Definition
- Glucose combines with Hb continuously and nearly irreversibly during the life span of RBC (120 days). Therefore, glycosylated Hb (GHb) will be proportional to mean plasma glucose level during previous 6 " “12 weeks. May be reported as HbA1c or as total of A1b, A1a, or A1c.
Values may not be comparable with different methodologies and even different laboratories using same methodology. ‚
- Normal range (ADA 2010 recommendations):
- <5.6%
- 5.7 " “6.4% increased risk for diabetes
- >6.5% diabetic range
- The interpretation of HbA1c levels is not intuitively obvious to many patients with DM, who are accustomed to thinking in terms of blood glucose levels. The American Diabetes Association (ADA) has called for laboratories to express HbA1c results as estimated average blood glucose (eAG). ADA believes that eAG will be easier for patients to understand and will lead to improved management of DM.
- The formula recommended to calculate eAG is
View OriginalView Original ‚
Use
- Monitoring compliance and long-term blood glucose level control in patients with diabetes.
- Index of diabetic control (direct relationship between poor control and development of complications).
- Predicting development and progression of diabetic microvascular complications.
- Possibly for diagnosis of DM. Usefulness is still to be determined.
Interpretation
- A1C test should be performed at least two times a year in patients who are meeting treatment goals (and who have stable glycemic control).
- A1C test should be performed quarterly in patients whose therapy has changed or who are not meeting glycemic goals.
- Lowering A1C to below or around 7% has been shown to reduce microvascular and neuropathic complications of type 1 and type 2 diabetes. Therefore, for microvascular disease prevention, the A1c goal for nonpregnant adults in general is <7%.
- Dietary preparation or fasting is not required.
- An increase almost certainly means DM if other factors (see below) are absent (>3 SD above the mean has S/S = 99%/48%), but a normal value does not rule out impaired glucose tolerance. Values less than the normal mean are not seen in untreated DM.
- May rise within 1 week after rise in blood glucose due to stopping therapy but may not fall for 2 " “4 weeks after blood glucose decrease when therapy is resumed.
- Mean blood glucose in first 30 days (days 0 " “30) before sampling GHb contributes approximately 50% to final GHb value, whereas days 90 " “120 contribute only approximately 10%. Time to reach a new steady state is approximately 30 " “35 days.
- When fasting blood glucose is <110 mg/dL, HbA1c is normal in >96% of cases.
- When fasting blood glucose is 110 " “125 mg/dL, HbA1c is normal in >80% of cases.
- When fasting blood glucose is >126 mg/dL, HbA1c is normal in >60% of cases.
- One percent increase in GHb is related to approximately 30 mg/dL increase in glucose.
- When mean annual HbA1c is <1.1 times ULN, renal and retinal complications are rare, but complications occur in >70% of cases when HbA1c is >1.7 times ULN.
Increased In
- Fetal Hb above normal or 0.5% (e.g., heterozygous or homozygous persistence of HbF, fetomaternal transfusion during pregnancy)
- Chronic renal failure with or without hemodialysis
- Iron deficiency anemia
- Splenectomy
- Increased serum triglycerides
- Alcohol ingestion
- Lead and opiate toxicity
- Salicylate treatment
Decreased In
- Shortened RBC life span (e.g., hemolytic anemias, blood loss)
- Following transfusions
- Pregnancy
- Ingestion of large amounts (>1 g/day) of vitamin C or vitamin E
- Hemoglobinopathies (e.g., spherocytes), which produce variable increase or decrease depending on assay method.