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Expert Q&A

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Why do my A1C results from different sources vary?

Asked by Peter, Denver

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What can I do when I am getting A1C test results that vary widely from multiple sources?

Expert Bio Picture

Conditions Expert Dr. Otis Brawley Chief Medical Officer,
American Cancer Society

Expert answer

Dear Peter: You ask a fascinating question.

The amount of sugar or glucose in the blood varies throughout the day. When a person eats a meal with a lot of starches or sugars, the glucose level blood goes up. In a nondiabetic person the glucose level goes down to normal, which is generally 80 to 110 milligrams per deciliter, in less than two hours. People with severe diabetes may require oral drugs or an insulin injection to lower their blood sugar.

Hemoglobin is a protein that is found in red blood cells. Its primary purpose is to carry oxygen from the lungs to organs throughout the body. Red blood cells are made in the bone marrow and move to the blood stream where they live for about 120 days. While in the bloodstream, glucose can penetrate the red blood cell wall and bind to hemoglobin.

Hemoglobin A1C (also called HgA1C or A1C) is a measure of glycated hemoglobin. This is the percentage of one's hemoglobin that has glucose bound to it. It can be used as a measure of what a persons' blood sugars have been over the previous three months or so. An HgA1C of 6.5 percent is an average blood sugar of 135 mg/dl. Someone with A1C results of 6.8 to 7.4 percent has reasonable control of his or her blood sugar, but I would want most patients to have even better control. A measure of 8 percent or greater really needs to be under better control.

We see high HgA1Cs that are accurate measurements, but not reflective of glucose control in people with low red cell turnover from iron, vitamin B12 or folate deficiency. HgA1C values may be elevated or decreased in those with chronic kidney disease. These disorders can generally be diagnosed through some simple, highly available blood tests.

There are a few drugs that can change hemoglobin distribution and cause A1C readings that are not representative of blood sugar history. A review of your medicines may be in order. The red blood cell stimulator, erythropoietin can lower HgA1c levels by causing increased red blood cell production and diluting the amount of hemoglobin available to bind to glucose. Malaria-prevention drugs can affect HgA1C, as can the common sulfa antibiotics. These drugs tend to increase red blood cell destruction and production and cause low A1Cs that are not reflective of serum glucose status.

In the conditions above, the HgA1C that is measured is accurate, but the reading is not representative of glucose control. There are several HgA1c assays or tests that do not give accurate readings with patients who may have an underlying hemoglobin disorder, also called a hemoglobinopathy. There are several common hemoglobin disorders such as sickle cell trait and the thalassemias that are generally asymptomatic and generally of no consequence to the patient having it, but patients can have erroneous HgA1C measurements.

A blood test known as a hemoglobin electrophoresis may also be needed to determine a hemoglobin problem. In a patient with a known hemoglobin disorder, the doctor may want to specifically request that the HgA1C assay be done by boronate affinity chromatography. This is a method that is less affected by hemoglobin disorders. It is possible that the older HgA1Cs you got were this type of test and the newer results are from one of the tests that more affected by hemoglobin disorders.

The National Glycohemoglobin Standardization Program website contains current information about substances and conditions that interfere with A1C test results.

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