Date Published: March 31, 2017
Author(s): Patrick Orikiriza, Dan Nyehangane, Daniel Atwine, John J. Kisakye, Kennedy Kassaza, Juliet-Mwanga Amumpaire, Yap Boum.
To confirm presence of Mycobacterium tuberculosis complex, some tuberculosis culture laboratories still rely on para-nitrobenzoic acid (PNB), a traditional technique that requires sub-culturing of clinical isolates and two to three weeks to give results. Rapid identification tests have improved turnaround times for mycobacterial culture results. Considering the challenges of the PNB method, we assessed the performance of the SD Bioline TB Ag MPT64 assay by using PNB as gold standard to detect M. tuberculosis complex from acid-fast bacilli (AFB) positive cultures.
The aim of this study was to determine the sensitivity, specificity and turnaround time of the SD MPT64 assay for identification of M. tuberculosis complex, in a setting with high prevalence of tuberculosis and HIV.
A convenience sample of 690 patients, with tuberculosis symptoms, was enrolled at Epicentre Mbarara Research Centre between April 2010 and June 2011. The samples were decontaminated using NALC-NaOH and re-suspended sediments inoculated in Mycobacterium Growth Indicator Tubes (MGIT) media, then incubated at 37 °C for a maximum of eight weeks. A random sample of 50 known negative cultures and 50 non-tuberculous mycobacteria isolates were tested for specificity, while sensitivity was based on AFB positivity. The time required from positive culture to reporting of results was also assessed with PNB used as the gold standard.
Of the 138 cultures that were AFB-positive, the sensitivity of the SD MPT64 assay was 100.0% [95% CI: 97.3 – 100] and specificity was 100.0% (95% CI, 96.4 – 100). The median time from a specimen receipt to confirmation of strain was 10 days [IQR: 8–12] with SD MPT64 and 24 days [IQR: 22–26] with PNB.
The SD MPT64 assay is comparable to PNB for identification of M. tuberculosis complex and reduces the time to detection.
Proper diagnosis is the first step toward better management and prevention of tuberculosis transmission. The Stop TB Partnership has recently described the actions and resources needed to end tuberculosis in the world by 20301. The world is now focusing on a ‘paradigm shift’ that will see countries improve case finding and decrease tuberculosis incidence rates by at least 10% annually. In order to achieve this global plan to end tuberculosis, all healthcare practitioners will be required to ensure that 90% of vulnerable groups are screened, 90% of those diagnosed are started on treatment and 90% are successfully treated. As such, one of the global priorities is on diagnosis.1 Discovery, development and rapid uptake of new tools and interventions have been highlighted as major requirements for the success of this plan.
The median age (years) and interquartile range (IQR) was 38 (30–48) with a gender ratio (male/female) of 49/51 and HIV positivity of 58.6%. Of the 138 AFB-positive MGIT cultures among the 690 patients included in the study, 136 cultures (98.6%) were confirmed positive for M. tuberculosis complex by both SD MPT64 and PNB, and two as NTM (Table 1). This gave a sensitivity of 100% [95% CI: 97.3–100]. All the known negative cultures and NTM were reported as negative for M. tuberculosis complex, giving a specificity of 100.0% [95% CI: 96.4–100], respectively.
Few studies have evaluated the performance of the SD MPT64 assay in field settings with high tuberculosis and HIV burdens. This study confirms prior evaluations and increases the evidence that the test has excellent sensitivity and specificity in identifying M. tuberculosis complex using PNB as a gold standard, in a Ugandan field setting where there is high tuberculosis and HIV co-infection. There have been previous evaluations in various countries using different gold standards. All the investigations have shown that the technique has high sensitivity and specificity. One study used reference bacterial strains and Mycobacterium bovis field isolates from animals and found a sensitivity of 96.5% [95% CI: 91.2–99.0] and specificity of 100% [95% CI: 96.7–100].19 The same group also found a high positive predictive value of 100% [95% CI: 96.7–100], and a negative predictive value of 92.9% [95% CI: 82.7–98.0].