Background: South Africa’s implementation of XpertMTB/RIFR as the initial diagnostic test for pulmonary tuberculosis (TB) reached national coverage in September 2013. XpertMTB/RIF’s use for the diagnosis of extrapulmonary tuberculosis (EPTB) was investigated with the aim of establishing whether EPTB testing should be incorporated into the national testing algorithm.
Methodology: EPTB specimens from 7916 hospitalized patients received over a 6-month period at a high throughput TB referral laboratory in Johannesburg, South Africa were investigated. Large volume specimens were centrifuged, tissue biopsies homogenised and all specimens checked for growth of contaminating bacteria on blood agar. Contaminated samples received NALC-NaOH decontamination prior to liquid culture. Residual specimens (volumes >1ml) after inoculation of culture (n=1175) were tested using XpertMTB/RIF as per the sputum protocol recommended by the manufacturer.
Results: Using culture as the reference standard, XpertMTB/RIF’s overall sensitivity was 59% (95% confidence interval [CI] 53, 65) and specificity 92% (CI 90, 94) with the highest sensitivity of 91% (CI 78, 97) for pus, 80% (CI 56, 94) for lymph node aspirates and 51% (CI 44, 58) for fluids (ascitic 59% and pleural 47%). A difference in sensitivity of Xpert was noticed between specimens classified as thick 87% (CI 76, 94) versus clear (watery) 48% (CI 36, 61) appearance. The sensitivity was unchanged when traces of blood 52% (CI 44, 60) or pre-centrifugation 57% (CI 28, 82) were documented among the clear specimens. An additional 124 specimens generated valid results by XpertMTB/RIF but were contaminated by MGIT (10.5%). There was an early diagnosis of rifampicin (RIF) resistance (9.6%) by XpertMTB/RIF in 25/260 cases.
Conclusions: XpertMTB/RIF’s performance on EPTB specimens provides very promising results and should be considered for incorporation into National TB guidelines. XpertMTB/RIF is less affected by contaminating bacteria and could significantly reduce laboratory labour and the diagnostic delay of culture for EPTB specimens.