As a diabetes specialist, I frequently assess hemoglobin (Hb)A1c values. It is well known that anemia of chronic disease associated with end-stage renal disease renders the HbA1c test invalid. At what level of anemia is the HbA1c affected? In which disease states or at what stages of chronic kidney disease is the HbA1c test no longer reliable? — Pamela A. Tetro, MSN, FNP, NDE, Charlottesville, Va.

Since the HbA1c assay is dependent on a naturally functional and structural Hb molecule, any alteration in that molecule or its integrity will diminish the assay’s accuracy. Similarly, any condition altering the size or longevity of the red blood cells (RBCs) will also change the results because the assay measures the amount of glycosylation (or binding) of serum glucose and the hemoglobin molecules in the red corpuscle.

Since these cells live an average of 60 to 90 days in a healthy person, an index is derived to infer a daily glucose level based on the degree of saturation (or binding) that exists.

When either the hemoglobin or the RBC itself is impaired, the result can be erroneously high or low. Conditions known to interfere with hemoglobin glycosylation include genetic variants or hemoglobinopathies, renal failure, and large amounts of ingested aspirin. Conditions affecting RBC survival include recovery from acute blood loss, anemia (specifically, the iron-deficient variety), hemolytic anemia, and increased RBC turnover attributable to other pathologies.

Additional factors affecting the HbA1c assay are ingestion of large amounts of vitamin C and/or vitamin E, chronic alcoholism, and chronic opiate use. There is no direct linear scale to use for correlation of values, so clinical assessment is key. Consistently elevated serum glucoses that seem to oppose a low or normal HbA1c should be given consideration in light of any other compounding factors, and the HbA1c assay should not be used as the sole diagnostic indicator. — Sherril Sego, FNP-C, DNP (184-4)

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