Accuracy of smear microscopy and incremental yield of Xpert MTB/RIF in detection of pulmonary tuberculosis in laboratories in Rwanda
Abstract
Background: The National tuberculosis control Program (NTP) of Rwanda is currently phasing
in the use of light emitting diode-fluorescent microscopy (LED-FM) as an alternative to the
Ziehl-Neelsen technique (ZN). This, alongside the Xpert MTB/RIF technique is expected to
improve diagnosis of tuberculosis (TB) and detection of Rifampicin resistance in patients at the
various health facilities. There is limited information in Rwanda on the incremental yield of
these new techniques in comparison to the ZN technique which has been in use for many years.
Objective: To determine the accuracy of routine sputum smear microscopy and assess the
incremental yield of Xpert MTB/RIF test in detection of pulmonary tuberculosis in peripheral
and intermediate health facility laboratories in Rwanda.
Methods: The study was conducted at four intermediate and four peripheral health facility
laboratories across Rwanda. This study enrolled 648 participants with suspected pulmonary
tuberculosis who visited any of the eight health facilities from April 1st to June 21st, 2014. Each
participant gave one sputum sample which was processed as per the individual laboratory
procedures and the results obtained from the direct sputum smear recorded. The remaining
sample was processed using N-acetyl-cystein-Sodium hydroxide and inoculated a Lowenstein
Jensen slant and mycobacterium growth indicator tube (MGIT) at the National Reference
Laboratory (NRL). The pellet was used to prepare two smears, one for ZN and the other for
LED-FM staining and microscopy. The original health facility slide was re-examined at NRL
and the results recorded. The pellet was also used for testing using Xpert MTB/RIF.
Results: The study found the overall sensitivity and specificity of smear microscopy to be 51.5%
and 99.8% respectively, and positive and negative predictive values of 98% and 91.4%
respectively. The increment of Xpert MTB/RIF test over the smear microscopy was 40.3% an
47.6% respectively in HIV-negative and HIV-positive tuberculosis suspects. The overall
prevalence of negative pulmonary tuberculosis was 39.2%.
Conclusion: This study has provided the baseline data on the performance of currently used
routine methods and the incremental yield of Xpert MTB/RIF test compared to MTB culture as
the gold standard. It is therefore imperative to implement measures which could increase the
sensitivity of LED-FM microscopy before replacing ZN techniques in health facility laboratories.
The use of smear microscopy alone misses more than 50% of pulmonary tuberculosis cases,
consequently all smear negative cases with prominent signs and symptoms of pulmonary
tuberculosis should be tested using Xpert MTB/RIF test in order to increase tuberculosis case
detection and reduce the chain of transmissio
Publisher
University of Nairobi
Description
Master of Science in Medical Microbiology