Why Do Some People Need to Fast Before a Blood Test?

fast before a blood test

When scheduling a blood test, doctors often instruct patients to do a fast or refrain from eating for a certain number of hours, prior to undergoing the lab work. Have you ever wondered the reason behind that requirement?  Certain food and drinks impact our blood glucose or blood sugar levels, affecting the results of your fasting blood tests.  Not everyone has to fast, while some people have to fast for more hours than others. Why is that? Many factors determine doctors’ decisions.

Requirements on Fasting for Blood Work

What is fasting for bloodwork? It’s simple – you can’t eat or drink anything except for water, for a certain length of time, until you’ve gotten the test done.  The number of hours you will need to fast will depend on the particular blood test you’re getting done. But typically fasting times go from 9 to 12 hours. Say, for example, you need to do a 12 hour fast. If you eat your last morsels at 10 o’clock at night, you’ll do your test at 10:00 AM the following day. The physician ordering the blood tests will specify the time you should do it. Once your blood is drawn, you can eat immediately following the test. Not all blood tests require fasting. Here are the ones that typically do, along with the required length of fasting:

how long should i fast before a blood test

  • Fasting blood glucose – Measures the amount of glucose (sugar) in your blood to test for diabetes or prediabetes. (Minimum of 8 hours)
  • Blood cholesterol tests/Lipid Profile – Used to check cholesterol levels and other levels of fat in the blood. (9 to 12 hours)
  • Basic or comprehensive metabolic panel –  This is often part of a routine physical exam which measures blood sugar, electrolyte, and fluid balance, and kidney function. (10 to 12 hours)
  • Iron Tests – Measures your iron levels to determine if they’re too high or too low. The latter could mean you have anemia.  (12 hours)
  • Renal function panel – The main test to check the health of your kidneys. (8 to 12 hours)
  • Vitamin B12 test – Tests your vitamin B12 levels.  (6-8 hours)
  • Gamma-glutamyl transferase (GGT): GGT is an enzyme found in many organs in the body, but this test diagnoses alcohol-related liver disease.  (8 hours)

Tips for Preparing for Blood Tests

In addition to refraining from food and drinks, you should also refrain from smoking, chewing gum (even sugarless), and exercising.  These things can boost your digestion, which can affect your results. Ask your doctor about taking medications – prescribed or over-the-counter – prior to your test. None of us is perfect, and sometimes we slip up. If you’ve eaten or drank something besides water, be sure to tell the person who’s drawing your blood. This is important.

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Visit us – the affordable clinics that make up the UrgentMED network – for blood tests.  We’re fast and convenient, and with 16 locations, we’re the largest comprehensive urgent care network in Southern California. We accept most insurance plans, and if you don’t have insurance, we have affordable rates for uninsured patients. We look forward to helping you stay on a healthy track!