Pooling sputum samples for the Xpert MTB/RIF assay: a practical screening strategy for highly infectious tuberculosis cases

Abstract

The Xpert MTB/RIF test (Xpert) can help in the accurate screening of tuberculosis, however, its widespread use is limited by its high cost and lack of accessibility. Pooling of sputum samples for testing is a strategy to cut expenses and enhance population coverage but may result in a decrease in detection sensitivity due to the dilution of Mycobacterium tuberculosis (Mtb) by sample mixing. We investigated how the mixing ratio affected the detection performance of Xpert. We used frozen sputum samples that had been kept after individual Xpert assays of the sputa from Mtb-confirmed TB patients and non-TB patients. Our results showed that the overall sensitivity of the Xpert pooling assay remained higher than 80% when the mixing ratio was between 1/2 and 1/8. When the mixing ratio was raised to 1/16, the positive detection rate fell to 69.0%. For patients with either a high sputum Mtb smear score ≥ 2⁺, a time-to-positive culture ≤ 10 days, or an Xpert test indicating a high or medium abundance of bacteria, the pooling assay positivity rates were 93.3%, 96.8%, and 100% respectively, even at a 1/16 mixing ratio. For participants with cavities and cough, the pooling assay positivity rates were 86.2% and 90.0% at a 1/8 ratio, higher than for those without these signs. Our results show that the Xpert pooled assay has a high overall sensitivity, especially for highly infectious patients. This pooling strategy with lower reagent and labor costs could support TB screening in communities with limited resources, thereby facilitating reductions in the community transmission and incidence of TB worldwide.

Keywords: Active screening; Infectivity; Pooling assay; Tuberculosis; Xpert.

Fig. 1

Flow chart of the Xpert pooling assay of frozen samples from confirmed TB and non-TB patients

Fig. 2

Positivity rates of Xpert pooling assays for the diagnosis of tuberculosis. (a) All patients. (b) Patients with (n = 18) or without cavitation (n = 65 ). (c) Patients with (n = 70) or without cough (n = 16). (d) Patients with very low (n = 17), low (n = 26), medium (n = 17), and high (27) Mtb load defined by the CT values from Xpert. (e) Patients with negative (n = 26), 1⁺ (n = 10), 2⁺ (n = 10), 3⁺ (n = 9), and 4⁺ (n = 26) grade of smear. (f) Patients with < 5 (n = 8), 5–10 (n = 24), 10–20 (n = 27), 20–42 (n = 15), > 42 (n = 7) time-to-positive culture (TTP).