sugar hasn’t changed at all. Fingerstick glucose meters can be thrown off too, though by a different mechanism.

Why Anemia Messes With HbA1c

HbA1c measures how much sugar has attached itself to hemoglobin inside your red blood cells over the previous two to three months. The test works because red blood cells live about 120 days, so they accumulate glucose steadily over their lifespan. Anything that changes how long red blood cells survive, how many of them are circulating, or how hemoglobin itself is structured will shift the HbA1c result without your actual blood sugar changing at all.

Research has confirmed that variation in red blood cell lifespan is one of the main causes of HbA1c variation that has nothing to do with blood glucose. One study of people with type 2 diabetes found a significant negative correlation between red blood cell lifespan and HbA1c discrepancy: the shorter the cells lived, the more the HbA1c reading drifted from what blood sugar levels would predict.¹ When red blood cell lifespan dropped below about 74 days, the cells’ shortened life accounted for roughly 14% of the HbA1c test value, a clinically meaningful distortion.²

Iron Deficiency Anemia Pushes HbA1c Up

Iron deficiency is the most common form of anemia worldwide, and it tends to make HbA1c read falsely high. The reason is twofold. First, when you’re low on iron, your body produces fewer new red blood cells, so the cells already in circulation are older on average. Older cells have had more time to accumulate glucose on their hemoglobin, inflating the reading. Second, iron deficiency changes the structure of hemoglobin itself and promotes a chemical process that speeds up glycation, the sugar-binding reaction the test measures.³

The practical fallout can be significant. In one study of people with iron deficiency anemia and no diabetes, iron supplementation alone dropped their HbA1c from about 5.5% to about 5.0%. Before treatment, 70% of the subjects whose HbA1c had put them in the prediabetes range returned to normal glucose tolerance after their iron was corrected. Some who had HbA1c values in the diabetes range reverted to prediabetes or normal status, again purely from fixing their iron.⁴ That means some people may be told they have prediabetes or diabetes when they really just have an iron problem.

Hemolytic Anemia Pushes HbA1c Down

Hemolytic anemias, where red blood cells are destroyed faster than normal, do the opposite. Because the cells are dying prematurely, they never get the chance to accumulate the usual amount of glucose. The result is a falsely low HbA1c.⁵ In severe cases, hemolysis can make HbA1c essentially unmeasurable.⁶

This is the more dangerous direction of error for someone with diabetes. A falsely reassuring HbA1c might convince you and your doctor that blood sugar is well controlled when it isn’t, leading to under-treatment. The risk extends to anyone with conditions that shorten red blood cell life, including autoimmune hemolytic anemia, certain genetic conditions, and even mechanical heart valves that physically shear red blood cells as blood flows past them.

Vitamin B12 and Folate Deficiency

Megaloblastic anemia, caused by a shortage of vitamin B12 or folate, also inflates HbA1c. These deficiencies impair red blood cell production, leading to fewer but larger and longer-lived cells that accumulate more glucose. Researchers found that after treating B12 deficiency, a rise of about 0.94 g/dL in hemoglobin was associated with a 0.24-percentage-point drop in HbA1c. Before treatment, over a third of the patients had HbA1c values in the prediabetes range; after three months of B12 supplementation, 35% of them no longer met the prediabetes threshold at all.⁷

A separate study looking at both severe and mild-to-moderate B12 deficiency anemia found the same pattern. Before treatment, average HbA1c values in the severe group were about 5.9% and dropped to around 5.0% after treatment; the mild-to-moderate group went from about 5.6% to 4.9%. Compare those to the control group’s average of 5.1%, and the pre-treatment inflation becomes clear.⁸ The takeaway from both studies is the same: correcting the vitamin deficiency brought HbA1c back in line with what blood sugar was actually doing.

Sickle Cell Disease, Thalassemia, and Other Hemoglobinopathies

Inherited hemoglobin variants add another layer of complexity. Sickle cell disease shortens red blood cell lifespan, which can lower HbA1c in the same way other hemolytic conditions do. But the picture gets messier depending on which lab method is used. One study found that sickle cell trait (carrying one copy of the gene, not full-blown disease) did not affect HbA1c results on a particular immunoassay platform, but beta-thalassemia minor pushed HbA1c values falsely high, into the diabetic range, even in people without diabetes.⁹

The fact that different lab assays respond differently to hemoglobin variants makes this an especially tricky area. Some newer assay methods handle hemoglobinopathies better than older ones, but no single method is immune to all variants. For people living with sickle cell disease, researchers have evaluated fructosamine, a blood test that measures glycated serum proteins instead of glycated hemoglobin, as an alternative. Because fructosamine reflects average blood sugar over roughly two to three weeks and doesn’t rely on red blood cells at all, it sidesteps the hemoglobin problem entirely. Studies have found it to be a reliable stand-in for HbA1c in this population.¹⁰

How Anemia Tricks Fingerstick Glucose Meters

HbA1c isn’t the only test affected. Standard fingerstick glucose meters, the kind people with diabetes use at home every day, can also give misleading results when hematocrit (the percentage of blood made up of red blood cells) is abnormally low. Anemia lowers hematocrit, and when hematocrit drops below about 35%, many glucose meters read falsely high.¹¹

The mechanism is different from the HbA1c problem. Most meter test strips work by measuring a chemical reaction in a thin layer of blood. When hematocrit is low, the blood sample contains proportionally more plasma (the liquid part where glucose is dissolved) and fewer packed cells. The strip’s sensor sees more glucose per unit volume, not because glucose is truly elevated but because the sample is plasma-heavy. An older study demonstrated this dramatically: at a hematocrit of 20%, one meter brand produced readings that averaged 33% higher than the true value.¹² Newer meters have improved their hematocrit correction algorithms, but the effect has not been entirely eliminated, and people with severe anemia should be aware that their readings may skew high.

Continuous Glucose Monitors Are Less Affected

Continuous glucose monitors measure glucose in interstitial fluid, the liquid between your cells, rather than in blood directly. Because the measurement doesn’t depend on red blood cells or hematocrit, you might expect CGMs to be immune to anemia interference. Early data supports this expectation. A study in diabetes patients on peritoneal dialysis, a group commonly affected by anemia, found no correlation between hemoglobin levels and the monitor’s accuracy.¹³ That makes CGM data particularly valuable when anemia is complicating HbA1c interpretation. Clinical guidelines have started to incorporate this, recommending that clinicians use CGM metrics alongside or instead of HbA1c when there is a mismatch between the HbA1c number and what day-to-day glucose values suggest.¹⁴

The Kidney Disease Problem

People with chronic kidney disease often face a perfect storm of factors that make HbA1c unreliable. Their kidneys may not produce enough erythropoietin, the hormone that stimulates red blood cell production, so they become anemic. They may also have shortened red blood cell lifespan from the uremic toxins that accumulate when the kidneys aren’t filtering properly. And if they receive erythropoietin therapy (EPO) to treat the anemia, the treatment itself introduces yet another distortion.

When EPO kicks red blood cell production into gear, a flood of young, newly minted red blood cells enters the bloodstream. These young cells haven’t been around long enough to accumulate much glucose, so HbA1c plummets even if blood sugar hasn’t budged. One study of dialysis patients found that EPO therapy dropped HbA1c significantly without any corresponding change in fasting blood glucose.¹⁵ A case report illustrated the real-world danger of this effect: a 64-year-old man with diabetes and kidney disease had his insulin repeatedly reduced over three years because his HbA1c kept falling to as low as 4.4%, despite elevated blood glucose readings. After EPO was stopped, his HbA1c rebounded to 8.8%, revealing that his diabetes had been undertreated the entire time.¹⁶

Pregnancy and Iron Deficiency

Pregnancy deserves its own mention because iron deficiency anemia is extremely common in pregnant women, and HbA1c is sometimes used to screen for or monitor gestational diabetes. A systematic review found that across six of seven included studies, iron deficiency anemia in pregnant women was associated with higher HbA1c levels. When those women received iron replacement therapy, their average HbA1c dropped from about 5.1% to 4.89%.¹⁷ Other researchers have concluded that iron deficiency anemia undermines HbA1c reliability in pregnant women whether or not they have diabetes.¹⁸

There is some nuance, though. Not all degrees of anemia cause the same distortion. One study that stratified pregnant women by anemia severity found that mild and moderate anemia produced HbA1c differences too small to matter much in clinical practice, falling within the normal measurement variability of less than 0.5 percentage points.¹⁹ So mild anemia probably isn’t going to fool your obstetrician, but severe anemia very well could.

Blood Donation and Acute Blood Loss

Even a single blood donation can temporarily alter HbA1c. A study of blood donors found that over half of non-diabetic donors and a similar proportion of donors with type 2 diabetes had a significant drop in HbA1c after donating. The maximum reductions were about 12% relative to the pre-donation value in both groups. When donors returned at the time they were eligible to donate again, roughly half still had HbA1c levels significantly lower than before.²⁰ The effect happens because donating blood triggers the body to replace lost red blood cells with new ones. The younger average cell population carries less accumulated glucose, lowering the reading. If you’re managing diabetes and donate blood regularly, it’s worth scheduling HbA1c tests well before your next donation, or at least letting your doctor know.

Medications That Change the Picture

Several drugs affect red blood cell dynamics in ways that can muddy HbA1c readings. EPO therapy, as discussed in the kidney disease context, is one. Another class of medications worth knowing about is SGLT2 inhibitors, a newer group of diabetes drugs that work by making the kidneys excrete more glucose in urine. From the earliest clinical trials, researchers noticed that these drugs raised hemoglobin and hematocrit levels. Part of that was initially attributed to a mild diuretic effect concentrating the blood, but it became clear that SGLT2 inhibitors also boost erythropoietin levels and stimulate red blood cell production, while improving iron metabolism.²¹ The HbA1c drop you see with these drugs is partly a genuine glucose-lowering effect and partly an artifact of having more young red blood cells in circulation. Teasing apart how much of the HbA1c improvement is “real” versus “red blood cell math” is something researchers are still working through.

High Altitude and Erythrocytosis

People who live at high altitudes naturally produce more red blood cells to compensate for lower oxygen levels, a condition sometimes called high-altitude erythrocytosis. This creates an HbA1c distortion in the same direction as hemolytic anemia: falsely low. A cross-sectional study in China found that individuals with hemoglobin above 160 g/L had significantly lower HbA1c values than those with hemoglobin at or below that threshold, about 6.3% compared to 7.0%, even after accounting for other variables.²² The proposed explanation is straightforward: when your blood contains more hemoglobin overall, the proportion of it that gets glycated is diluted, so HbA1c reads lower than what your glucose levels would warrant. For the millions of people living above 2,500 meters worldwide, this is a real consideration in diabetes management.

What to Do If You Have Anemia and Need Accurate Glucose Monitoring

The first step is simply being aware of the problem. If you’ve been told your HbA1c is creeping into prediabetes or diabetes territory and you also have untreated anemia, the right move is to correct the anemia first and then recheck. Multiple studies have shown that treating iron deficiency or B12 deficiency can drop HbA1c by half a percentage point or more, enough to shift you from one diagnostic category to another.⁴

When anemia can’t be corrected quickly or at all (as in chronic hemolytic conditions or kidney disease requiring ongoing EPO), alternative monitoring strategies become essential. These include:

Fructosamine: Measures glycated serum proteins rather than glycated hemoglobin, reflecting average blood sugar over about two to three weeks. Unaffected by red blood cell lifespan or hemoglobin variants.
Glycated albumin: Similar concept to fructosamine, reflecting roughly two to three weeks of glucose levels. Some labs report this instead of or alongside fructosamine.
Continuous glucose monitoring: Provides real-time glucose data that doesn’t depend on red blood cells at all. Especially useful for dose adjustments when HbA1c can’t be trusted.
Fasting and postprandial glucose tests: Standard blood draws that measure glucose directly. They give a snapshot rather than an average, but they’re unaffected by anemia.

Clinical frameworks now emphasize that when there’s a mismatch between what HbA1c says and what day-to-day glucose readings show, clinicians should investigate potential sources of HbA1c interference rather than reflexively adjusting diabetes medications.¹⁴ The worst outcome, illustrated by the kidney disease case described earlier, is chasing a false HbA1c with medication changes that either overtreat or undertreat actual blood sugar.

Anemia, Glycated Hemoglobin, and Oxygen Delivery

There is one connection between anemia, blood glucose, and physiology that goes beyond test accuracy. When hemoglobin becomes glycated, the sugar molecule attached to it locks oxygen in place on the hemoglobin, making it harder for the cell to release oxygen to tissues. The higher the proportion of glycated hemoglobin, the less efficiently oxygen gets delivered.²³ In someone who already has fewer red blood cells because of anemia, this impairment in oxygen delivery compounds the oxygen deficit the anemia itself causes. The combination of anemia and poorly controlled diabetes can create a degree of tissue oxygen deprivation that neither condition would produce alone, which is part of why managing both conditions simultaneously matters.

1
PubMed Central. The influence of shorter red blood cell lifespan on the rate of HbA1c target achieved in type 2 diabetes patients with a HbA1c detection value lower than 7%
2
PubMed Central. Red Blood Cell Lifespan < 74 Days Can Clinically Reduce Hb1Ac Levels in Type 2 Diabetes
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PubMed Central. Increased Levels of Glycated Hemoglobin A1c and Iron Deficiency Anemia: A Review
4
PubMed. Effect of iron deficiency anemia and iron supplementation on HbA1c levels – Implications for diagnosis of prediabetes and diabetes mellitus in Asian Indians
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PubMed Central. Low Hemoglobin A1c (HbA1c) Revealing Hemolytic Anemia in a Growth Hormone-Treated Child: A Case Report
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PubMed Central. Immeasurable glycosylated haemoglobin: a marker for severe haemolysis
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PubMed Central. Relationship between glycosylated hemoglobin and vitamin B12 deficiency anemia
8
PubMed Central. Glycated Haemoglobin (HbA1c) Variations in Nondiabetics With Nutritional Anemia
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Saudi Medical Journal. Effect of sickle cell trait and B-Thalassemia minor on determinations of HbA1c by an immunoassay method
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PubMed Central. Serum fructosamine and glycemic status in the presence of the sickle cell mutation
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PubMed Central. Hematocrit Interference of Blood Glucose Meters for Patient Self-Measurement
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PubMed. Effect of hematocrit concentration on blood glucose value determined on Glucometer II
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PubMed. Impact of Body Composition and Anemia on Accuracy of a Real-Time Continuous Glucose Monitor in Diabetes Patients on Continuous Ambulatory Peritoneal Dialysis
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PubMed. When HbA1c Lies: Recognizing and Managing HbA1c-Glucose Discordance in Clinical Practice
15
The Egyptian Journal of Internal Medicine. Assessment of hemoglobin A1c in hemodialysis patients with diabetes before and after erythropoietin treatment
16
PubMed. Class effect of erythropoietin therapy on hemoglobin A(1c) in a patient with diabetes mellitus and chronic kidney disease not undergoing hemodialysis
17
PubMed Central. Exploring the Impact of Iron Deficiency Anaemia on Glycated Haemoglobin A1c Levels in Pregnant and Non-Pregnant Women: A Systematic Review
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Clinical and Experimental Obstetrics & Gynecology. Impact of Iron Deficiency Anemia on Hemoglobin A1c Levels in Diabetic and Non-Diabetic Pregnant Women
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PubMed. Effect of anemia and erythrocyte indices on hemoglobin A1c levels among pregnant women
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PubMed Central. Whole Blood Donation Affects the Interpretation of Hemoglobin A1c
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Research Square. Impact of Altitude on the Accuracy of HbA1c in Reflecting Glycemic Status in Patients with Type 2 Diabetes: A Cross-Sectional Study in China
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Open Access Text. Excess insulin and hypoxia, linkages to obesity and type 2 diabetes