How long do you have to fast before a blood test?

Patients should fast for at least 12 hours before they have a fasting blood test, according to St. John Providence Health System. Not all blood tests require patients to fast, but those that do require that patients abstain from food or any type of beverage besides water before their test.

The blood tests that require fasting include a fasting glucose test, a fasting lipid panel, a fasting metabolic panel, a fasting cholesterol test, an HDL test or a triglyceride test. Patients who do need to fast may not be able to get their tests completed if they accidentally break their fast, states St. John Providence Health System.