Type of test Blood

 Normal findings

 Nondiabetic adult or child: 4% to 5.9%

Good diabetic control: < 7%

Fair diabetic control: 8% to 9%

Poor diabetic control: > 9%

(Values vary with laboratory method used.)

Test explanation and related physiology

 This test is used to diagnose and monitor diabetes treatment. It measures the amount of hemoglobin A1c (HbA1c ) in the blood and provides an accurate long-term index of the patient’s average blood glucose level. In adults, about 98% of the hemoglobin in the RBCs is hemoglobin A. About 7% of hemoglobin A con sists of a type of hemoglobin (HbA1 ) that can combine strongly with glucose in a process called glycosylation. When glycosylation occurs, it is not easily reversible.

HbA1 is actually made up of three components: A1a , A1b , and A1c . HbA1c is the component that combines most strongly with glucose. Therefore HgA1c is the most accurate measurement because it contains the majority of glycosylated hemoglobin.

The amount of glycosylated hemoglobin depends on the amount of glucose available in the bloodstream over a RBC’s 120-day life span. Therefore determination of the GHb value reflects the average blood sugar level for the 100- to 120-day period before the test. The more glucose the RBC was exposed to, the greater the GHb percentage. One important advantage of this test is that the sample can be drawn at any time because it is not affected by short-term variations (e.g., food intake, exercise, stress, hypoglycemic agents).

As mentioned, the average life span of an RBC is 120 days, so the GHb may not reflect more recent changes in glucose levels. Because the turnover rate of proteins is much faster than hemoglobin, the measurement of serum glycated proteins (e.g., glycated albumin or fructosamine) provides more recent information about glucose levels. Glycated proteins reflect an average blood glucose level of the past 15 to 20 days. Although an initial single glycated protein result may not separate good glucose control from poor control, serial testing provides a much better indication of glucose control.

The GHb or glycated protein tests are particularly beneficial for the following:

• Evaluating patient compliance and success of treatment

• Comparing and contrasting the success of forms of diabetic therapy

• Determining the duration of hyperglycemia in patients with newly diagnosed diabetes

• Providing a sensitive estimate of glucose imbalance in patients with mild diabetes

• Individualizing diabetic control regimens

• Providing a feeling of reward for many patients when the test shows achievement of good diabetic control

 • Evaluating the diabetic patient whose glucose levels change significantly day to day (brittle diabetic)

• Differentiating short-term hyperglycemia in patients who do not have diabetes (e.g., recent stress or myocardial infarction) from those who have diabetes (where the glucose has been persistently elevated)

A diagnosis of diabetes mellitus can be made on the basis of the results from two tests—fasting blood glucose (p. 464) and GTT—performed on separate days that are close in time. By a relatively simple calculation, GHb can be accurately correlated with the daily mean plasma glucose (MPG) level, which is the average glucose level throughout the day. This has been very helpful for diabetics and health care professionals in determining and evaluating daily glucose goals. Each 1% change in GHb represents a change of approximately 35 mg/dL MPG. See Table 1.

Table1. Correlation between glycosylated hemoglobin and mean plasma glucose

 Interfering factors

• Hemoglobinopathies can affect results because the quantity of hemoglobin A (and, as a result, HbA1 ) varies considerably.

• Falsely elevated values occur when the RBC life span is lengthened.

 • Abnormally low levels of proteins may falsely indicate normal glycated protein levels despite high glucose levels.

 • Ascorbic acid may falsely indicate low levels of fructosamine.

 Procedure and patient care

 • See inside front cover for Routine Blood Testing.

 • Fasting: no

• Blood tube commonly used: gray or lavender

Abnormal findings

 Increased levels

 -Newly diagnosed diabetic  patient

 -Poorly controlled diabetic  patient

 - Nondiabetic hyperglycemia (e.g., acute stress response, Cushing syndrome, pheochromocytoma, glucagonoma, corticosteroid therapy, or acromegaly)

 - Splenectomized patients

 - Pregnancy

Decreased levels

 - Hemolytic anemia

 - Chronic blood loss

 - Chronic renal failure

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