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Any induration
15%
6/39
6 mm
31%
12/39
11 mm
26%
10/39
16 mm
23%
9/39
21 mm
0%
0/39
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Because this patient does not meet any specific criteria for smaller minimum values of induration (see Incorrect Answers), 15 mm is the cutoff. PPD remains the primary investigative tool for latent tuberculosis (TB) infection (LTBI). Placement involves a 0.5 mL injection intradermally on the volar surface of the arm. The transverse induration is measured at 48-72 hours. Of note BCG vaccinations render a patient's PPD positive for a least one year. Patients with a negative reaction with negative controls (typically Candidia) implies anergy from immunosuppression, old age, or malnutrition, and this does not rule out TB. According to the CDC, there are 2 options for treatment of LTBI with INH: a 9-month regimen and 6-month regimen. When the two are compared, the 9-month regimen is regarded as being more efficacious while the 6 month regimen may be more cost-effective and result in greater adherence by patients. Regardless of the regimen that is chose, patients should be strongly counseled to adhere to latent TB treatment for at least 6 months. Arias discusses recent advances in the diagnosis of tuberculosis, principally in vitro quantification of the immune cell response, the interferon gamma release assays (IGRA). In contrast to the PPD test, IGRAs have been used in the detection of TB exposure with patients with immune system alterations (HIV, rheumatologic diseases). As it currently stands, the recommendation is that IGRAs are a complement, not a substitution for PPD testing. Illustration A demonstrates a positive PPD result. Of note, it is difficult to determine from a photograph as positivity is not determined by the area of erythema but rather the diameter of the induration. Incorrect Answers: Answer 1: This is not true in any circumstance. Answer 2: Reaction > 5 mm is considered positive in patients with HIV, close TB contacts, or CXR evidence of disease. Answer 3: Reaction > 10 mm is considered positive in healthcare workers, homeless patients, immigrants from or living in developing nations, IVDUs, chronic disease patients, or residents of health or correctional institutions. Answer 5: Anyone with a reaction > 15 mm is considered positive.
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