Prospective cross-sectional evaluation of the small membrane filtration method for diagnosis of pulmonary tuberculosis

Abstract

Smear microscopy has suboptimal sensitivity, and there is a need to improve its performance since it is commonly used to diagnose tuberculosis (TB). We prospectively evaluated the diagnostic accuracy of the small membrane filtration (SMF) method, an approach that uses a vacuum manifold and is designed to concentrate bacilli onto a filter that can be examined microscopically. We enrolled hospitalized adults suspected to have pulmonary TB in Kampala, Uganda. We obtained a clinical history and three spontaneously expectorated sputum specimens for smear microscopy (direct, concentrated, and SMF), MGIT (mycobacterial growth indicator tube) 960 and Lowenstein-Jensen (LJ) cultures, and Xpert MTB/RIF testing. We performed per-specimen (primary) and per-patient analyses. From October 2012 to June 2013, we enrolled 212 patients (579 sputum specimens). The participants were mostly female (63.2%), and 81.6% were HIV infected; their median CD4 cell count was 47 cells/μl. Overall, 19.0%, 20.4%, 27.1%, 25.2%, and 25.9% of specimens tested positive by direct smear, concentrated smear, MGIT culture, LJ culture, and Xpert test, respectively. In the per-specimen analysis, the sensitivity of the SMF method (48.5%; 95% confidence interval [CI], 37.4 to 59.6) was lower than those of direct smear (60.9%; 51.4 to 70.5 [P = 0.0001]) and concentrated smear (63.3%; 53.6 to 73.1 [P < 0.0001]). Subgroup analyses showed that SMF performed poorly in specimens having a low volume or low bacterial load. The SMF method performed poorly compared to standard smear techniques and was sensitive to sample preparation techniques. The optimal laboratory SMF protocol may require striking a fine balance between sample dilution and filtration failure rate.

FIG 1

(A) Small membrane filtration (SMF) manifold (version 5.0) evaluated. The SMF manifold device consists of a cylindrical manifold connected to a single vacuum and uses separate control valves at each station for independent operation. (B) The prototype manifold shown contains 10 workstations, each fitted with a stainless steel screw-on port that holds an SMF filter apparatus. (C) Sputum processing and aliquoting protocol.

FIG 2

Study profile. SMF, small membrane filtration method; MTB, Mycobacterium tuberculosis; NTM, nontuberculous mycobacteria.

FIG 3

(A to D) Sensitivity of the SMF method compared to those of direct and concentrated AFB smear microscopy in a per-specimen analysis with MGIT culture as the reference standard. Only MGIT culture-positive specimens were included. Results show sensitivity by HIV infection status (n = 168; one HIV result was missing) (A), sputum volume (n = 168; one volume was missing) (B), sputum viscosity (visual determination) (C), and number of M. tuberculosis CFU in Lowenstein-Jensen (LJ) culture (n = 146; 23 specimens were negative or contaminated on LJ culture) (D).