Tuberculosis is a leading cause of death in the Philippines, and the prevalence of MDR-TB, a specific form of TB infection, is 4.3 percent. A patient suspected to have tuberculosis has to submit sputum samples in health facilities. The sputum will then be evaluated under microscope to detect presence of TB bacilli.
However, for patients who are suspected to have MDR-TB, they will have to undergo culture which would take about 6-8 weeks and additional three weeks for drug susceptibility testing. This technology is available in selected tertiary hospitals and is not affordable. Because of the high prevalence of pulmonary tuberculosis, our country needs a faster, cheaper and reliable diagnostic tool for detection of Mtb (Mycobacterium tuberculosis) in clinical specimens.
The microscopic observation drug susceptibility (MODS) assay utilizes liquid culture broth in a plastic microwell plate where specimens are inoculated and growth is observed through an inverted microscope. This method also provides easy Mtb detection and determination of drug resistance. With limitations of existing diagnostic tests for TB, we need to validate the usefulness of MODS because of its promising accuracy rates as reported from the literatures.
We aim to compare the performance of MODS assay in detecting Mtb with liquid and solid media. We collected 795 sputum samples from the out-patient clinic of the Philippine General Hospital (UP-PGH) and patients referred by the Department of Health PMDT (Programmatic Management for Non-resistant Tuberculosis).
All laboratory procedures were done at the UP-PGH TB research laboratory. We decontaminated the specimen, collected the sediment, and inoculated this is assay plates, supplemented with broth and antibiotic. We sealed the plates with films to prevent contamination and provide safety to the technician, and incubated them for 40 days. After this, the plates were read under the inverted microscope to look for pattern formation of Mtb. We found out that MODS assay can detect TB bacilli as early as the first week of incubation.
For the drug resistance, the assay provided a comparable result with the gold standard. We also observed that despite on-going treatment, Mtb can be isolated in all cultural techniques, and 40 to 80 percent of the isolates were MDR regardless of smear result. We also compared the cost of the three techniques for Mtb detection. Through the solid media was the cheapest, MODS lessened the waiting time of doctors for the result of drug susceptibility, thus providing early treatment for the patient. However, we cannot approximate the cost of transmission of MDR-TB to those in contact with the patient. Our research study was limited to clinical isolates.
We did not take into account the clinical condition of patients, especially those on treatment. We were also limited by the skill of the operator because extensive training is needed to differentiate the characteristic microscopic findings and to prevent contamination. This will also be a challenge to technologists who will set up MODS assay in their laboratories.
By using this technology, appropriate treatment can be provided early to patients especially to MDR-TB suspects. We expect that this technology can be effectively scaled up and implemented in regional TB laboratory centers in our country. Eventually, we see this assay being available in an off-the-shelf kit form that will not require an inverted microscope, hence can be used at point of care.
This will actually lessen the waiting time for the results in a cheaper way. Since MODS can be applied to direct samples, we believe this will give health practitioners the edge of knowing the culture and sensitivity results earlier.
Therefore, it will impact positively on public health in terms of timely reduction of isolation and early contact investigation. This test can also be included as a standard test for detecting MDR-TB for the national tuberculosis program if policy-makers will be persuaded to use MODS.
Written by: Myrna T. Mendoza and Marc Agnew M. Cajucom University of the Philippines Manila Published by: Department of Science and Technology-Science and Technology Information Institute (DOST-STII)