You may also be told not to smoke before your test. For more information about a wider range of blood tests, go to Lab Tests Online. Page last reviewed: 25 February Next review due: 25 February Home Common health questions Operations, tests and procedures Back to Operations, tests and procedures. A person will typically fast overnight and do the test early in the morning. Fasting helps ensure that the blood test records an accurate measure of fasting blood sugar levels.
The results help a doctor diagnose or rule out diabetes. Cholesterol is a fatty substance in the blood. High cholesterol can lead to an increased risk of certain health conditions. Blood cholesterol tests, also known as lipid profiles, assess the quantities of fats in the blood. The different fats tested for include:. The amounts of these fats will increase if a person has recently eaten food. That is why people are asked not to eat for 9—12 hours before the test, which helps give an accurate profile of the amounts of these fats in the blood.
Research has suggested that fasting may not be necessary before all cholesterol and triglyceride tests. However, people who are having these tests should refrain from drinking alcohol for 24 hours before the test. It is always best for individuals to check with a doctor to see whether these new guidelines apply to them.
A gamma-glutamyl transferase GGT test helps diagnose liver disease. GGT is an enzyme in the liver that helps it work effectively. A person may need to avoid all eating and drinking for 8 hours before the test, as well as avoid drinking alcohol and taking some prescription medications.
An iron blood test measures the levels of the mineral iron in the blood. This test helps identify conditions that are caused by a lack of iron in the blood, such as anemia. Iron is contained in some types of food and is absorbed very quickly from food into the blood.
Therefore, if a person eats food before the iron blood test, the results may show inflated levels of iron. To ensure accurate results, a person should fast from midnight the night before the test until after the test is done. When people eat food and drink alcohol, the food and liquid get broken down in their stomach and absorbed into the bloodstream.
This can affect the levels of certain substances in the blood, such as blood glucose or cholesterol. For correct diagnosis of these conditions, it is important that a person fasts.
Eating or drinking before the test may raise the levels of a particular substance in the blood, leading to inaccurate results. Affected results could then lead to an inaccurate diagnosis. If you forget and eat or drink something, call your doctor or lab and ask if the test can still be done. They can then tell you if you need to reschedule your test. Be honest with your doctor or lab technician if you mistakenly ate or drank.
Most fasting lab results are available for your doctor in about 24 hours. However, some tests may take longer to complete. The lab will provide a report to your doctor, and your doctor will use the report to help with next steps in your diagnosis and treatment.
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