Why is one screened for tuberculosis prior to starting TNF blockers?

Tuberculosis (TB) is an infection caused by bacteria. It most commonly affects the lungs. When a patient of pulmonary TB coughs, the bacteria are discharged in the air. If someone inhales these bacteria, they then find a way into the lungs. Once in the lungs- the bacteria face the following-

  • The bacteria are killed by body’s immune system. They are destroyed & cannot harm the body.
  • They lodge in the body, overcome the immune system & cause an active infection- tuberculosis.
  • The immune system does not kill them but instead jails them (literally; it forms a granuloma—a jail wherein the bacteria remain in dormant- inactive form). So the bacteria are in the body, but are jailed in dormant state & do not cause harm. Anything that frees them from the jail can make them active & lead to an infection.

The bacteria remain in the granuloma jail for years in a dormant phase. They are inactive & do not cause an active infection while in this phase. This is called ‘Latent TB’. The person does not have an active infection & does not spread it to anyone.

TNF alpha is a cytokine (chemical in the body) that is required for maintaining the integrity of the granuloma & also for other defence mechanisms that keep these bacteria dormant. TNF alpha blockers (Remicade, Infimab, Inflectra, Enbrel, Etacept, Intacept, Benepali, Humira, Exemptia, Cimzia, Simponi) are used to treat autoimmune ailments like rheumatoid arthritis, ankylosing spondylitis.

If we start TNF blockers in a person who has latent TB, the granulomas would open up, setting the dormant TB bacteria free & thus leading to active TB infection. This is called for reactivation of TB. This is why, one is always tested for active/ latent TB before TNF blockers are thought of. Active pulmonary TB can be diagnosed by examination & X-ray of the chest along with sputum test. TNF blockers are not given if one has active TB.

Tests for Latent Tb include Mantoux test & IGRA (Interferon gamma release assay commonly known as the TB gold test). A positive test (either) would suggest presence of latent TB.

In case of latent TB, one needs to be treated before TNF blockers can be given. The treatment recommendations vary from country to country. TNF blockers can be given after 1-2 months of this treatment.

Thus the chance of reactivation of TB with TNF blockers is real but can be minimised by proper testing & treatment for latent TB.

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