Antigen detection tests for TB
A systematic review calls for more research into this area
More studies are needed to determine the diagnostic accuracy of using antigen detection tests for active tuberculosis (TB). This is the conclusion of a systematic review and meta analysis that concluded that there are too few studies targeting too few specific antigens. Tests that detect Mycobacterium tuberculosis (TB) antigens in specimens such as urine could provide rapid direct evidence of active disease, especially in remote settings where blood and sputum tests are difficult to administer.
The World Health Organization (WHO) estimates that in 2009, 9.4 million new cases of TB occurred and 1.7 million people died of the disease. The vast majority of these patients live in low- and middle-income countries where TB diagnosis depends primarily on smear microscopy from sputum. Microscopy has low sensitivity and does not detect smear negative TB, which may account for 24% to 61% of all pulmonary cases in HIV-infected individuals. Improved diagnostic tests such as mycobacterial culture and nucleic acid amplification (NAA) tests are available in high-income countries but are often too expensive and complex for routine use by TB control programmes in resource-constrained TB-endemic settings.
There have been considerable efforts over the past 50 years to devise a rapid TB test based on antibody detection using blood samples. However, substantial published evidence suggests that the current commercial serological (blood) tests are inaccurate for diagnosis of active TB.
Antigen detection tests using urine appear to offer several advantages. Immunochromatographic tests, for example, are rapid (results may be available within minutes) and easy to operate. If developed into a point-of-care test, it could allow for immediate initiation of TB treatment. A urine specimen would be particularly useful for children, who can have difficulty providing sputum. Finally, in patients suspected of extrapulmonary TB, an antigen detection test might prevent the use of more invasive tests.
The authors call for more studies to determine the value of detecting lipoarabinomannan (LAM), a major glycolipid component of the cell wall of M. tuberculosis to diagnoise other mycobacteria for TB meningitis in high HIV prevalence settings. Future studies are also needed that evaluate the performance of antigen detection tests in HIV-infected individuals with smear-negative TB. Accurate, rapid, inexpensive, simple diagnostic tests are urgently needed for TB care and control.
For more information, contact Dr Andy Ramsay.