Almost all of us have experienced the tuberculin skin test, also known as a PPD (purified protein derivative). This skin test has been the cornerstone of diagnosing exposure to tuberculosis for decades. When positive, we can be reasonably confident that the person has been exposed to Mycobacterium tuberculosis, the bacteria that causes TB, and that the tuberculosis bacteria are in the lungs of the skin test positive person.
Once inhaled, the M. tuberculosis remains in the lungs, usually contained by the person’s immune system. However, over the course of one’s lifetime, there is an approximate 15% chance of the bacteria overcoming this immune response and causing active tuberculosis. This usually is in the form of pulmonary tuberculosis, which can cause fevers, night sweats, weight loss, cough, production of sputum, hemoptysis (coughing up blood). Rarely, the bacteria can disseminate from the lungs and involve most any organ in the body. If a person needs an organ transplant or requires immunosuppressive therapy, these latent bacteria can start to grow and cause serious illness in the otherwise asymptomatic person with a positive PPD. When in the latent phase, the person is not contagious to any of his contacts. It is only when the bacteria start to grow and cause actual active infection and illness that the person becomes infectious to those around him.
Fortunately, if we can identify those who have been exposed, early enough, we can prescribe therapy to prevent the activation of active tuberculosis. The traditional therapy has been 9 months of daily isoniazid therapy taken daily (a total of 270 doses). This reduces the risk of activation of the bacteria within the lungs to close to 0. A new regimen has now received approval. This new approach consists of INH and rifapentine (an antibiotic active against M. tuberculosis), each given one day a week for only 12 weeks – a total of 12 doses in all! It is recommended that the 2 drugs be administered by health professionals, so that the patient can be observed taking the required pills, in other words, DOT, directly observed therapy. Missing even one dose of this weekly therapy may seriously degrade its ability to protect people from reactivating the latent bacteria in their lungs. This new regimen, in addition to being as effective as 9 months of INH alone, appears to have less toxicity, especially with respect to hepatitis, the most serious complication of taking INH.
We will be instituting this new regimen in those people with positive PPDs, who are eligible to take this shortened course of therapy for latent tuberculosis.
In a future entry, I will discuss the new blood tests for diagnosing tuberculosis exposure and how these should be used vis-à-vis the PPD.
Posted by DrSugar on 12/09/2011 at 11:15 AM