TB Redux

By Peter Galvin, MD

Tuberculosis (TB) is a significant health issue worldwide, but here in the U.S., it is a more limited health issue. TB is classified into two types: active TB and latent TB. TB is highly contagious and is spread through the air by inhaling droplets an infected person emits when coughing. In most healthy people with intact immune systems, infection with the TB organism does not lead to active TB. Rather, the immune system sequesters, or “walls-off”, the nascent TB infection in the lung, thus giving the individual a latent TB infection (LTBI). A LTBI may convert to active TB if a person’s immune system becomes compromised, either by disease or immune system suppressing (or modulating) medications.

Immune system modulators are often prescribed to treat autoimmune diseases or prevent rejection of transplanted organs. Since suppressing the immune system can cause a LTBI to convert to active TB, patients must be screened for a LTBI prior to starting these medications. This is because, unlike active TB which usually causes a cough (often bloody), fever, and weight loss, latent TB has no symptoms. Screening, which is looking for a disease in a person without symptoms, can be done in several ways. Most people are familiar with a TB skin test, also called a Mantoux test. In this test, a purified protein derivative (PPD), based on the TB organism (or tubercle), is injected under the skin. If the patient’s immune system has been exposed to TB, swelling (a wheal) will appear at the injection site within 48 hours. A patient with a positive Mantoux must undergo additional testing, including a chest X ray and possibly a sputum test to look for the TB organism, to determine whether the patient has active or latent TB.

Most of the world, except for the U.S., uses a TB vaccine called Bacille Calmette-Guerin, or BCG. This vaccine will cause the patient to have a permanently positive Mantoux, which is why the U.S. doesn’t use it. The Mantoux, therefore, cannot be used to screen people born outside of the US. A relatively new test can be used in this case. It is a blood test that looks for antibodies to TB called the QuantiFERON-TB Gold In-Tube (QFT-GIT) test. BCG does not cause a positive QFT-GIT, however it is a blood test and not a pin prick. It does not require the follow up visit that the Mantoux does. Thankfully, TB is rare in the U.S., however screening should be done if a person is at higher risk of TB exposure or is at risk of their immune system being compromised by either disease or medications. Those at higher risk of TB exposure include people born in countries with high rates of TB infection, including Mexico, the Philippines, India, Vietnam, and China. Also, at risk are those who live in settings of close contact with others, such as homeless shelters or correctional facilities.

Finally, latent TB, if found, is usually not treated because of the potential toxicity of treatment. A mainstay of treatment is isoniazid (INH), which has a significant possibility of liver toxicity. It is thought that the potential toxicity of treatment outweighs the risk of latent TB converting to active disease.

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