Question

A 35 year old man with a history of intravenous drug use presented to the local health clinic with complaints of a wet, persistent cough, fever, malaise(general discomfort) and anorexia. Over the past 4 weeks he states he has lost 15 pounds, has chills, night sweats and has noticed some hemoptysis. Chest radiographs revealed numerous patchy infiltrates throughout both lung fields, especially in the hilar and apical(upper) regions. Sputum cultures were positive for TB after a week of culturing. The patients additional blood work revealed he was HIV positive.

Answer 1

PPD stands for purified protein derivative test. This test is commonly known as the Mantoux test or the tuberculin sensitivity test for TB diagnosis. Basically it is used to check whether a person is infected with Mycobacterium tuberculosis.

Tuberculin purified protein derivative (PPD) is injected intradermally (that is into the dermis, the layer between the epidermis and hypodermis) inner side of the forearm. Based on the prior exposure to TB you either see a flat skin or an induration. That is

when you inject the tuberculin protein and you are not previously exposed to Mycobacterium tuberculosis. the macrophages present it to T cells. but the T cells do not recognize and move along, leading to the skin getting flattened

in scenario 2 when you inject the tuberculin protein and you are previously exposed to Mycobacterium tuberculosis. T cell known that this is a threat when presented by the macrophages. this expresses a hypersensitivity reaction which brings in more immune cells at the point due to the release of more number of chemokines. thus the is an induration seen. The induration is a positive for PPD and is checked after 48h post-injection. (Analysed differently: for high-risk patients like suspects of TB a 5mm induration is positive likewise for medium risk people like the ones coming from endemic region 10mm or more is positive and others its 15mm and more)

once a PPD test is positive the doctor goes for an X-ray or a CT scan to check whether the disease is active or latent.  as well as a sputum test that checks if Mycobacterium tuberculosis is present or not.

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