A blood test is usually performed after an overnight fast. To check the blood components, your doctor would ask you to come in the morning, the next day, after fasting for at least eight hours overnight and then he would draw the blood. But this may not be the case every time, since fasting is not necessary before all blood tests. For example, a blood test for pregnancy does not require fasting. Blood tests, performed mainly to measure blood sugar and lipids, require fasting before the test. Fasting before blood tests is necessary if you want accurate results. The recommended fasting period for blood test is of about 8 - 12 hours. You should religiously follow the fasting instructions given by your doctor.
Fasting Before a Blood Test
Blood test fasting period is determined by the doctor, taking into consideration various health issues. The diagnosis is seriously affected, if the patient eats or drinks before the test. Foods and drinks consumed, can significantly alter your blood sugar and lipid levels. You must have read that blood composition and lipid levels in your body change every hour as you eat or drink. Some foods are digested fast while some are digested very slowly. Fasting minimizes the risk of drastic changes in blood composition and ensures that the basic sugar and lipid values in the blood test results would be correct. The main types of blood tests that require fasting are described below.
Blood Sugar Test: The fasting blood glucose test is a routine blood test which is performed to monitor blood sugar levels in diabetic patients. It also helps diagnose diabetes (high blood sugar levels) and prediabetes (slightly high blood sugar levels). People are requested to maintain an 8 - 12 hour fast (normally overnight) and not eat anything until the blood is drawn for testing, early in the morning. As the body does not make any glucose from the starch (as it does not get food), you get an accurate measurement of the normal blood sugar level.
Triglycerides Levels Test: The levels of triglycerides in the body shoot up by 15 to 20% after eating. This is the reason doctors suggest about 12 hours fasting before the blood test. Fasting is necessary to get the normal levels of triglycerides. Fasting ensures correct results. Some patients may require 14 hours fasting for blood test, if their triglyceride levels are expected to reach the normal levels slowly. Information about your health problems and routine medication helps determine the fasting period for blood test.
Cholesterol Levels Test: Blood test fasting period for cholesterol testing may vary from patient to patient. The suggested fasting period is 6 - 12 hours; but it depends on various factors. Sometimes, doctors suggest patients to avoid high cholesterol foods for a week, before the blood test. This does not mean that you have to follow a totally different diet. Drastic changes in diet can force the body to use the available cholesterol. This can lead to incorrect results.
Significance of Fasting Before Blood Test
As explained above, diagnoses of diabetes and prediabetes are based on the established blood sugar values that are found at standardized situations. Same is the case with lipid levels. The values which provide guidelines and help predict increased risk for a disease and urgency of treatment, are based on 'fasting' values for LDL and HDL cholesterol and triglycerides. For example, if the fasting blood sugar is greater than 125 mg/dl, the doctor may conclude that the patient has diabetes. So, if you don't fast and eat a snack bar; or drink a sweetened fruit juice before your test, your blood sugar level may naturally rise above 125 mg/dl. The doctor may conclude that you are a diabetic; when you are actually not. Similarly, if a blood test is performed after having breakfast or lunch, the cholesterol values could be elevated by as much as 20 points. The doctor will assume that you really need aggressive treatment. This will lead to a problematic situation. To avoid such confusions, blood test fasting rules should be strictly observed.
Remember, during the fasting period, smoking, alcohol consumption, chewing gum, etc. is not allowed. Some medications and dietary supplements influence blood test results, so you are expected to discuss with your doctor if you are taking any. You may drink or rather, you should drink plenty of 'plain' water before the test, to keep yourself hydrated. Do not hesitate to ask questions, your health care provider will clear all your doubts before the test.