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How Long to Fast Before a Blood Test

How Long to Fast Before a Blood Test

Fasting is instructed before a blood test for checking blood sugar, cholesterol, and lipid value. However, the fasting duration depends on the type of test performed for the candidate.
Ningthoujam Sandhyarani
Last Updated: Apr 22, 2018
Blood tests are performed with or without fasting. If the doctor mentions a fasting blood test, the first thing that comes to our mind is the duration for which to fast before the test and how important it is. Well! The duration of fast is not the same for all blood tests, and there is a medical objective behind starving for some hours before drawing out the blood sample.
Why Fasting is Important Before a Blood Test?
The biochemistry and chemical balance of blood vary according to recently ingested foods. For better understanding about the importance of fasting prior to a blood test, let's talk about ingestion of foods and their digestion. When we eat food, it gets digested yielding glucose, vitamins, minerals, water, and wastes. The useful nutrients get absorbed in the gastrointestinal tract; while some amount of glucose and lipids are released in the blood for distribution to the body parts. Thus, the sugar and lipid levels in the bloodstream are higher when measured after meal.
Two of the blood tests that are affected by recently consumed foods are blood sugar test and lipid profile (including low density cholesterol, high density cholesterol, and triglycerides). In order to find standard readings for sugar concentration and lipid level in the blood, fasting before a blood test is indicated by doctors. Comparison is made between the patient's readings and the normal levels, so as to find out whether the individual is at the risk of developing diabetes and heart diseases in the near future or not.
Fasting Duration
The starving time varies in accordance to the specific approach for which the blood work is being performed. In general, the fasting period for blood test is 8 hours, and the maximum duration does not exceed 12 hours. During the starving time, a candidate can drink water to reduce hunger pangs. Also, according to the main approach for blood test, certain beverages and current medications are allowed during fasting. All these blood test fasting rules are explained to the candidates at the time of consultation. The exact duration for the fast before a blood test is given below.
Fasting Period Before a Blood Sugar Test
For blood glucose test, 8 hours of fasting is enough. As mentioned before, blood sugar reading is higher after food consumption. The rise in value will depend on the type of food ingested and time interval after having meal. To avoid all these confusions, fasting for blood work is directed to the patients.
Fasting Period for a Cholesterol Test
The cholesterol screening test is done, if the doctor suspects the patient for heart and cardiovascular diseases. An individual participating in blood cholesterol test will be asked to remain without foods or drinks for 6-12 hours. Early morning is a convenient time for a fasting blood cholesterol test, so that the fasting period overlaps with sleep time.
Fasting Period for Lipid Panel
By lipid panel test, we mean testing for all types of lipids, high density cholesterol (HDL, the good cholesterol), low density cholesterol (LDL, the bad cholesterol) and triglycerides. The fasting duration for lipid panel is 10-12 hours, after which, the physician will draw blood sample for testing.
So, this was an overview concerning the importance of fasting before a blood test. Remember to follow the instructions to get accurate test results. In case, you fail to fast for the suggested period (say 1 or 2 hours less) for any reason, feel free to talk to the doctor. Most probably, the health specialist will postpone the blood test to a later date.