There are thousands of blood tests, and quite a few of them require a fasting blood specimen. The instructions for the test give a time frame such as overnight, nothing after midnight or something similar. The rules for fasting before a blood test can be complicated. There are rules that require specific foods to be eaten or avoided. Each test should come with specific instructions.
General Fasting Rules
A blood glucose test is the most commonly ordered fasting blood test. The fast is required to minimize the effects of food and drugs on the blood chemistry. The physician wants to eliminate outside influences by requiring a fasting state.
Eat a healthy dinner the night before, but do not eat or drink anything else until after the blood test. A healthy dinner would include protein, vegetables and complex carbohydrates. Desserts should consist of fruits, rather than sweets.
Schedule the test for early in the morning. After the test, normal eating and drinking can resume. Small sips of water are allowed during the fast. Brushing your teeth is permitted. Coffee, herbal teas and chewing gum are not allowed. Basically, nothing except water should be eaten or drunk during the fasting time frame. No alcoholic beverages should be consumed for 24 hours before the blood test. Continue to take prescription medicines unless directed not to by the physician ordering the tests. Taking vitamins and supplements are discouraged. Vitamin C is a frequent culprit in erroneous blood tests.
Special Fasting Rules
Some foods affect the body chemistry for several days. Some blood and urine tests are very sensitive and will detect the effects of these foods for a week or longer. These blood and urine tests come with very specific fasting requirements that should be followed exactly. For instance, if the instructions say to wait a week after eating avocados, and an avocado was eaten the day before the blood test, postpone the test until the instructions are met. If the doctor recommends that the test be done right away without meeting the requirements, be sure to inform the laboratory about the current fasting status.
Broken Rules
There are rules that are frequently broken because commonly taken drugs or foods are consumed without thought of the effects on a blood test. Common drugs include aspirin, alcohol, caffeine, acetaminophen, ibuprofen, cough syrup, vitamins and allergy medication. All of these items should be avoided during the fast before a blood test.
Pre-Fasting Rules
Some blood tests require that specific foods or drugs be consumed ahead of time in preparation for the test. For example, carbohydrate loading is encouraged before a glucose tolerance test, or GTT. The physician is looking at a timed assay that shows the body's use of glucose over a period of hours.
Always follow the instructions and rules for fasting blood tests. More details can be found on specific blood tests at LabTestsOnline.org, a public resource on clinical lab testing from the laboratory professionals who do the testing.
