Which of the following diagnostic tests is definitive for TB?
A. Chest x-ray
B. Mantoux test
C. Sputum culture
D. Tuberculin test
Correct Answer: C. Sputum culture
The sputum culture for Mycobacterium tuberculosis is the only method of confirming the diagnosis. Mycobacterial culture is the gold standard for diagnosis. Mycobacterial culture should be performed on both the solid and liquid medium. Liquid media culture can detect very low bacterial load and is considered a gold standard. Culture essential for drug susceptibility testing.
Option A: Lesions in the lung may not be big enough to be seen on x-ray. A chest x-ray is indicated to rule out or rule in the presence of active disease in all screening test positive cases. In pulmonary tuberculosis, initial testing includes a chest X-Ray, sputum evaluation.
Option B: Skin tests may be falsely positive or falsely negative. The Mantoux reaction following injection of a dose of PPD (purified protein derivative) is the traditional screening test for exposure to Tuberculosis. The result is interpreted taking into consideration the patient’s overall risk of exposure. Patients are classified into 3 groups based on the risk of exposure with three corresponding cut-off points.
Option D: Note that a Mantoux test indicates exposure or latent tuberculosis. However, this test lacks specificity, and patients would require subsequent visits for interpreting the results as well as chest x-ray for confirmation. Although relatively sensitive, the Mantoux reaction is not very specific and may give false-positive reactions in individuals who have been exposed to the BCG-vaccine.
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