Propidium monoazide and Xpert MTB/RIF to quantify Mycobacterium tuberculosis cells

Propidium monoazide (PMA) penetrates non-viable cells with compromised membranes. PMA has been proposed to improve the specificity of Xpert MTB/RIF (Xpert) for the detection of viable Mycobacterium tuberculosis. This study assessed the effect of PMA on Xpert cycle thresholds (CT) of M. tuberculosis made non-viable under antibiotic pressure. In vitro, we measured the difference between CT with and without PMA (ΔCT) in liquid cultures treated with one of six anti-tuberculosis drugs (isoniazid, rifampin, pyrazinamide, ethambutol, streptomycin, moxifloxacin) and found significant ΔCT only with isoniazid and ethambutol for pan-susceptible M. tuberculosis and only with ethambutol for extensively drug-resistant M. tuberculosis. In the clinic we assessed ΔCT in sputum samples collected from patients with pulmonary tuberculosis before and at regular intervals over 12 weeks after initiation of treatment. Before treatment start, estimated CT were 19.3 (95% CI: 17.1–21.4) and 19.8 (95% CI: 17.6–22.1) without and with PMA, respectively. Under treatment CT increased by 2.54 per √√day (95% CI: 1.38–3.69) without PMA and an additional 0.55 per √√day (95% CI: 0.37–0.74; p < 0.0001) with PMA. We conclude that PMA increases the specificity of Xpert for viable M. tuberculosis but the effect is small and dependent on the antibiotics used.