For most comprehensive blood work including glucose and lipid panels, you should fast for 8–12 hours before the test, ideally overnight. This means no food, beverages (except water), gum, or tobacco during that period.
Fasting is particularly important for tests measuring blood sugar, triglycerides, and cholesterol because eating or drinking anything other than water can significantly elevate these values and produce inaccurate results. A fasting state typically begins about 8 hours after your last meal, which is why blood draws are often scheduled early in the morning.
However, not all blood tests require fasting. Complete blood counts (CBC), thyroid panels, liver function tests, and many others can be performed without fasting. Your healthcare provider or the laboratory will specify fasting requirements when ordering your tests—always check the instructions provided in your lab order or call ahead to confirm.
If you're uncertain whether to fast, contact your doctor or the lab directly rather than guessing. Arriving unfasted when fasting was required means you'll need to reschedule. Conversely, fasting when unnecessary causes unnecessary inconvenience. Most labs include specific preparation instructions with appointment confirmations to prevent confusion.