Diagnosis of TB and MDR-TB
Early diagnosis is key for MDR-TB for two reasons:
- Practitioners can ensure that the proper treatment is used on MDR-TB patients, avoiding medications where the patient is immune (Nathanson, Nunn, Uplekar, Floyd, Jaramillo, Lonnroth, Raviglione, 2010). In 2007, approximately 15% of MDR-TB cases were predicted to actually be XDR-TB strains (Shah, Wright, Bai, Barrera, Boulahbal, et al., 2007). As MDR-TB cases are resistant to isoniazid and rifampicin (first-line TB drugs), XDR-TB strains are resistant to first-line drugs, as well as fluoroquinolones and at least one of the second-line injectable drugs.
- The sooner proper treatment commences, the sooner MDR-TB will be contained in the community.
In order to diagnose MDR-TB, drug-resistance testing should be conducted to determine if the TB strain is resistant to any drugs other than rifampicin and isoniazid (Nathanson et al., 2010). This is can be done a few different ways. We will begin with the most efficient, which are the rapid molecular tests.
Rapid Molecular Tests
Rapid molecular tests are now available to detect resistant strains. These tests are not universally available, but WHO strongly recommends that these be used in the first-line of diagnosis when MDR-TB is suspected or when working with HIV-infected patients. the Xpert MTB/RIF diagnostic test produces results within the same day so patients can begin treatment quickly. Implementation of this test and other technologies are slowly being integrated into practice, but will greatly impact MDR-TB case management. Additional Information is available at http://www.who.int/tb/features_archive/factsheet_xpert.pdf (Nathanson et al., 2010).
Other rapid molecular tests in the field include NRA, MODS, Genotype MTBDR, and Genotype MTBDRplus. These diagnostic tests can take 2-29 days to provide results (Bwanga, Hoffner, Haile, and Joloba, 2009). Additional information about the sensitivity and specificity of these tools can be found at: https://learn.bu.edu/courses/1/13fallsphph709_a1/groups/_25651_1//_1765668_1/Direct%20Susceptibility%20Testing%20of%20MDR-TB.pdf.
Conventional Drug-Susceptibility Tests
Conventional drug-susceptibility tests can be conducted using solid or liquid culture from the patient. Diagnosis using this route can take more time since it typically cannot be conducted on-site. The common turn around time using this method is about two months (Bwanga et al., 2009). The use of these tests are crucial among patients at risk of MDR-TB (Nathanson et al., 2010). According to the Center for Disease Control (CDC), these groups include:
- Prior TB disease treatment;
- Contact with a patient with known anti-TB drug resistance;
- Demonstrated resistance to first-line anti-TB drugs; or
- Positive cultures after more than 3 months of treatment.
If possible, it is best to conduct both molecular and conventional drug-susceptibility testing. Although molecular testing can provide a quick diagnosis for many disease mutations, others still remain unknown. Therefore, it is important to conduct both tests when working with patients suspected of contracting MDR-TB.
