Finding the right tests…
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Finding the right tests…
Molecular
GeneXpert MTB/XDR extends the standard cartridge-based TB test by checking for resistance to isoniazid, fluoroquinolones, ethionamide, and amikacin, drugs that sit outside what the standard rifampicin-only test can tell you. It's typically ordered as a follow-on test once TB is confirmed, especially when rifampicin resistance has already been found and your doctor needs to know which second-line drugs will still work. In pooled diagnostic accuracy studies, the test correctly identified isoniazid resistance with 94.2% sensitivity and fluoroquinolone resistance with 93.2% sensitivity, both measured against slower reference methods like culture-based drug testing. Because fluoroquinolones are central to several drug-resistant TB regimens, knowing this resistance status early meaningfully changes which medicines your doctor can rely on.
This test reports resistance results separately for each drug it checks: isoniazid, fluoroquinolones, ethionamide, and amikacin, alongside confirmation of rifampicin resistance status carried over from the standard test. A drug marked resistant means your doctor should avoid relying on it in your treatment regimen. A drug marked susceptible, meaning no resistance mutation was found, suggests it should still work, though your doctor weighs this alongside your treatment history and clinical response. Some results may come back indeterminate if the bacterial load in your sample was too low to test a specific drug reliably.
The standard GeneXpert test checks only rifampicin, which is a strong flag for multidrug-resistant TB but doesn't say anything about isoniazid or the second-line drugs used once rifampicin resistance is confirmed. Treating drug-resistant TB well depends on knowing which of those additional drugs will still work, not just that resistance exists somewhere.
This test adds isoniazid, the other cornerstone first-line drug, along with fluoroquinolones and the injectable agents amikacin and ethionamide, both important building blocks of drug-resistant TB regimens. Running all of these from one sputum sample avoids the delay of sending separate tests or waiting on slower culture-based drug susceptibility testing.
Pooled data from multiple diagnostic accuracy studies found 94.2% sensitivity and 98.5% specificity for isoniazid resistance, and 93.2% sensitivity and 98.0% specificity for fluoroquinolone resistance, using culture-based methods as the comparison standard. A separate study from an Indian tertiary care setting found isoniazid resistance in about 15% of patients tested, and fluoroquinolone resistance in a similar share, with resistance to both being especially common among patients who already had confirmed rifampicin resistance.
This is generally not a first test - it's ordered once TB is already confirmed, commonly right after a rifampicin-resistant result on the standard GeneXpert test, so your doctor can plan a second-line regimen with actual resistance data instead of guessing.
The standard test checks only for rifampicin resistance alongside TB detection. This extended version adds isoniazid, fluoroquinolones, ethionamide, and amikacin, giving your doctor a much fuller picture of which drugs will still work.
Usually yes, in sequence rather than together. The standard test confirms TB and checks rifampicin first, and this extended panel is typically ordered afterward, often specifically because rifampicin resistance was found.
Pooled studies have found around 94% sensitivity for isoniazid resistance and around 93% sensitivity for fluoroquinolone resistance, measured against culture-based reference testing, which is considered strong performance for a rapid molecular test.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
Fluoroquinolones are a key building block of several drug-resistant TB treatment regimens, so knowing whether the bacteria resist them early changes which medicines your doctor can safely rely on.
Not necessarily. It's a fast, reliable screen for the specific drugs it covers, but your doctor may still use culture-based testing for drugs outside this panel or to confirm results in complex cases.
It means the bacterial load in your sample was too low to reliably test that specific drug. Your doctor may ask for a repeat sample or rely on other test results to guide your treatment.