Harvard Men's Health Watch

Ask the doctor: What blood tests require fasting?

Q. Sometimes my doctor tells me it's okay to eat before a blood test, and sometimes it isn't. Why is that?

A. Actually, fasting affects the results of very few blood tests. For example, measurements of kidney, liver, and thyroid function, as well as blood counts, are not influenced by fasting. However, fasting is required before commonly ordered tests for glucose (blood sugar) and triglycerides (part of the cholesterol, or lipid, panel) for accurate results.

You should not eat for eight hours before having your blood glucose measured to check for diabetes or to see how well treatments are working. An alternative test for glucose level that does not require fasting measures a substance called hemoglobin A1c, which reflects average blood sugar over the previous three months.

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