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List of Foods to Avoid Eating Before a Blood Test

In the clinic, after the physical examination if the doctor suspects a disease in an individual or else to get to know the root cause of any particular condition, the doctor advices blood tests. Blood tests are the most frequently performed investigations.1 In the pathological laboratory, the technicians take the blood from a vein in the arm using a needle.

For some blood tests, just one prick in the finger will do.2 Usually blood tests are done in the morning before taking the food. The patient must be in fasting at least for eight to twelve hours. Fasting of more than twelve hours can reduce the reliability on the result.

List of Foods to Avoid Eating Before a Blood Test

List of Foods to Avoid Eating before a Blood Test

The following list of foods should be avoided before a blood test:

  • Coffee and caffeinated drinks have their effects on digestion and in turn change the result of blood test.
  • Alcohol should be avoided as it alters the fat and blood sugar levels.
  • Herbal formulae can also alter the result of these tests and hence should be avoided.
  • Any food supplements given for iron, vitamins should be avoided.
  • Chewing gum can elevate digestion hence has influence on blood test and must be avoided.

In case if the person is on medication for some illness, it is left to doctor’s decision to advice the person whether to avoid the medicine 12 hours prior to blood test or allow taking. Since the doctor knows the kind and dosage of medicine, he/she can interpret the results of blood test even if the patient has taken the medicine before going to blood test.

Artificial sugars are completely to be stopped because they have adverse effect on the blood test for measuring glucose level.

Foods rich in fat should not be eaten at least 24 hours prior to blood test as they are slowly digested. It is also found that these foods have their impact on blood test even if they are taken 12 hours before and the patient is fasting after that.

Smoking also affects the result of the blood test. Hence, smoking is also prohibited, if he/she is told to have a blood test. Even exercise fastens the metabolism and changes the concentration of different substances like glucose, fat, etc. Therefore, exercises are not to be performed before blood test.

Blood Tests Which Require Fasting or Restriction of Food

Some blood tests require fasting. It means that the patients who are undergoing blood test are not allowed to eat anything or drink any other liquid than water. All blood tests do not expect fasting. The test during which fasting is needed are listed here:

  • Fasting Blood Sugar: During Fasting Blood Sugar test, the patient is said to come in empty stomach that means he/she should have the food before 8 to 10 hours. This gives the correct measurement of sugar (glucose) level in the blood.
  • Test for Iron: In fasting condition, the level of iron can be perfectly measured and any variation can be easily identified. In case if a patient is already under medication with iron supplements, it is also avoided before 24 hours of test otherwise may alter the result.
  • GGT: It is a test conducted to diagnose any disease related to liver. Gamma-glutamyl transferase is an enzyme which is found in the liver and needed for effective functioning of liver. Normally fasting is not required here but if the patient has habit of consuming alcohol or have smoking habit, doctors advise not to do so before 24 hours of testing.
  • Cholesterol Blood Tests: Cholesterol includes HDL cholesterol (usually called as good cholesterol) test, LDL cholesterol (also referred to as bad cholesterol) test and test for triglycerides in the blood. The patient is asked to fast for about 9 to 12 hour prior to test. Some new studies tell that fasting is not needed while testing for triglycerides.
  • Other Blood Tests needing Fasting: Test for renal function panel, vitamin B12 and kidney function and electrolyte balance need fasting state.

How is Blood Test Done?

The workers in the laboratory collect the blood sample from the patient with the help of a needle or finger prick. Blood from the finger is collected over a slide or if it is drawn from the arm vein, is collected in the syringe and immediately transferred to respective apparatus to conduct different tests. Drawing the blood from the vein does not usually hurt the patient but cause a very small discomfort. In most of the cases, blood drawing does not cause any severe reaction. Before the procedure, the technicians have to assess the patients whether they are:

  • Aged
  • Small children
  • Afraid of pricking
  • Nervous
  • Not able to see the blood and
  • Already having less volume of blood.

In case, if the patients are nervous or afraid of pricking, the attention of them is distracted by talking with them. The technician himself or his/her assistants can do this job. Even the people who accompany the patients sometimes are allowed to draw the attention of them in case the patients are young children.

After the procedure, the lab worker applies pressure on the spot of pricking. Sometimes the patient is told to do this. A small bandage or a plug of cotton is kept on the place to avoid swelling or to stop bleeding. Depending upon the test suggested by the doctor, the professionals conduct the various tests. Hence the time taken for the report cannot be same for all kinds of blood tests. Procedure of taking the blood from patients’ body would take usually less than 3 minutes. Results can be expected as early as few minutes or would take a few hours to even a few days.

What is the Use of Blood Test?

There are a number of uses of doing blood test. Though blood test alone cannot help a doctor to diagnose many diseases it is used as a first step of diagnosis. The other important uses are as follows:

  • The risk factors of heart abnormalities or diseases can be found out.
  • The normal functioning of the organs like heart, liver and kidneys can be assessed.
  • Deficiency or excess of most of the hormones in the blood can be measured.
  • To check the working of medicines on the body.
  • The diseases like anemia, AIDS, cancer, diabetes, etc can be diagnosed.
  • To know the rate of sedimentation of erythrocytes inside the body.
  • To measure the clotting time of blood.

Complication of Blood Test

Usually there would not be any complications and if present are negligible. Commonly expected risks are a small bruising or swelling at the site of pricking, bleeding to a very little extent and a short time discomfort.

Results of a Blood Test

The results of the test are usually expressed in numerical values. They suggest the presence of any disease or show any kind of unwanted depositions, excretory substances in the blood. Here are the major blood tests to be done to detect the illness.

CBC (Complete blood count) which involves the measurement of RBC, WBC, platelets, mean corpuscular volume, hemoglobin, hematocrit

Lipoprotein which includes total cholesterol level, HDL and LDL cholesterol levels

Blood glucose level.


Nowadays most of the blood tests do not require any restriction of normal diet except in some particular tests like testing for blood glucose or fat. In these, strict avoidance of sugar and fat is required. Other than specific tests like this, normally fasting is not required for many tests. Alcohol and smoking for any instance is prohibited. Overlooking the guidelines given by the health professionals leads changes in the results which intern guides to wrong diagnosis and wrong medication.


  1. Venipuncture – the extraction of blood using a needle and syringe”. Retrieved June 21, 2012.
  2. MacLennan CA, van Oosterhout JJ, White SA, Drayson MT, Zijlstra EE, Molyneux ME (July 2007). “Finger-prick blood samples can be used interchangeably with venous samples for CD4 cell counting indicating their potential for use in CD4 rapid tests”. AIDS. 21 (12): 1643–5.
Sheetal DeCaria, M.D.
Sheetal DeCaria, M.D.
Written, Edited or Reviewed By: Sheetal DeCaria, M.D. This article does not provide medical advice. See disclaimer
Last Modified On:November 11, 2022

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