Last updated on 25th Oct 2021 - By Dwayne Michaels

Does drinking water before blood test affect results?

While not all lab tests require it, some tests require fasting for the most accurate results.


Most of the time, this means you’ll be asked to stop having any food or liquid between eight to 24 hours before your lab test or bloodwork, depending on the test.

Fasting is a standard procedure for patients to conduct prior to a blood draw. Fasting before a blood test helps improve the accuracy and reliability of the test. Accurate test results are a critical component of diagnosing various health conditions and monitoring the effectiveness of treatment plans.

Why do some tests require fasting?

Nutrients and ingredients in the food and beverages you consume are absorbed into your bloodstream and could impact factors measured by certain tests.

Fasting improves the accuracy of those tests. For example, if a patient is hypoglycemic, eating foods before a blood test could spike their blood sugar levels, inhibiting the physicians ability to detect abnormalities and correctly diagnose their condition.

Can I Drink Coffee While Fasting?


Any beverage other than water should not be consumed before a blood test, unless approved by your healthcare provider.

Alcohol, soda, and juice contain high quantities of sugar, which can interfere with many common test results. It’s fine to drink water before a blood test, unless you’re instructed otherwise by your doctor.

Most other blood tests, like hemoglobin levels, renal function, liver function, thyroid hormones, sodium and potassium levels dont need to be done on an empty stomach because they do not change before or after meals to any meaningful degree.

These tests typically require fasting:

Blood test

1. Blood glucose test :

Typical fasting time for Fasting blood glucose test is atleast 8 hours. Blood glucose testing is the process used to measure the concentration of glucose in your blood.

Many types of glucose tests exist and they can be used to estimate blood sugar levels at a given time or, over a longer period of time, to obtain average levels. The body needs glucose for energy, and glucose comes from the food we eat. The fasting plasma glucose test is the preferred method for diagnosing diabetes, because it is easy to do and less expensive than other tests.

2. Liver function test :

Your doctor may ask you to fast for at least 8 hours before Gamma-glutamyl transferase or GGT test. GGT measures the amount of the enzyme GGT in your blood.

GGT is predominantly used as a diagnostic marker for liver disease. Latent elevations in GGT are typically seen in patients with chronic viral hepatitis infections often taking 12 months or more to present. Elevated serum GGT activity can be found in diseases of the liver, biliary system, pancreas and kidneys.

GGT is elevated by ingestion of large quantities of alcohol. However, determination of high levels of total serum GGT activity is not specific to alcohol intoxication.

3. Lipid profile :

Lipid profile is a panel of blood tests that serves as an initial screening tool for abnormalities in lipids, such as cholesterol and triglycerides.

You may not need fasting if you’re younger than 25 or if you require only a partial lipid panel or if your doctor is looking for a “non-fasting result. A total cholesterol reading can be used to assess an individual's risk for heart disease, however, it should not be relied upon as the only indicator.

The individual components that make up total cholesterol readingLDL, HDL, and VLDL—are also important in measuring risk. High cholesterol usually causes no signs or symptoms. A complete cholesterol test is done to determine whether your cholesterol is high and to estimate your risk of heart attacks and other forms of heart disease and diseases of the blood vessels.

4. Basic metabolic panel :

You may be asked to stop eating and drinking for 10 to 12 hours before you have this blood test. The basic metabolic panel test is a blood test that measure levels of certain substances in your blood. The basic metabolic panel (BMP) is a test that includes eight different measurements.

It provides information about your body’s energy use, which is known as metabolism. Usually, eight distinct measurements are included in the basic metabolic panel. This panel measures the blood levels of blood urea nitrogen (BUN), calcium, carbon dioxide, chloride, creatinine, glucose, potassium, and sodium.

5. Vitamin B12 test :

Typical fasting time for this test is 6-8 hours. Vitamin B12, also known as cobalamin, is a water-soluble vitamin involved in metabolism.

Vitamin B12 deficiency can potentially cause severe and irreversible damage, especially to the brain and nervous system. At levels only slightly lower than normal, a range of symptoms such as fatigue, lethargy, difficulty walking, depression, poor memory, breathlessness, headaches, and pale skin, among others, may be experienced especially in people over age 60.

Vitamin B12 is included in multivitamin pills; in some countries grain-based foods such as bread and pasta are fortified with B12. In the US, non-prescription products can be purchased providing up to 5,000 g each, and it is a common ingredient in energy drinks and energy shots, usually at many times the recommended dietary allowance of B12. The vitamin can also be a prescription product via injection or other means.

6. Iron tests :

A serum iron test measures how much iron is in your blood. Typical fasting time for these type of tests is 12 hours. Iron level can change, depending on how recently you ingested iron.

Your health care provider will likely have you do this test in the morning or after fasting. The primary purpose of the test is to check whether a person has abnormally high or low levels of iron in their blood, both of which can cause serious health complications. 65% of the iron in the body is bound up in hemoglobin molecules in red blood cells.

About 4% is bound up in myoglobin molecules. Around 30% of the iron in the body is stored as ferritin or hemosiderin in the spleen, the bone marrow and the liver. Small amounts of iron can be found in other molecules in cells throughout the body. None of this iron is directly accessible by testing the serum.

The test for serum iron uses blood drawn from veins to measure the iron ions that are bound to transferrin and circulating in the blood. This test should be done after 12 hours of fasting.

Are lab test results always right?

While they do have to meet very high standards, they can be wrong sometimes. For example, you might get a false positive (the results say you have the condition you were tested for, but you really dont) or a false negative (the results say you don’t have a condition, but you really do).

Lab test failures contribute to delayed or wrong diagnoses and unnecessary costs and care. For context, a 2014 study estimated that diagnostic errors happen about 12 million times per year in U.S. outpatients. This represents 1 in 20 adults. In case you think your blood test result doesn't make any sense, we recommend that you do the test again from HealthLabs as they perform testing using verified methods and they test blood only in the best CLIA certified labs.

Note : No insurance or doctor prescription is required.

HealthLabs also provide result interpretation. Their certified health specialists can walk you through your results and provide clarification if needed.

Find A Lab Near You

Besides Lab test failures, there are many other reasons for inaccurate blood results. Know the common reasons for inaccurate blood test results?

Dr. Dwayne Michaels
Guest Author
Dr. Dwayne Michaels
Guest Author