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New Rapid TB Test Reduces Time to Diagnosis


A rapid molecular test for sputum (Xpert MTB/RIF; Cepheid) can detect tuberculosis (TB) in the span of 2 hours rather than the 4 to 8 weeks needed for TB culture methods which is considered the gold standard or the 2-3 days a Mantoux skin test takes.

Catherina C. Boehme, MD, of the Foundation for Innovative New Diagnostics (FIND) in Geneva, Switzerland, and colleagues, reported the results of their study online September 1 in the New England Journal of Medicine.

Currently, global control of tuberculosis is hampered by slow, insensitive diagnostic methods, particularly for the detection of drug-resistant forms and in patients with human immunodeficiency virus infection. The current rapid test feasible in poorer nations is sputum-smear microscopy which routinely misses half of all TB cases and can not report on rifampin resistance as the rapid test Xpert MTB/RIF can.

Boehme and colleagues studied specimens from 1,730 patients from Peru, Azerbaijan, South Africa and India who were suspected of having tuberculosis. The samples were subjected to conventional analysis for comparison to the rapid test Xpert MTB/RIF.

The test accurately identified 98% of all confirmed smear-positive TB cases with only a false positive rate of 0.8%. The automated Xpert MTB/RIF test yielded false-negative or indeterminate results in about 27.5% of samples from smear-negative, culture-positive patients. But running the test three times on such samples increased the sensitivity to 90.2%

The test also accurately identified 98% of those samples resistant to rifampin. Smears cannot determine drug resistance.

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There are some limitations to the study. The tests were actually performed in labs in developed countries, and need to be verified in local laboratories in the third world countries.

Cost may be a barrier with bringing the technology to third world countries. The GeneXpert machine costs about $30,000 and each test costs about $63. Even Cepheid’s discounts may not be enough to allow truly poor countries to implement this technology. Cepheid has said it will sell the machine in developing countries for $20,000 and tests for about $32.

Adoption of the rapid TB test could allow earlier treatment of active cases which would greatly aid in preventing the spread of the disease.

TB is short for tuberculosis. TB disease is caused by a bacterium called Mycobacterium tuberculosis. The bacteria usually attack the lungs, but TB bacteria can attack any part of the body such as the kidney, spine, and brain. If not treated properly, TB disease can be fatal.

TB is spread through the air from one person to another. The TB bacteria are put into the air when a person with active TB disease of the lungs or throat coughs, sneezes, speaks, or sings. People nearby may breathe in these bacteria and become infected.
TB is NOT spread by shaking someone’s hand, sharing food or drink, touching bed linens or toilet seats, sharing toothbrushes, or kissing.

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Source reference:
Boehme C, et al "Rapid molecular detection of tuberculosis and rifampin resistance" New Engl J Med 2010; DOI:10.1056/NEJMoa0907847.



A Rapid Active test done on a boy for tuberculosis showed "IgG reactive" please advise if it could be diagnostic for Active Tuberculous lesion?