A 36-year-old woman comes to the office for a tuberculin skin test prior to volunteering at her daughter's school, which is required by state law. The patient lives in the southern United States. She feels well and has never been tested for tuberculosis. The patient has no ill contacts and no underlying medical conditions. She was born in the United States, is not a health care worker, and has never been incarcerated. Two days after intradermal injection of the Mycobacterium tuberculosis purified protein derivative, there is a 12-mm induration at the injection site. What is the best next step in management of this patient?
Tuberculin skin test (TST) thresholds | |
Induration size | Patient population |
≥5 mm |
|
≥10 mm |
|
≥15 mm |
|
PPD = purified protein derivative; TB = tuberculosis. | |
Tuberculin skin testing (TST) is used to identify patients who have been exposed to Mycobacterium tuberculosis and may have latent tuberculosis infection (LTBI). This test injects M tuberculosis purified protein derivative (PPD) into the dermis on the inner surface of the forearm (Mantoux technique). Patients that have been exposed to M tuberculosis will develop a delayed hypersensitivity reaction in response to PPD exposure, leading to induration at the injection site 48-72 hours after administration.
The cutoff for a positive TST depends on patient risk factors. In those who are healthy with no risk factors who live in the United States (as in this case), the threshold for a positive TST is ≥15 mm induration. This threshold is higher than in some other countries (eg, the Canadian threshold for this population is ≥10 mm) due to greater likelihood of exposure to nontuberculous mycobacteria in the United States, which increases false-positive results at lower cutoff values.
LTBI testing is not generally recommended for patients at low risk for exposure but may be required by law (as in this case). In these individuals, the following is recommended:
When TST is negative (eg, <15 mm), no further management or work-up is recommended.
When TST is positive, patients should generally undergo repeat TST or an interferon-gamma release assay (IGRA) to exclude a false-positive result. Repeat testing is warranted in low-risk, low-prevalence settings; it is not recommended in high-risk or high-prevalence settings.
IGRA can be used for initial or repeat LTBI screening but may not always be available.
(Choice A) A detailed history, physical examination, and chest x-ray should be obtained in patients with a positive TST to rule out active TB. Patients with a positive TST and no signs of active TB infection (negative x-ray) have LTBI. Given her low pretest probability of TB, this patient's TST result <15 mm is considered negative.
(Choice B) Active pulmonary TB can be treated with isoniazid, rifampin, ethambutol, pyrazinamide for 8 weeks followed by isoniazid and rifampin for an additional 16 weeks. This patient has negative LTBI screening; she does not require treatment.
(Choices C and E) Isoniazid monotherapy (with pyridoxine added for those at high risk for neuropathy) and rifamycin-based regimens are first-line treatments for LTBI. Rifamycin-based regimens (eg, daily rifampin for 4 months) are generally preferred due to fewer adverse effects and shorter treatment duration, which improves adherence. This patient with a negative TST does not require further evaluation or treatment.
Educational objective:
Tuberculin skin testing can be used to identify patients with latent tuberculosis infection. In the United States, an induration size <15 mm is considered negative (ie, ≥15 mm is considered positive) in healthy patients with a low likelihood of tuberculosis infection.