When diagnosing diabetes, an elevated level should be confirmed with repeat testing, except in symptomatic patients and who have plasma glucose >200 mg/dl. American Diabetes Association (ADA) recommends HbA1c should be measured 3-4 times per year for type 1 and poorly controlled type 2 diabetes and 2 times per year for well-controlled type 2 diabetic patients.
HbA1c assay reflects long-term fluctuations in blood glucose concentration, a diabetic patient who may have come in recent weeks under good control may still have a high concentration of HbA1c and conversely a diabetic patient previously under good control now poorly controlled may still have low concentrations of HbA1c.
In conditions with shortened erythrocyte lifespan (hemolytic anemia), HbA1c results will be decreased and affect will be dependent on the severity of anemia. Patients with polycythemia or post-splenectomy may exhibit increased HbA1c levels due to longer erythrocyte lifespan. HbA1c values in recently transfused patients may be falsely elevated.
Capillary electrophoresis HbA1c method does not shows any interference for hemoglobin variants, heterozygous HbS, HbC and HbD and HbF <=15%( Elevated HbF can be identified in capillary electrophoresis methods but not in immunoassays). Levels of Hb F up to 15% in the blood sample do not interfere with HbA1c quantification. Samples that contain high amounts of Hb F (>15%), usually found in some people with thalassemia, infants, and some pregnant women, may yield a lower than expected HbA1c result with this assay. In patients with homozygous forms of variant Hb (e.g. SS, CC, EE) SC), there is no HbA thus no HbA1c values cannot be quantitated using this method. In such situations fructosamine should be used as an alternate method for monitoring glycemic control.
Individuals with recent significant blood loss exhibit falsely low HbA1c values due to a higher fraction of young erythrocytes.
Abnormal life span of red blood cells as found in hemolytic anemia, polycythemia or post splenectomy may affect the levels of HbA1c.