Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (979 page)

   Possibly for diagnosis of DM. Usefulness is still to be determined.

   Interpretation

   A
_1C
test should be performed at least two times a year in patients who are meeting treatment goals (and who have stable glycemic control).

   A
_1C
test should be performed quarterly in patients whose therapy has changed or who are not meeting glycemic goals.

   Lowering A
_1C
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 A
_1c
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, HbA
_1c
is normal in >96% of cases.

   When fasting blood glucose is 110–125 mg/dL, HbA
_1c
is normal in >80% of cases.

   When fasting blood glucose is >126 mg/dL, HbA
_1c
is normal in >60% of cases.

   One percent increase in GHb is related to approximately 30 mg/dL increase in glucose.