The Genotype MTBDRplus and INNO-LiPA Rif.TB assays are tests that can detect if a TB bacterium is resistant to two of the most common TB drugs, rifampicin and isoniazid.
An assay is a procedure in biochemistry to detect a substance on a molecular level. Genotype MTBDRplus was developed by Hain Lifescience, a company in Germany. INNO-LiPA Rif.TB was developed by a Belgium company called Innogenetics.
Both are quick tests to detect the TB bacterium and to see if it is resistant to drugs. They can detect this within one day. The tests can be used on specimens that are smear-positive (see how to diagnose TB) to test for drug resistance. It can also be used on special groups of mycobacterium tuberculosis which have been grown in TB cultures. It is currently used in many wealthy countries like Australia, Denmark, France, Germany, Italy, the Netherlands, Portugal, Spain, the UK, Sweden and Switzerland. It is very important to test for drugs resistance, especially in countries like South Africa, where drug-resistant TB is common.
How it works
Both tests are so-called line probe assays. Line probe assays work in the following way: the DNA (the genes, DNA stands for desoxyribonucleid acid) of the TB bacterium is extracted with biochemical methods. It is then multiplied by the hundreds with a technique called Polymerase Chain Reaction (PCR) to identify if the genes of the TB bacterium have been altered. All DNAs are pinned onto a surface with another biochemical method (all of this can only be seen with special equipment). Finally, a colouring agent is added to the pinned-down DNA to highlight certain areas that might look different than expected. TB bacteria that are resistant to certain drugs, like rifampicin or isoniazid, have had mutations (changes) in their DNA. The tests can detect if there are changes in regions of the affected genes. Because there are millions of genes, researchers have mapped them all and given names to specific regions, like names for countries on a map. If there is resistance to rifampicin, then the mutation is in a region called the rpoB gene. If there is resistance to isoniazid, the mutations are in the katG or inhA region. The tests find the mutations by screening these regions – if one of them looks different, then it is very likely to be because of resistance. This will then show up as a different colour on the test – similar to the stripes on pregnancy tests if a person is pregnant.
The Genotype MTBDRplus test can see if a TB bacterium is resistant to rifampicin or isoniazid. It can thus test for resistance to two drugs. Researchers have found out that it is very sensitive and very specific for rifampicin resistance, meaning that it is very good at detecting resistance if it is there, and also at detecting when they are not there (see Diagnostic tests for active tuberculosis). It is not quite so good at detecting isoniazid resistance, although the MTBDRplus version is already the second version of the test, and was improved for better isoniazid resistance detection. (The first version was simply called MTBDR.)
The INNO-LiPA Rif.TB test can only test for resistance to one - rifampicin. In studies researchers have proved that it is highly sensitive and specific when testing isolated TB bacteria (97% sensitive and 99% specific in laboratory settings). On actual clinical specimens (like sputum, urine, tissue, etc.) its sensitivity was between 80 and 100% (depending on the study), and its specificity remained at 100%. This means that it is a good test to rule out rifampicin resistance, but it is less good at detecting it.
Pros and Cons
Both tests are much quicker at detecting TB drug resistance than the usual methods of drug resistance testing in TB cultures. They both only take about a day to give a result, which means that patients can be treated with the correct drugs immediately, and don't have to wait for the drug resistance testing results for weeks. The important thing to look at when it comes to speed is called laboratory turnaround time (TAT): the time that a lab takes after receiving a patient's specimen to report a result.
In a pilot study in South Africa in 2008, the TAT was about 8 days with the Genotype MTBDRplus – compared to 41 days by conventional drug resistance diagnostics with culture. Unlike the GeneXPert, the tests need to be done in a laboratory with adequate facilities and equipment. They have mostly been used in wealthy countries, but recently they have also been tested in Russia, Nepal and South Africa.
The Genotype MTBDRplus costs 3.50€ per test (approximately R40). In South Africa, this is 30-50% cheaper than conventional drug resistance testing. The once-off cost for equipment is 9,990€ for the machine, 2,200€ for the incubator and 11,600€ for a machine that interprets all results. The total of 23,790€ is about R260.000.
The GenotypeMTBDRplus in South Africa
In 2008, the Genotype MTBDRplus was tested in a large pilot study in laboratories in Cape Town and Johannesburg. This study was carried out by FIND (the Foundation for Innovative New Diagnostics), the SA Medical Research Council and the SA National Health Laboratory Service (NHLS). It found that in countries with many TB cases and a lot of drug-resistant TB (like South Africa), the Genotype MTBDRplus is faster and more precise than conventional drug resistance testing by culture. It is also cheaper. All South African laboratories should use the Genotype MTBDRplus to ensure that TB patients get quick and correct results about the drug resistances of their TB bacterium.