Throughout the red blood cell's life span, the hemoglobin molecule incorporates glucose onto its chain. Glycosylation is irreversible and occurs at a stable rate. The amount of glucose permanently bound to hemoglobin depends on the blood sugar level. Thus, the level of glycosylated hemoglobin, designated Hgb A_1c, reflects the average blood sugar over a period of several weeks.

The test is used to evaluate the overall adequacy of diabetic control and provides information that may be missed by individual blood and urine glucose tests. Insulin-dependent diabetics, for example, may have undetected periods of hyperglycemia alternating with postinsulin periods of normoglycemia or even hypoglycemia. High Hgb A_1c levels reflect inadequate diabetic control in the preceding 3 to 5 weeks.

In addition to providing a more accurate assessment of overall blood glucose control, the test is more convenient for diabetic clients because it is performed only every 5 to 6 weeks and because there are no dietary or medication restrictions before the test.

• Hgb A_1c is 3 to 6 percent of hemoglobin.
• Hgb A_1c is 7 to 11 percent in diabetes under control.

• Individuals with hemolytic anemia and high levels of young red blood cells may have spuriously low levels.
• Individuals with elevated hemoglobin levels or on heparin therapy may have falsely elevated levels.

• Monitoring overall blood glucose control in clients with known diabetes, because the test aids in assessing blood glucose levels over a period of several weeks and provides data that may be missed by random blood or urine glucose tests:
  • With prolonged hyperglycemia, levels of Hgb A_1c may rise to as high as 18 to 20 percent.
  • After normoglycemic levels are stabilized, Hgb A_1c levels return to normal in about 3 weeks.¹⁶
• Monitoring adequacy of insulin dosage for blood glucose control, especially that administered by automatic insulin pumps
• Evaluating the diabetic client's degree of compliance with the prescribed therapeutic regimen, because fasting or adjusting medications shortly before the test will not significantly alter results

Nursing Care Before the Procedure

Client preparation is the same as that for any test involving collection of a peripheral blood sample (see Appendix I).

• The client should be informed that fasting or adjusting medications for diabetes shortly before the test will not significantly alter results.

A venipuncture is performed and the sample obtained in a lavender-topped tube. The sample must be mixed adequately with the anticoagulant contained in the tube and transported promptly to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving collection of a peripheral blood sample.

• Complications and precautions: A value of greater than 15 percent of total Hgb A_1c indicates that the diabetes is out of control. Notify the physician at once.