A 35-year-old woman comes to the office due to anorexia and unintentional weight loss of 10 kg (22 lb) over the past 3 months. She has also had occasional cough, subjective fevers, and night sweats. The patient has no prior medical problems and takes no medications. She was previously homeless and used injection drugs but has not done so for 4 years and now has stable housing and employment. The patient does not use tobacco or alcohol. Temperature is 37 C (98.6 F), pulse is 90/min, blood pressure is 120/70 mm Hg, and respiratory rate is 16/min. Physical examination reveals inspiratory crackles over the right upper lung field. Chest x-ray reveals an infiltrate with small cavitation in the apical-posterior segment of the right upper lobe. Rapid HIV testing is negative. Three sputum smears are negative for acid-fast bacilli but tuberculin skin testing shows a 15-mm induration. Which of the following is the most accurate statement about the patient's current condition?

Question

A 35-year-old woman comes to the office due to anorexia and unintentional weight loss of 10 kg (22 lb) over the past 3 months.  She has also had occasional cough, subjective fevers, and night sweats.  The patient has no prior medical problems and takes no medications.  She was previously homeless and used injection drugs but has not done so for 4 years and now has stable housing and employment.  The patient does not use tobacco or alcohol.  Temperature is 37 C (98.6 F), pulse is 90/min, blood pressure is 120/70 mm Hg, and respiratory rate is 16/min.  Physical examination reveals inspiratory crackles over the right upper lung field.  Chest x-ray reveals an infiltrate with small cavitation in the apical-posterior segment of the right upper lobe.  Rapid HIV testing is negative. Three sputum smears are negative for acid-fast bacilli but tuberculin skin testing shows a 15-mm induration.  Which of the following is the most accurate statement about the patient's current condition?

Quizplus · Accepted Answer

Explanation: Sputum acid-fast bacillus (AFB) smear is the most rapid, least expensive, and widely used diagnostic test for pulmonary tuberculosis (TB). However, this test cannot be used to definitively rule in/out the diagnosis due to: Low sensitivity (45%-80%) - microscopic detection of AFB requires a high burden of organisms (>10,000 bacilli/mL), so false negatives are common Poor differentiation - cannot distinguish between tuberculous and nontuberculous mycobacterium (both are AFB positive), so confirmatory testing with mycobacterial culture or nucleic acid amplification (NAA) testing is required Given this patient's positive tuberculin skin testing (indicating likely Mycobacterium tuberculosis exposure) and characteristic TB risk factors and clinical manifestations, active pulmonary TB remains the most likely diagnosis. TB can be definitively established or excluded with one or both of the following: Mycobacterial culture - takes 2-6 weeks but is considered the gold standard for microbial confirmation and drug susceptibility NAA testing - highly sensitive (95%) and specific (98%), and results return rapidly (<48 hours); unlike AFB smears, NAA testing can distinguish between M tuberculosis and nontuberculous mycobacteria (Choices A and B) Positive tuberculin skin testing indicates TB exposure and necessitates further evaluation with symptom review and chest x-ray to determine if a patient has latent or active disease. Those with symptoms and/or characteristic x-ray findings (eg, cavitary infiltrate) are considered likely to have active pulmonary TB and require microbial confirmation. Due to the low sensitivity of sputum AFB smear, this test cannot be used to exclude the diagnosis. She now requires mycobacterial culture or NAA testing. (Choice C) Anergy to tuberculin antigen can be caused by immunosuppression (eg, HIV) or waning immunity and may lead to false-negative (not false-positive) tuberculin skin testing. (Choice D) This patient has risk factors for TB exposure, a cavitary infiltrate, and positive tuberculin skin testing; a diagnosis of active pulmonary TB is likely. Her AFB sputum testing is likely false negative; additional testing with mycobacterial culture or NAA is likely to confirm the diagnosis. Educational objective: Sputum acid-fast bacilli smear is a common initial diagnostic test for active pulmonary tuberculosis. Positive testing supports the diagnosis and may lend credence to empiric treatment. However, this test is limited by low sensitivity and a lack of distinction between tuberculous and nontuberculous mycobacterium, so it cannot be used to definitively rule in/out the diagnosis. Additional testing with mycobacterial culture or nucleic acid amplification is required for confirmation. References: Updated guidelines for using interferon gamma release assays to detect mycobacterium tuberculosis infection - United States, 2010. (http://www.ncbi.nlm.nih.gov/pubmed/20577159)

A 35-Year-Old Woman Comes to the Office Due to Anorexia