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What is a TB test?

A TB test is used to detect antibodies in blood against TB bacteria (Mycobacterium tuberculosis). It can be performed in two ways:

  • TB skin test  
  • TB blood test.

TB blood test involves interferon-gamma release assays (IGRAs). It is of two types:

  • TB gold test
  • TB spot test

TB is a bacterial disease caused by Mycobacterium tuberculosis, which usually infects lungs, causing symptoms of chronic cough, high-grade fever, weight loss and blood-stained sputum (haemoptysis). Occasionally, it also infects other organs, such as kidneys, meninges, bones and joints. A TB blood test checks for the presence of interferons produced against TB bacteria, which helps in screening for exposure to TB bacteria.

  1. Why is a TB test performed?
  2. How do you prepare for a TB test?
  3. How is a TB test performed?
  4. What do TB test results indicate?

A TB blood test is performed to evaluate exposure against Mycobacterium tuberculosis. TB blood test is also performed:

  • As a routine screening test in people who are at high risk of developing TB
  • In health-workers working in a high-risk area
  • In a person who is in close contact with an active TB patient (usually test is performed about 6-8 weeks later from the exposure)
  • If a person shows signs and symptoms of TB

The signs and symptoms of TB include:

A TB blood test helps not only in diagnosing active TB infection but also in detecting a latent TB infection. Extra-pulmonary TB infections (infection in joints, bones, kidneys, meninges etc.) can also be detected using this test.

Symptoms of extra-pulmonary TB are dependent on the organs affected. Some of these include:

  • Lymphadenitis
    • Persistently swollen lymph nodes
    • Pus formation in lymph nodes 
  • Intestinal TB
    • Abdominal colic
    • Vomiting
    • Fever
    • Weight loss
  • Meningitis
  • Pott’s spine
  • Renal TB
    • Haematuria (blood in urine)
    • Pyuria (pus in urine)
    • Dysuria (painful or difficult urination)
    • Fever

No special preparations are needed for the TB blood test, Mantoux test and AFB test.

The procedures followed in various TB tests are as follows:

  • TB blood test

This is a simple test where a healthcare provider collects a blood sample from a vein in the arm using a sterile needle. There is a minimal risk of light-headedness and bruising at the site of injection. However, at most times, these symptoms disappear quickly. Rarely, an infection may occur at the site of blood withdrawal.

  • Mantoux test

A small amount of tuberculin is injected in the lower part of arm. The individual is asked to return within 48 to 72 hours to measure the reaction to tuberculin.

  • AFB test

Three early morning sputum samples are collected on different days. A bronchoscope is used if the individual is not able to produce sputum. Gastric aspirates or washings are collected in children for this test.

Normal results:

  • TB Blood test: A negative IGRA signifies that a person does not have a TB infection.
  • Mantoux test: A negative Mantoux test indicates the absence of TB infection or disease. It can also denote that the person has not reacted to the test or has a latent TB disease or infection.
  • AFB test: A negative AFB smear indicates the absence of TB infection and mycobacteria in the body.

Abnormal results:

  • TB blood test: A positive IGRA indicates exposure to TB bacteria. It also indicates that the person might have latent, active or extra-pulmonary tuberculosis.
  • Mantoux test: A positive Mantoux test denotes the presence of TB infection in the body. Additional tests are recommended to confirm if the person has latent TB infection or disease.
  • AFB test: A positive AFB smear indicates the possibility of a mycobacterial infection in the body. A culture is performed to identify the species of mycobacteria and confirm the diagnosis of the infection.

A TB blood test (IGRA test) detects TB infection in its latent as well as active phase; however, the two phases cannot be distinguished with this test. A clinical judgement along with other investigations, such as sputum evaluation, chest X-ray or sputum culture, may be needed for clear discernment.

A false positive TB blood test is sometimes seen in persons who are exposed to other Mycobacterial species viz. Mycobacterium kansasii; In such cases, the medical history of the patient and other investigations (eg, X-ray chest) are used for confirming the diagnosis.

The Mantoux test may show positivity to Bacillus Calmette–Guérin (BCG) vaccine (vaccine given at the time of birth for preventing TB); however, a TB blood test does not show positive results to the BCG vaccine, making it more reliable over skin test. AFB testing can be used as a confirmatory diagnostic test after the blood and skin test.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational perspective and is in no way a substitute for medical advice from a qualified doctor.

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  1. Center for Disease Control and Prevention [internet], Atlanta (GA): US Department of Health and Human Services, Testing for TB Infection
  2. MedlinePlus Medical Encyclopedia: US National Library of Medicine; Tuberculosis
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