Fasting Blood Work Preparation Guide for Patients in Palm Beach County, FL

What it involves

Fasting for a blood test means no food and no drink except plain water for several hours before the sample is taken. Your provider decides whether a particular test needs it. Fasting matters because what you eat and drink is absorbed into the bloodstream, which can change the levels of sugar, fat, and other substances the test is trying to measure.

Who this is for

• Patients who were told a glucose or cholesterol test needs fasting and want clear, step-by-step instructions
• Patients with diabetes or prediabetes who have follow-up blood work coming up
• New patients who have been asked to bring recent lab results or get new ones
• Anyone unsure whether the test on their order slip requires fasting at all

What to expect

• Instructions from your provider on whether to fast and for how many hours
• A blood draw that takes only a few minutes once you are in the chair
• A conversation about your medicines before the test so nothing is stopped by mistake
• A review of the results with your provider afterward

How long to fast

MedlinePlus says you usually need to fast for 8 to 12 hours before a test, and that your provider will tell you exactly how long based on the test ordered. If a test requires fasting, scheduling it early in the morning means most of the fast happens while you sleep. A basic metabolic panel is one of the tests that may require about eight hours without food, and the basic metabolic panel guide lists the eight results it reports.

What you can and cannot have

While you are fasting, MedlinePlus says you may drink plain water and nothing else. These are off limits: Vitamin D tests need no special preparation, while a B12 level may need a fast of about six to eight hours, and the vitamin D and B12 blood tests guide explains who usually needs each one. A cholesterol test may need a fast of nine to twelve hours, and the lipid panel numbers guide explains how to read the report once you have it.

• Food of any kind, including snacks and mints
• Juice, coffee, soda, and other drinks, which can get into your bloodstream and change the results
• Flavored or lemon water and flavored carbonated water, which may contain sugars or artificial sweeteners
• Chewing gum
• Smoking
• Exercise

Tests that usually need a fast

MedlinePlus gives these as common examples, and adds that not every version of each test requires fasting, so always ask: A complete blood count by itself usually needs no fasting, but it is often drawn with other tests that do, and the complete blood count guide explains what it counts.

• Blood glucose tests, used to help diagnose and monitor diabetes and prediabetes
• Cholesterol level tests, called a lipid panel
• A basic metabolic panel, which shows how well organs such as the heart, kidneys, and liver are working
• Sometimes liver function tests or a renal function panel, depending on how they are ordered

Medicines, vitamins, and supplements

MedlinePlus says to ask your provider whether it is fine to take your usual prescription and over-the-counter medicines during the fast, because for some tests they may ask you to stop one. Do not stop a medicine unless your provider tells you to. Mention your vitamins and supplements too, since those can also affect results. The best time to ask is at least a full day before the test. Some medicines can also change liver numbers, so if your blood work includes a liver panel, the liver function tests guide explains what each result measures and why your medicine list matters.

If you ate or drank by mistake

Tell your provider. MedlinePlus says you may need to reschedule for a day when you can fast properly, because an inaccurate result gives an inaccurate view of your health. A rescheduled draw costs less than a decision made on a wrong number.

Do not fast unless you are told to

MedlinePlus is clear that you should not fast if your provider has not told you to, and that fasting for some tests can itself affect the result. Read the order, and if it does not say, call the office. After the draw you can eat right away. Light-headedness is common after a fasting draw, so bring a snack, and know that the in-house lab services page lists which office draws on site.

Sources

• MedlinePlus — Fasting for a Blood Test
• NIDDK — The A1C Test & Diabetes