Tuberculosis Diagnosis – Time for Innovation
An alarming increase in the global incidence of drug-resistant Mycobacterium tuberculosis infection has created a critical need for methods that can rapidly detect M. tuberculosis and identify drug-resistant cases. Failure to quickly and effectively identify and treat patients with drug-resistant tuberculosis (TB), particularly multidrug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis, leads to increased mortality, nosocomial outbreaks, and resistance to additional antituberculosis drugs. i
In patients with active pulmonary TB, only an estimated 45%-85% of infections are detected by sputum microscopy. An estimated 10,000 organisms/ml of sputum are required for smear positivity.
Culture is still considered the gold standard with an estimated 10-100 organisms/ml of sputum required for a positive culture. Samples of sputum or tissue require initial decontamination to remove fast-growing nonmycobacterial organisms. Liquid broth cultures require 1 to 3 weeks of incubation for detection of organisms but my take as long as 6 weeks. Once mycobacteria have been identified in a clinical specimen, speciation is necessary for clinical diagnosis.Testing of M. tuberculosis isolates for drug susceptibility is important to guide therapy. Liquid broth systems, such as MGIT, can be used and provide results in 5 to 14 days.
Can we afford to wait for weeks for culture and sensitivity results in a country with MDR and XDR TB?
Nucleic acid amplification assays offer a new RAPID technique for the direct detection of M. tuberculosis in clinical specimens. In the September Online Editorial of the NEJM it states that if an enhanced rapid nucleic acid-amplification test is adopted globally, it could help prevent more than 15 million tuberculosis-related deaths by 2050ii.
The Cepheid GeneXpert MTB/RIF is a single use sample-processing cartridge system, with integrated multicolour real-time PCR capacity. This assay employs a novel six-color dye set to detect M. tuberculosis and identify rifampicin resistance as a surrogate for MDR directly from a patient’s sputum in hours. A recent (September 2010) study published in the NEJM by Boehme et al states that with one test the GeneXpert can identify 98% of patients with smear-positive and culture-positive tuberculosis (including more than 70% of patients with smear-negative and culture positive disease) and correctly identify 98% of bacteria resistant to rifampicin iii.
LANCET has improved the TB work flow to incorporate these new rapid molecular diagnostic techniques as illustrated below.
|SPUTUM for TB MC&S specimen 1|
|MTB Rifampicin Sensitive||MTB Rifampicin Resistant||MTB not detected|
|Report and no culture||Report and submit for culture and additional sensitivity testing||Report and no culture|
|Subsequent SPUTUMS submitted for TB MC&S within 180 days of first specimen|
|Culture Negative||Culture Positive|
|Report||Previous Rifampicin S/R||Previous GE NOT detected|
|Refer to previous report||Rapid TB antigen
|Rapid TB antigen
Please do not hesitate to contact the LANCET microbiology team if we can be of any assistance regarding the new TB testing or other related issues.