Reports of multidrug resistant tuberculosis in every region of the world have raised red flags over the past few years, but clinicians have a far more basic problem on their hands: diagnosing the disease. The World Health Organization (WHO) reported last year that one-third of tuberculosis infections go undetected or are poorly treated, leading to disease spread and evolution of drug resistance.
The most accurate diagnosis is made by culturing the bacteria that causes the disease, Mycobacterium tuberculosis. However, this process takes around two months — far too long for clinicians to respond quickly in the field. Serological tests that detect anti-tuberculosis antibodies in the blood take less than an hour. But last month, the WHO, in its first-ever explicit ‘negative’ policy recommendation, cautioned against using these tests after researchers showed that they are neither accurate nor cost-effective.
The apparent need for a fast and cheap diagnostic test has pushed researchers to develop novel methods. One such approach was published online today in the Journal of Applied Microbiology. The researchers measured the heat cast off by growing bacteria — a technique known as isothermal microcalorimetry — to diagnose tuberculosis and related infections. Policy makers and doctors have historically ignored this method because the required equipment is too expensive. But this study provides evidence that a low-cost version of the instrumentation can diagnose the disease in less than two weeks.
A team of scientists from Switzerland and Mexico briefly cultured strains of M. tuberculosis and the related bugs M. smegmatis and M. avium. Using their cheaper calorimeter, they then measured the heat generated by the growing bacteria. Since each species has its own heat signature, the researchers managed to diagnose the fast-growing M. smegmatis in less than three days and the slow-growing strains, including M. tuberculosis, in just under two weeks.
Two weeks still feels like a long time — but compared to the gold standard culturing technique, it’s not too shabby. However, the researchers note that they used lab-adapted strains of the bacteria in this proof-of-concept study; these tests may take longer in the field due to lower levels of cultivable bacteria. And although the machine the authors used costs less than the standard set-up, it certainly isn’t cheap, priced at around $40,000. The authors note, however, that the instrument could be produced for $1,000, but did not provide details on how to get the price down 40-fold.
Image: via Wikimedia Commons
