TB Blood Test: Why and How is it Done?

A TB blood test shows if you have tuberculosis (TB). This test is most commonly called Interferon Gamma Release Assay (IGRA). But let us first discuss, what is TB?

Tuberculosis is a very contagious bacterial infection that is an air-borne disease spread through respiratory droplets. It is possible that you may have inactive (latent) TB and not experience symptoms. Or you may have active tuberculosis with symptoms. Generally, patients with latent TB are not contagious.

You may get your IGRA results within 24 hours. An IGRA test is more specific and accurate than TB skin test.

You can undergo this blood test even if you are vaccinated against TB with a vaccine called Bacille Calmette-Guerin (BCG). This vaccination does not interfere with IGRA, but might interfere with the skin test, causing a false-positive result.

An important point to know is these tests do not tell you for sure whether you have active TB. Many people in the Philippines are infected with TB bacteria, yet do not develop symptoms. This is known as latent TB. These tests do not differentiate active TB from latent, and should be interpreted cautiously.

Why Is It Done?

Your doctor may recommend a TB blood test if you have recently come in contact with someone who has TB. They may also suggest this test if your doctor thinks you may have tuberculosis. 

Your doctor will suggest an IGRA if you see the following symptoms of TB:

Usually, tuberculosis affects your lungs. But it can spread to your other body parts such as your kidneys, spine, joints, and brain.

Your doctor may recommend this test if you:

• Have HIV or another health condition that weakens your immune system.
• Are a healthcare professional who needs this test as part of your facility’s infection control program.
• Need to start taking a drug that can suppress your immune system.
• Work or live in an area with a high rate of TB infection.
• Use illegal drugs.

Prerequisites

Before the TB blood test, your doctor will give a few instructions that you must follow. This helps to prevent false IGRA results. 

You must also tell your doctor or nurse if you:

• Use medicines that affect your immune system.
• Had TB before, came in contact with someone with TB, received the BCG vaccine, travelled or lived overseas.
• Have any immune-suppressing illness.
• Had a fever or infection in the past month.
• Have received any vaccinations in the past month.

Understanding TB Blood Test Results

You will receive your TB blood test results within 24 hours. Ensure you visit your doctor’s clinic after receiving your test results. Your doctor will do the reading for you and tell if your results are positive or negative.

Your test results might vary due to your age, gender, medical history, method of the test, and other factors.

You must know that your test results may not always indicate a health problem. Your health expert will be the best person to guide you after looking at your test results. 

Depending on your test results, your doctor will prescribe medications and recommend treatment.

Negative TB Blood Test Result

A negative test result indicates that you do not have tuberculosis or TB germs in the body.

Positive tuberculosis blood test result

If you receive a positive TB blood test result, this indicates that you have been infected with TB bacteria. Your doctor may suggest a few tests to determine if you have latent TB infection or active TB disease.

• Active TB or TB disease: This condition means you are contagious as TB bacteria are active in your body. 
• Inactive (Latent) TB: This condition means that you have TB bacteria which are not active and cause no symptoms. If you have latent TB, you are not contagious, although there are possibilities that you may develop TB in future.

According to your IGRA and other tests that help determine the type of TB, your doctor will recommend treatment and prescribe medication.

When Should It Be Repeated

Your doctor may ask you to repeat this test if he/she feels the results are false-positive due to any factor.

Ensure you tell your doctor if you have any tumors or are using any medicine that might affect your test results. Your doctor may recommend repeating a TB blood test if you have tumors that need treatments and medicines that can suppress your immune system.

Your IGRA test results may also be affected if you have conditions like HIV/AIDS or any other blood disorder.

They may also ask repeating this test to ensure the medication and treatment are working fine on your tuberculosis and administer future treatment.

Procedure for TB Blood Test

A TB blood test is a test for which you have to visit a pathologist’s lab to give a sample of your blood.

In the pathologist’s lab, the professional will ask you to sit on a chair and relax. The professional will clean the inside of your elbow with an antiseptic solution to kill possible bacteria around the area.

The professional will tie an elastic band around your upper arm, limiting blood circulation to swell your vein. A swollen vein will become prominent, helping the professional to draw blood easily.

Now, your doctor will gently insert the needle in the selected vein and draw a small amount of blood. Once collected enough blood to pass for analysis, the professional will remove the needle and clean the punctured area. The professional will also apply a bandage on it.

The lab technician will put the drawn blood into a labelled test tube or vial.

The professional will remove the elastic band tied around your upper arm and that’s it

This test hardly takes 5-7 minutes.

You may feel slight pain or bruise around the punctured area, but it will go away quickly. If you feel the pain for the next few days, you can take your doctor’s help.