Diagnostic Accuracy of the Small Membrane Filtration Method for Diagnosis of Pulmonary Tuberculosis in a High-HIV-Prevalence Setting

Abstract

g Sputum acid-fast bacilli (AFB) smear microscopy has suboptimal sensitivity but remains the most commonly used laboratory test to diagnose pulmonary tuberculosis (TB). We prospectively evaluated the small membrane filtration (SMF) method that concentrates AFB in a smaller area to facilitate detection to improve the diagnostic performance of microscopy. We enrolled adults with suspicion of pulmonary TB from health facilities in southwestern Uganda. Clinical history, physical examination, and 3 sputum samples were obtained for direct fluorescent AFB smear, SMF, Xpert MTB/RIF, and MGIT culture media. Sensitivity and specificity were estimated for SMF, AFB smear, and Xpert MTB/RIF, using MGIT as the reference standard. Direct acid-fast bacilli (AFB) smear microscopy of unconcentrated sputum has been the mainstay of tuberculosis (TB) diagnosis for more than a century, especially in resource-constrained settings, where more than 90% of the cases and TB-related deaths occur (1). Direct AFB smear microscopy is inexpensive, is specific in high-TB-prevalence settings, and can rapidly identify the most infectious patients (i.e., sputum AFB positive) (2, 3). However, sputum AFB has suboptimal sensitivity, particularly in populations with a low Mycobacterium tuberculosis bacillary load in sputum (i.e., paucibacillary TB), such as children, those with extrapulmonary TB, and persons with HIV infection (4). Overall, the diagnostic sensitivity of the direct sputum AFB smear ranges between 30% and 75%, often leading to delayed diagnosis, continued transmission, and poor outcomes (5, 6).Several methods have been evaluated to increase the sensitivity of smear microscopy. These include the use of chemicals, such as sodium hydroxide (NaOH), N-acetyl-L-cysteine (NALC), or sodium hypochlorite (bleach), to liquefy and concentrate sputum (7-9), use of light-emitting diodes to improve visualization (10, 11), and physical measures, such as centrifugation or gravity sedimentation (12, 13). A systematic review showed that sputum centrifugation plus a chemical (NALC or NaOH) can improve the yield of smear microscopy up to 39% (14). However, centrifuges are expensive, require electricity, and are biosafety level 3, as they carry a potential risk of aerosolization, and some bacilli may be lost in the supernatant. In general, given the widespread use and operational benefits of smear microscopy, any technology that increases its yield would be of great value.The small membrane filtration (SMF) is a novel method that concentrates bacilli from clinical samples by filtrating the sputum digests through a 25-mm-diameter polycarbonate membrane filter (15). After filtration, the membrane is fixed to a glass slide that can be stained and examined for AFB with standard methods. In the initial proof-of-concept studies that used individual filters connected to a rudimentary, plastic-made vacuum manifold, SMF showed an absolute incremental yield of up to 40% compared to direct smear, but the filtration failure rate (5% to 20%) was unac-

Effect of previous treatment and sputum quality on diagnostic accuracy of Xpert® MTB/RIF

“…13 A detailed description of the study population, experimental methods and results of the parent study are published elsewhere. 13 Participants and clinical data We enrolled adults with suspicion of pulmonary TB admitted to the Mbarara Regional Referral Hospital, Mbarara, Uganda, or attending out-patient clinics of the Municipality Health Centre in Mbarara, Uganda. HIV-infected and non-HIV-infected presumptive TB patients were enrolled independently.…”

“…11 Spot samples were randomized in a 1:1 ratio using a block size of 4; one sample was tested using Xpert and one by the experimental smear method. 13 A direct smear was performed on the specimen used for Xpert before decontamination of the sample using N-acetyl-L-cysteine (NALC)/sodium hydroxide solution and then processed for culture and Xpert. The other two samples were decontaminated using NALC, and then processed for AFB smear microscopy and culture ( Figure 1).…”

Detection and Quantification of Mycobacterium tuberculosis in the Sputum of Culture-Negative HIV-infected Pulmonary Tuberculosis Suspects: A Proof-of-Concept Study

“…The selection of the study population was solely based on when the TOP TB assay was ready for clinical testing (October 2013) rather than selection bias, as shown by the results of the parent study that included the entire population of HIV-infected patients. [ 24 ] The lack of clinical follow-up of subjects limits the clinical interpretation of certain results. For example, TOP was positive in numerous patients with a current or past history of treatment for TB, which complicated the clinical interpretation of negative cultures; interestingly, a similar phenomenon has recently been described with Xpert MTB/RIF, although a discrepant analysis was not performed.…”

“…Participants for this study were enrolled into a prospective cross-sectional study designed to independently evaluate the diagnostic accuracy of a new AFB smear microscopy method [ 23 ] and Xpert MTB/RIF, with liquid media culture (manual MGIT 960) as the reference method. [ 24 ] From September 4 th 2012 to April 11 th 2014, the parent study enrolled 1,047 (737 HIV-infected and 310 HIV-uninfected) consecutive TB suspects admitted to the wards or attending any of the outpatient clinics of the Mbarara Regional Referral Hospital or the Municipality Health Centre in Mbarara city. Eligible participants were adult (≥18 years), TB suspects (≥2 weeks of cough + at least one other symptoms of TB) [ 25 ] willing to follow the study protocol.…”

“…Participants had a standardized TB evaluation, HIV testing and provided three spontaneously expectorated (≥2 mL) sputum samples that included one early morning and two spot samples in a 24-hr period. One of the spot samples (selected by randomization) [ 24 ] was tested for direct AFB smear, Xpert MTB/RIF and culture.…”