Not the only way to assess glucose control
Can you believe I’ve had patients show me a week’s worth of home glucose tests showing great numbers, tell me they’ve been that good for the last three months, and then I find a sky high Hemoglobin A1c test? How can that be?
Hemglobin A1c (or HgbA1c) is a standard measure of glucose control, or lack thereof, over the three months preceding the blood test.
It’s also used for diagnosis of diabetes and prediabetes. Levels between 5.7 and 6.4% suggest prediabetes. Levels of 6.5% of higher indicate diabetes.
Hemoglobin is the oxygen-carrying protein in red blood cells. HgbA1c tells us if many sugar molecules are stuck to the hemoglobin, a process called glycosylation. HgbA1c is sometimes referred to as glycated hemoglobin. About half of the HgbA1c value is determined by blood sugar levels in the month before the blood draw.
But the HgbA1c test is not always an accurate reflection of blood sugar levels.
Many factors unrelated to serum glucose (sugar) levels can alter the HgbA1c value. Here they are:
Pregnant women tend to have lower than average HgbA1c.
Certain Types of Anemia
Iron-deficiency anemia may yield falsely low or high HgbA1c, depending on whether it’s being treated or not.
Acute bleeding and hemolytic anemia give falsely low HbA1c values.
The unifying feature here is that young red blood cells, called reticulocytes, take some time to get glycosylated.
Lack of a Spleen
HgbA1c will be falsely high. Your spleen removes old red blood cells. Not having a spleen increases the life span of red blood cells, so they can accumulate more glucose molecules.
Various Hemoglobin Types or Congenital Abnormalities
Hemoglobin S and hemoglobin C may lead to deceptively low HgbA1c. Hemoglobin F tends to overestimate.
Recent red blood cell transfusions will lower the HgbA1c if it was elevated to begin with, especially if lots of blood is transfused.
It’s complicated; talk to your kidney specialist.
HgbA1c values can be unreliable in chronic alcoholism, chronic narcotic users, severely high triglyceride or bilirubin levels, kidney failure, vitamin and mineral deficiencies (particularly the vitamins and minerals needed to make red blood cells).
Hispanics, Asians, and Blacks tend to have higher HgbA1c’s than Whites who have the same blood sugar levels. The difference is about 0.3% (absolute, not relative.
Wild Glycemic Excursions
What’s this? You might call it labile diabetes: dramatic swings between sugars too low and way too high. For example, this patient may have daily glucose swings between 40 and 210 mg/dl (2.2 and 11.7 mmol/l). His HgbA1c may turn out near normal or acceptable, but many experts worry that the wild oscillations may contribute to diabetic microvascular complications like eye and kidney disease.
Are There Alternatives to HgbA1c?
Yes. If you think the HgbA1c test is inaccurate, consider other tests such as continuous glucose monitoring, fructosamine, glycated albumin, 1,5-anhydroglucitol, and more frequent home glucose monitoring.
Steve Parker, M.D.
Reference: Bazerbachi, F., et al. Is hemoglobin A1c an accurate measure of glycemic control in all diabetic patients? Cleveland Clinic Journal of Medicine, vol. 81, #3, March 2014: 146-149