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.

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