Fasting before diagnostic tests

Dr RK Manglem *

Fasting before a blood test or a radiological investigation is when people are asked not to eat or drink anything other than water before the test. Fasting is not always necessary before a test.

In fact, majority of the tests do not require fasting. But when fasting is required, even if it is for a short duration of time, it can be quite daunting for some patients. Understanding when and how to fast before a test can help reduce unnecessary worry.

Common blood tests that require fasting

Whether someone needs to fast or not before a blood test depends on the type of blood test or radiological investigation they are having. The following are the blood tests that require fasting:

Fasting blood sugar

A fasting blood glucose test can diagnose diabetes. A fasting blood glucose test measures levels of sugar in the blood to see if they are healthy.

It is important that a person has not had anything other than water for 8 to 10 hours prior to the test. Fasting helps ensure that the blood test records measure of fasting blood sugar levels. The results help a doctor to diagnose or rule out diabetes. It also helps in monitoring a patient already with diabetes.

Blood cholesterol tests or lipid profile

Cholesterol is a fatty substance in the blood. High cholesterol can lead to an increased risk for certain cardiovascular conditions. Lipid profile assesses the quantities of fats in the blood.

The different fats tested for include:
o HDL cholesterol, also known as “good cholesterol”.
o LDL cholesterol, also known as “bad cholesterol”
o Triglycerides.

The amounts of these fats will increase if a person has recently eaten food. So, people are asked not to eat for 9-12 hours before the test. Fasting before the test helps to give an accurate profile of the amounts of these fats in the blood.

There are some recent guidelines that suggest fasting is not necessary before all cholesterol and triglyceride tests. However, people who are going to have these tests are asked to refrain from drinking alcohol for 14 hours before the test. It is always best for individuals to check with their doctor to see whether these new guidelines apply to them or not.

Gamma-Glutamyl Transferase test

A gamma-glutamyltransferase test (GGT) test helps to diagnose liver disease. GGT is an enzyme in the liver that helps it to work effectively. Eating does not affect GGT levels, but drinking alcohol and smoking can. People having this test are asked not to consume alcohol or smoke for 24 hours before the test.

Vitamin B12 test

Vitamin B12 test measures how much of the vitamin is in the blood. The test helps diagnose a specific type of anaemia and other problems. Some medications can interfere with this test, and hence it is necessary to take the advice of the doctor about it before taking the test. Typical fasting time for this test is about 8 hours.

Why fasting is required before blood tests

When people eat food and drink alcohol, the food and liquid gets broken down in their stomach and absorbed into the bloodstream, which can affect the levels of certain substances in the blood, such as:
o Sugar
o Cholesterol and other fats
o Enzymes, such as GGT
o Vitamins, such as B12

Measuring levels of these substances is crucial to diagnose certain conditions, such as:
o Diabetes
o Anaemia
o High cholesterol
o Liver disease

Eating or drinking before the test may raise the levels of a particular substances in the blood that may interfere with the test results, which may lead to wrong diagnosis.

Common radiological investigations that require fasting

X-ray: Investigations for the abdomen, KUB, Barium Swallow, IVP series, etc. require that patients come with fasting before undergoing the procedure.

Ultrasound: Ultrasonography of the abdomen (whole or upper abdomen) require fasting. Patients are asked to fast at least 5-6 hours prior to the test. For abdominal ultrasound, patients are asked to drink ½ – 1 litre of water and to hold the urine till the bladder is full.

CT scans: CT scans of the abdomen require fasting. However, for CT scans for other investigations that require administration of contrast, some patients may be asked to fast in order to reduce the vomiting sensation.

MRI: Abdomen and pelvic scans require fasting.

Conclusion

Some people find it difficult to avoid food or drink during the mandatory fasting period before the test. This may happen because either they become too hungry or get the timings wrong. In such cases, it is a good idea to speak to the doctor to reschedule the test appointment. Tests without proper preparation interfere with the test results. That is why following the laboratory’s instructions before undergoing tests are very important.