Fasting blood test : What is forbidden to eat before it?

Some blood tests require several hours of fasting. However, not everyone knows which ones and what it means that you have to come to them on an empty stomach.

What does it mean to be fasting blood test ? This means that at least 6 hours must pass from the last meal, and preferably 8 hours. Some sources even give 12 hours. During this time, you are only allowed to drink water (but in limited quantities and unsweetened and non-carbonated).

You can drink a small amount of liquid before the test, which can only be pure boiled or non-carbonated spring water. We do not drink mineralized water, because it can also affect changes in the composition of blood, e.g. electrolyte, and thus falsify test results.

In practice, it is best to come to such tests in the morning – a night’s rest will guarantee an adequate break, and the results will be more reliable. However, if the date of the examination falls in the evening or afternoon, eat an easily digestible, small meal in the morning, remembering to keep a six-hour break.

Before the test, do not smoke, drink coffee, chew gum or take medicines that you normally take (unless your doctor tells you otherwise).

How to prepare a child for an fasting blood test?

In the case of children, the matter is more complicated. In toddlers, it is difficult to keep a long distance from the meal. Much depends here on the age of the child.

Babies do not need to be tested without food, however, they should not be fed just before blood is drawn. Children up to 3 years of age (if necessary) can eat a light meal before the test.

The only exception is a glucose test, before which it is forbidden to eat anything, because the result will be disturbed. Older children are treated as adults in the case of examinations, i.e. they should be fasting, and they can eat their last meal the previous day in the evening.

What tests should be done on an empty stomach?

  • Complete blood count
  • erythrocyte sedimentation rate
  • reticulocytes and platelets
  • bleeding time + coagulation
  • total protein
  • albumin
  • bilirubin with fractions
  • liver tests (AST, ALAT transaminase
  • lipidogram (total cholesterol and its HDL and LDL fractions)
  • triglycerides
  • urea
  • Creatinine
  • uric acid
  • glucose
  • sugar curve
  • iron
  • calcium
  • phosphorus
  • magnesium
  • sodium
  • Potassium
  • Chlorides
  • Thyroid
  • Hormones
  • c-peptide
  • Iinsulin
  • hbA1C (glycated hemoglobin),
  • ferritin
  • vitamin B12
  • folic acid
  • sex hormones
  • aldosterone
  • cortisol
  • viral diseases (jaundice, toxoplasmosis, mononucleosis, pertussis, chickenpox, etc
  • monitoring the concentration of certain drugs.

Blood tests should be done on an empty stomach, because the food consumed stimulates the release of certain hormones and enzymes, which affects changes in blood parameters



4th Professional Medical Student. Karachi Medical and Dental College.

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