Fasting before certain medical tests is usually required to obtain accurate and reliable results. The purpose of fasting is to ensure that the test results are not influenced by recent food intake, as it can affect the levels of certain substances in the body, such as glucose, lipids, and hormones. Fasting allows the healthcare provider to get a baseline measurement of these substances, which can aid in the diagnosis and monitoring of various medical conditions.

Here are a few examples of medical tests that typically require fasting:

1. Blood glucose test: Fasting blood glucose test is commonly performed to diagnose diabetes or monitor blood sugar levels. Fasting for at least 8 hours prior to the test helps to ensure accurate results, as consuming food can raise blood sugar levels and affect the interpretation of the test.

2. Lipid profile: A lipid profile measures cholesterol and triglyceride levels, which are important indicators of cardiovascular health. Fasting for 9-12 hours before the test is usually recommended to obtain accurate results, as recent fat intake can temporarily elevate lipid levels.

3. Certain hormone tests: Hormone tests, such as thyroid function tests or cortisol levels, often require fasting. For example, measuring thyroid-stimulating hormone (TSH) levels is crucial for diagnosing thyroid disorders. Fasting is necessary as food intake can affect the release and regulation of hormones, potentially leading to misleading results.

Fasting guidelines may vary depending on the specific test and the healthcare provider's instructions. It is important to follow the fasting instructions provided by your healthcare professional to ensure accurate test results.

References:
1. American Association for Clinical Chemistry. (2021). Fasting Blood Tests. Retrieved from https://labtestsonline.org/articles/fasting-blood-tests
2. National Institute of Diabetes and Digestive and Kidney Diseases. (2020). The A1C Test and Diabetes. Retrieved from https://www.niddk.nih.gov/health-information/diagnostic-tests/a1c-test
3. American Heart Association. (2021). Cholesterol Tests. Retrieved from https://www.heart.org/en/health-topics/cholesterol/about-cholesterol/why-cholesterol-matters/cholesterol-tests