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

Summary 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 ( C T ) of M. tuberculosis made non-viable under antibiotic pressure. In vitro , we measured the difference between C T with and without PMA (Δ C T ) in liquid cultures treated with one of six anti-tuberculosis drugs (isoniazid, rifampin, pyrazinamide, ethambutol, streptomycin, moxifloxacin) and found significant Δ C T 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 Δ C T 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 C T 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 C T 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 M. tuberculosis but the effect is small and dependent on the antibiotics used.