What is drug susceptibility testing?
Drug susceptibility testing means testing to find out if a person has got drug resistant TB. That means finding out which drugs the TB bacteria in their body are sensitive to. It is essential that if a person might possibly have drug resistant TB, that this is discovered as soon as possible, in order that the patient can be provided with effective TB treatment.
Historically drug susceptibility testing has been done through culturing bacteria. It has needed specific laboratory facilities and trained personnel. In addition it is a very lengthy process. There are however now some new tests available one of which is the Genexpert test. This is much easier to use, but it only provides limited information about drug resistance.
Different types of drug susceptibility tests
Drug susceptibility tests for TB are basically of two different types. One type of test is culture, which involves looking at how the bacteria behave. For example, do the bacteria grow in the presence of anti TB drugs? Another type of test involves looking at genetic mutations.
Molecular drug susceptibility tests
Since resistance arises from genetic mutations, this approach is to detect the mutations themselves. Many mutations associated with resistance have been identified and molecular tests to detect them have been developed. The advantages of molecular methods of drug susceptibility testing include rapid turnaround times, but the disadvantages include a low sensitivity for some compounds, and a major issue is cost.
It is generally perceived that specialist staff are required in order to perform molecular assays. However, some assays such as the Genexpert are extremely easy to use. They can even be taken out of the laboratory setting and used as a “near” point of care test. The Genexpert is however limited in the resistance that can be detected.
There is a strong possibility that scientific developments in the area of TB diagnostics may enable resistance to be identified more easily in future. In August 2019 it was announced that a new test, the SPIT SEQ test has been developed in India. It is a sputum test, which can detect in 10 days all the resistance mutations in a sample. It is not yet commercially available but could be fairly soon. There could also be a difficulty with the cost, as it may initially cost $100 for one sample.1"MedGenome unveils novel DNA test for all DR mutations in TB", https://www.biospectrumindia.com/news/78/14407/medgenome-unveils-novel-dna-test-for-all-dr-mutations-in-tb.html
Beacon assays detect M. tuberculosis complex and associated rifampicin resistance directly from sputum samples using ultra sensitive PCR. The Genexpert TB assay is an automated real time based system that has a number of advantages including the fact that it is a closed tube system. The WHO is encouraging the use of the Genexpert TB test, but it has a number of disadvantages including cost.
Line Probe Assays
Line probe assays are drug susceptibility tests that use PCR and reverse hybridization methods for the rapid detection of mutations associated with drug resistance. Line probe assays are designed to identify M. tuberculosis complex and simultaneously detect mutations associated with drug resistance. One of the disadvantages with these assays is that they have an open-tube format, which can lead to cross contamination and an increased risk of false positive results.
Line Probe Assay (LPA) technology is suitable for use at national/central reference laboratories, or at laboratories where there is proven capacity to conduct molecular testing. There must be adequate and appropriate laboratory infrastructure and equipment. This must also include the necessary biosafety precautions and the prevention of contamination. Appropriate laboratory staff also need to be trained to conduct LPA procedures.2“Molecular Line-Probe Assay for the Detection of Resistance to Second Line Anti-TB Drugs“, WHO, www.who.int/tb/areas-of-work/laboratory/policy_statements