Rapid TB Test Accuracy Compromised by Mycobacterial Diversity

Throughout the world, Mycobacterium tuberculosis (Mtb) is responsible for the vast majority of tuberculosis (TB) cases; however, there are several other closely related mycobacterial species that cause TB, all part of the Mycobacterium tuberculosis complex (Mtbc).

Diagnostics for rapid confirmation of positive liquid cultures presumptive of M. tuberculosis bacteria, based on the detection of the MPT64 antigen, and are being used in many TB diagnostic laboratories worldwide. The diagnostic performance of these tests in West Africa, where TB is uniquely caused by the geographically restricted Mycobacterium africanum (Maf 1 and 2) and M. tuberculosis lineages, has not been properly assessed.

An international team of scientists led by those from the Medical Research Council (MRC) Unit, (Serrekunda, The Gambia) collected sputum samples prospectively from individuals with suspected TB between April and October 2014 and were all initially screened for the presence of acid fast bacilli (AFB) by Auramine microscopy. They compared the abundance of mpt64 gene product in sputum samples of patients with untreated pulmonary TB caused by Maf 2, the strain common in The Gambia or Mtb.

The scientists then prospectively analyzed culture isolates from 173 patients with one of the rapid tests, the BD MGIT TBc Identification test kit (Becton Dickinson Microbiology Systems, Sparks, MD USA). They repeated the same analysis with all samples that had tested negative on day zero and a random set of those that had tested positive using the SD Bioline Ag MPT64 Rapid test (Standard Diagnostics, Yongin-si, Republic of Korea). They observed no significant difference between the two tests. Gene expression of mpt64 gene was performed using multiplex real-time polymerase chain reaction (qPCR).

The team found that 150 of the samples tested positive on day zero (the day when mycobacterial growth in culture was first recorded), with 23 (13.2%) testing negative at this time point. The accuracy was much higher (over 90%) for the Mtb samples, compared with less than 80% for the Maf 2 samples. At Day 10, 84% of Maf 2 samples tested positive compared with 98% of Mtb samples. By Day 90, 98% of both Mtb and Maf 2 samples tested positive. Based on these results, 22% of Maf 2 patients, and 10% of Mtb patients would have been wrongly classified as having non-TB mycobacteria if the tests had not been repeated after day zero. At the end of the 10-day window recommended by the BD MGITTM TBc ID manufacturer, 16% of all Maf 2 samples remained negative, compared with only 2% of Mtb samples.

The authors indicated that MPT64 tests need to be cautiously used in settings where Maf 2 is common. However, they also recognize that given the relatively low cost, limited technical expertise and shorter turnaround time associated with using rapid speciation tests compared to alternative speciation methods, MPT64 rapid tests will likely remain one of the preferred options for timely diagnosis of suspected TB despite the possibility of false negative results, and suggest that a negative MPT64 result would require confirmation by an alternative method. The study was published on July 7, 2016, in the journal PLOS Neglected Tropical Diseases.

Related Links:
Medical Research Council
Becton Dickinson
Standard Diagnostics