A fasting blood glucose test evaluates the patient’s ability to regulate glucose levels. A blood sample is taken by venipuncture after the patient has fasted for about 8 hours. Although values may vary slightly among laboratories, a finding between 70 and 120 mgjdl is considered normal in nonpregnant adults . A physician makes a diagnosis of diabetes if the finding is 126 mgjdl or more.A fasting blood glucose test can also be used to monitor the effectiveness of the patient’s treatment. If the glucose level is consistently elevated, the patient’s prescribed regimen may need to be changed. However, an elevated fasting blood glucose level in a patient with Type 1 diabetes doesn’t always signal a need for more insulin. If the patient has the Somogyi phenomenon, less insulin may be needed. This phenomenon results from insulin ­induced nocturnal hypoglycemia, which causes counter regulatory hormones to raise blood glucose levels through internal compensatory mechanisms. To alleviate the phenomenon, the physician will decrease the patient’s insulin dosage to eliminate nocturnal hypoglycemia.

Nursing Considerations

Before the test, tell your patient that she must fast for 8 hours. Make sure she understands that during the fast she may eat nothing and drink only water. Instruct her to fast for no more than 16 hours before the test. A fast of more than 16 hours or less than 8 hours can falsely increase glucose levels.

If the patient already uses insulin or an oral antidiabetic drug, her physician will withhold it during the fast. After the test, the patient can go back to her regular medication schedule. Instruct your patient to maintain a normal activity level during the fast. Any increase in activity can lower blood glucose levels and cause a false normal result.

If your patient is hospitalized before the test and her diabetic medication hasn’t been withheld during the fast, check her during the night for hypoglycemia. If she appears restless or diaphoretic while sleeping, wake her and check her blood glucose level. Nightmares and a headache in the morning also suggest nocturnal hypoglycemia. If you detect hypoglycemia, break the fast, appropriately treat the patient, and reschedule the test. If the patient isn’t hospitalized and her diabetic medication hasn’t been withheld, have a family member check her during the night.

If you draw the blood for a fasting blood glucose test to be performed at a laboratory, send the sample immediately. If the sample is left at room temperature, blood glucose levels will decrease. If you can’t send it right away, refrigerate it.

To monitor blood glucose levels at home, a patient can perform a fasting blood glucose test, using a fingers tick and a self-monitoring meter. Explain that if the test shows hyperglycemia, she should check the calibration of her meter. If it’s correct, she should repeat the test to verify her initial results. Reassure your patient that blood glucose levels fluctuate and an infrequent elevation in the fasting blood glucose level doesn’t indicate a loss of diabetic control. Elevated levels are significant only if they persist.