Genxpert and Line probe assay both are molecular/genotypic methods of drug-susceptibility testing. It is said to me that for the diagnosis of MDR-TB only one of them is carried out. So, if genexpert have been performed to diagnoses only RIF resistance does culture is done for detecting second line DST?
Can anyone suggest a workflow starting from AFB staining followed by genexpert, line probe assay, and culture DST.
Thank you!
Culture followed by susceptibility testing is the gold standard but time consuming. It is the ultimate phenotypic testing method. On the other hand, genotypic resistance can be determined by LIA and to some extent by GeneXpert MTB/Rif. Rarely there I have observed discordance. It could happen because there are some silent mutations too. However WHO advocates GeneXpert testing for rapid diagnosis of possible MDRTB. Still, for long term therapy like spinal and CNS TB we prefer Culture+DST.
Sourav Maiti
thank you so much for the informative reply...!
you said culture followed by susceptibility testing...is culture is not the drug-susceptibility testing method?...or you're trying to say that culture after the primary dug-susceptibility testing by molecular methods(GenXpert MTB/RIF).
- My query is that the GenXpert MTB/RIF and Line probe assay both are genotypic/molecular DST methods. The GenXpert MTB/RIF assay diagnoses only M.TB complex and RIF resistance. After confirmation of RIF resistance by GenXpert MTB/RIF assay...as it was said to me that only one of the molecular methods is used for DST purposes(Genxpert preferably).
Rifampicin is a surrogate marker for MDR-TB. Once detected by GeneXpert, second line DST can be done prefarably using molecular methods for rapid testing since Cultural method is the gold standard yet takes longer time. Some times discordant results are seen when comparing the phenotypic and genotypic methods. Better still, the idea is to quickly screen and treat to control the spread of TB
Mac-Fiberesima Gborieneomie
Thank you so much for the answer.
would you like to answer these queries?
1. No. We test for 10 drugs susceptibility. That is after culture, we get the original bacteria infecting.
2. INH resistance can be determined by DST or by LPA. Few LPA kits detect both INH and Rif resistance but they are validated on culture isolates and for pulmonary samples.
3. It depends on site. ZN staining is valuable in multibacillary samples whereas PCR will be helpful for paucibacillary samples also.
4. See point no.2
5. That includes all the agents approved for Mtb including PAS, injectables, Amoxyclav etc
Gene expert gives fast results.culture is an adjuvant.positive cultures are always helpful for conventional DST and line probe assay.i suggest both should be done.
Currently, GeneXpert is the point of care test for Tuberculosis because of its high specificity and sensitivity rate. Secondly because it requires minimal time frame for diagnosis as against Culture.
Smear microscopy is best for follow up not diagnosis. The reason is that it lacks the quality to distinguish viable and non viable baccili. Secondly, it requires about 10,000 baccili load for the sample to be considered positive. Decontaminating samples with the NaOH-Citrate NaLC before microscopy is ideal because it helps to liqufy the sample especially if its a pulmonary sample and also kill other normal flora that may be present in the sample thereby concentrating the TB baccili for better vision and quantification.
LPA is a genotypic test which is limited to INH and RIF for first line and then the fluoroquinolones and aminoglycosides for second line. Culture takes care of all first and second line drugs. Culture considers viable organisms whereas LPA concentrates on the DNA. So when the bacilli is dead, culture result will be negative while LPA will be positive because the DNA of the dead bacilli may still be present in the system. This why Culure is taken to be the gold standard.
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