Free NBME Shelf Exam Sample Questions

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Your Free NBME Sample Question

A 42-year-old nurse is coming into your clinic for follow-up of an abnormal purified protein derivative (PPD) test result. She was told she had an area of induration measuring 7 mm. She denies any contact with tuberculosis patients and denies any personal history of tuberculosis. She has never had a positive PPD in the past. She does have a history of lupus for which she follows with a rheumatologist. Her symptoms have been very well controlled on an immunosuppressing drug for the past few years. She is otherwise healthy with no other complaints today. On physical examination, her temperature is 37.1°C (98.8°F), pulse is 64/min, respirations are 14/min, and blood pressure is 122/68 mm Hg. The remainder of her physical examination is largely unrevealing. Her chest radiograph is shown in the attached image.

Which of the following would be the most appropriate next step in the management of this patient?

• A. Combination therapy with rifampin, isoniazid, pyrazinamide, and ethambutol
• B. Three months of isoniazid
• C. Four months of rifampin
• D. No treatment necessary as her purified protein derivative test is not considered positive
• E. Repeat the purified protein derivative test within 1 to 2 weeks

Incorrect Answer Explanations

Answer A: A combination of therapy with rifampin, isoniazid, pyrazinamide, and ethambutol would be indicated if the patient had active tuberculosis. Active TB would be confirmed with active disease found on a chest x-ray, which is the most appropriate initial test to perform if suspecting active TB. Findings are typically in the upper lobes. An image of an x-ray with signs of active TB disease is shown below. Compare this “positive” chest x-ray shown below, to the attachment in the question stem, which is “negative” or normal. Notice the difference of the infiltrate in the right upper lobe (demonstrated by the white arrows), as well as a cavitary lesion in the right upper lobe (demonstrated by the black arrows).

If treating active disease, all 4 medications are maintained for the first 2 months at which point ethambutol and pyrazinamide are discontinued. At that time, the rifampin and isoniazid are continued for the next 4 months for a total of 6 months of treatment. Treatment may be extended to 9 months in the setting of meningitis, osteomyelitis, miliary tuberculosis, or when pyrazinamide cannot be used (as in pregnancy). Active TB classically presents with fever, cough, sputum production, weight loss, hemoptysis, and night sweats.

In the above case, active TB can be excluded as her chest x-ray is normal and she is asymptomatic, making this choice incorrect.

Answer B: Three months of isoniazid would not be the correct answer in this case. A 9-month regimen of isoniazid or a 4-month regimen of rifampin are both viable options for treating latent tuberculosis. A three-month regimen of isoniazid, though, is not long enough to be fully effective and would likely lead to further antibiotic resistance, which is already an issue in treating tuberculosis. One of the main reasons why many clinicians are now favoring rifampin is that the recommended treatment course with isoniazid is so long that patients have trouble with compliance and there is also higher risk for hepatotoxicity with isoniazid. Because this course of isoniazid is not long enough, it is incorrect.

Answer D: This patient’s PPD test is considered positive because of her immunosuppression, making her cutoff for a positive PPD test 5 mm. As she is a health care worker, if she was not immunosuppressed, the area of induration must be at least 10 mm to be considered positive. The patient in question is a special case, though, considering she is on an immunosuppressing drug. As her cutoff for a positive PPD is 5 mm, and she has surpassed that, further management is necessary, which makes this answer incorrect.

Answer E: Two-stage PPD testing is necessary in patients who have never had a PPD test before. It is also often done when screening healthcare workers who have a negative initial test. While the above patient is a health care worker, her initial test was positive. This means that treatment is indicated as opposed to retesting, making this answer incorrect.

Bottom Line

A patient with a positive purified protein derivative (PPD) skin test as indicated by the area of induration, with a negative chest radiograph to exclude active tuberculosis, should be initiated on rifampin or isoniazid to treat latent tuberculosis.

For more information, see:

Bastos ML, Campbell JR, Oxlade O, et al. Health system costs of treating latent tuberculosis infection with four months of rifampin versus nine months of isoniazid in different settings. Ann Intern Med. Published online June 16, 2020. doi: 10.7326/M19-3741

Carvalho I, Goletti D, Manga S, Silva DR, Manissero D, Migliori G. Managing latent tuberculosis infection and tuberculosis in children. Pulmonology. 2018;24(2):106-114.

Ai JW, Ruan QL, Liu QH, Zhang WH. Updates on the risk factors for latent tuberculosis reactivation and their managements. Emerg Microbes Infect. 2016;5(2):e10.

Image sources:

Active Tuberculosis Xray: Wikimedia Commons

Normal Chest Xray: Wikimedia Commons

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