How do I manage diabetes when fasting for a blood test or a colonoscopy? I had to skip my colonoscopy last year because of low blood sugar. And now I have to have a cholesterol test. What do I do?
Nina Watson, RN, MSN, CDE, responds: Fasting can be a challenge for people with diabetes, especially when you are taking medications that have a direct effect on your blood glucose level. Always discuss how to adjust your medication regimen with your health care team before the fast. The changes you make will depend on what medications you are taking, what the dosages are, how your blood glucose control is, how long your fast will be, and why you are fasting. For instance, someone with diabetes who is having a procedure should request to have it done early in the day to avoid prolonged fasting.
Here are some general rules to follow for a morning fast. If you're on oral medications, hold them until the test is complete and take them again when you're ready to eat. If you are taking metformin and the test involves an intravenous injection, you may need to hold the metformin until at least 48 hours after the test (this is to ensure that your kidney function remains normal). Stop any injections such as Byetta and Symlin that are only to be taken with meals, and take those when you are ready to eat again.
Insulin changes are dependent on your prescribed regimen. Mealtime insulins, such as NovoLog, Humalog, and regular, should also be held until you are ready to eat. Dosing of background (long-acting) insulin should be discussed with your doctor. If you are on Levemir or NPH at bedtime only, there should be no need to hold your injections. If you take either one twice daily—depending on the length of your fast, control, and risk for hypoglycemia—your provider may want you to decrease the dose. If you are taking Lantus, your provider may or may not adjust your dose, based on how much of it you are taking and how long your fast is anticipated to be.
Regardless of what your diabetes management regimen is, check with your doctor for a detailed plan that you can follow easily when you're scheduled for a procedure that requires fasting. It is always a good idea to plan ahead.