How to Prepare for a Fasting Blood Test

November 12, 2012

How to Prepare for a Fasting Blood Test

There are two broad categories of blood tests – those that can be done anytime and fasting blood tests. In order to get accurate results, a person must not eat or drink a certain amount of time before the test. Foods or drinks with any fat, sugars, proteins or carbohydrates can radically change the composition of the blood. By fasting, you are purging your blood of any excess chemicals that get into the blood from recently digesting food. Fasting blood tests avoid false positives. Tests that require fasting overnight or at least 8 hours include:

  • complete blood cell counts

  • cholesterol blood tests

  • blood sugar or glucose level blood tests

  • most blood tests to check how the heart is performing

Some tests may require a 10 or 12 hour fast. If you add in traveling to the test site and waiting for the test, then you may wind up fasting hours longer than the minimum requirement. Longer fasting does not drastically change blood test results but it should be planned on in advance. Never Overeat Before Starting a Fast It can be very tempting to stuff your face in the hours leading up to the start of your fast. But overeating the day before will not make you any less hungry during the fast. You may become even hungrier because your body is expecting another feast. You may also experience painful gastrointestinal symptoms like cramps, constipation or vomiting if you overeat.

It’s always best to eat less than you normally would in the hours leading up to a fast. This helps prepare your body for the abnormal eating schedule of test day. The last snack before the fast starts should be rich in protein to help you stay full. Nuts, nut butter, lean meats or low-sugar protein bars are good choices. Usually, drinking water is allowed during the fast, but always check with your doctor about what you can drink and when before a fasting blood test. If you take daily medications, tell your doctor before the fasting blood test. Your doctor may ask you to not take the medications until after your test. Why Blood Tests Should Be Done Early Schedule your fasting blood test as early in the day as possible soon after you wake up. Some hospitals or doctors offices that perform fasting blood tests do not require you to schedule an appointment. You should still show up for the test within an hour or two of waking up. This will help you feel less hungry and less tempted to eat and ruin your test results. Many people also experience side effects if they do not eat every eight hours or so. These side effects can be painful. They include, but are not limited to the following:

  • Headaches or migraines

  • Nausea and dry heaves

  • Dizziness or light-headedness

  • Trouble concentrating

  • Constant fatigue

If you already know that you suffer from any of these symptoms during a 10 or 12 hour fast, get a taxi or a friend to drive you to and from the hospital or doctor’s office. You may be too incapacitated to drive safely.

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