Blood Glucose
Fasting blood sugar (FBS), drawn after at least an 8-hour fast, to evaluate circulating amounts of glucose; postprandial test, drawn usually 2 hours after a well-balanced meal, to evaluate glucose metabolism; and random glucose, drawn at any time, nonfasting.
Nursing and Patient Care Considerations
- For fasting glucose, make sure that patient has maintained 8-hour fast overnight; sips of water are allowed.
- Advise patient to refrain from smoking before the glucose sampling because this affects the test results.
- For postprandial test, advise patient that no food should be eaten during the 2-hour interval.
- For random blood glucose, note the time and content of the last meal.
- Interpret blood values as diagnostic for diabetes mellitus as follows:
- FBS greater than or equal to 126 mg/dL on two occasions
- Random blood sugar greater than or equal to 200 mg/dL and presence of classic symptoms of diabetes (polyuria, polydipsia, polyphagia, and weight loss)
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- Fasting blood glucose result of greater than or equal to 100 mg/dL demands close follow-up and repeat monitoring.
Oral Glucose Tolerance Test
The oral glucose tolerance test (OGTT) evaluates insulin response to glucose loading. FBS is obtained before the ingestion of a 50- to 200-g glucose load (usual amount is 75 g), and blood samples are drawn at ½, 1, 2, and 3 hours (may be 4- or 5-hour sampling).
Nursing and Patient Care Considerations
- Advise patient that for accuracy in results, certain instructions must be followed:
- Usual diet and exercise pattern must be followed for 3 days before OGTT.
- During OGTT, the patient must refrain from smoking and remain seated.
- Oral contraceptives, salicylates, diuretics, phenytoin, and nicotinic acid can impair results and may be withheld before testing based on the advice of the health care provider.
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- Diagnostic for diabetes mellitus if 2-hour value is 200 mg/dL or greater.
Glycated Hemoglobin (Glycohemoglobin, HbA1c)
Measures glycemic control over a 60- to 120-day period by measuring the irreversible reaction of glucose to hemoglobin through freely permeable erythrocytes during their 120-day lifecycle.
Nursing and Patient Care Considerations
- No prior preparation, such as fasting or withholding insulin, is necessary.
- Test results can be affected by red blood cell disorders (eg, thalassemia, sickle cell anemia), room temperature, ionic charges, and ambient blood glucose values.
- Many methods exist for performing the test, making it necessary to consult the laboratory for normal values.
C-Peptide Assay (Connecting Peptide Assay)
Cleaved from the proinsulin molecule during its conversion to insulin, C-peptide acts as a marker for endogenous insulin production.
Nursing and Patient Care Considerations
- Test can be performed after an overnight fast or after stimulation with Sustacal, I.V. glucose, or 1 mg of glucagon subcutaneously.
- Absence of C-peptide indicates no beta cell function, reflecting possible type 1 diabetes.
Fructosamine Assay
Glycated protein with a much shorter half-life than glycated hemoglobin, reflecting control over a shorter period, approximately 14 to 21 days. May be advantageous in patients with hemoglobin variants that interfere with the accuracy of glycated hemoglobin tests.
Nursing and Patient Care Considerations
- Note if patient has hypoalbuminemia or elevated globulins because test may not be reliable.
- Should not be used as a diagnostic test for diabetes mellitus.
- No special preparation or fasting is necessary.
