Prior to conducting some blood tests, doctors may ask patients to abstain from eating or drinking for several hours to ensure accurate test results. Unless they're given alternate instructions by their health care providers, patients should consume only water during the fasting period prior to a blood test.
While not all blood tests require a fasting period, fasting is often necessary before tests designed to detect blood sugar and cholesterol levels. A fasting blood sugar test, which is used to diagnose diabetes, checks the level of glucose in the bloodstream. Fasting cholesterol blood tests detect levels of triglycerides, good cholesterols and bad cholesterols. As diet can directly and immediately affect blood sugar and cholesterol levels, no food or drink should be present in the system when the test is conducted to ensure accurate test results. After the conclusion of the fasting period, which generally lasts 8 to 12 hours, the bloodstream should be free of anything that could adversely affect test results. During this fasting period, the only permitted beverage is water. The caffeine in coffee, tea and some sodas can affect test results by spiking blood sugar levels and stimulating the pancreas to produce more insulin. Drinking fruit juice and even chewing gum during the fasting period can also result in inaccurate tests.
Before scheduled blood tests, doctors should provide patients with specific information about fasting sessions, including the duration of the fast and what foods or beverages, if any, are permitted during this time. Patients should follow these instructions. Patients are often allowed to continue taking prescription medications during fasting periods, though doctors must be made aware of this. Often, doctors will ask diabetics to postpone scheduled insulin shots until after the test to avoid interfering with the results.