Current thinking is that latent TB is a life-long infection and can cause active TB at any point. Interesting new research questions this idea.

Researchers from the Perelman School of Medicine at the University of Pennsylvania reviewed numerous clinical studies and they suggest that patients testing positive with immunologic TB rarely develop active disease. They say that the standard TB tests show a positive result due to immunological memory.

The research team write: “It is thought that TB immunoreactivity is not a marker for the presence of continued TB infection. Rather, it serves as a sign of having been infected with TB at some point.”

Their analysis suggests that about 10% of people with TB immunoreactivity harbour viable organisms capable of causing disease in the right circumstances, such as immunosuppression; in the remainder, M tuberculosis bacteria are likely to be permanently dormant or dead. 

The information contributes to the urgent need for resources to be focussed on developing tests to identify infected patients who are asymptomatic. The team stress that detecting and treating those with active TB remains a high priority, as does providing TB preventive therapy for those around them.

Anthony Harries gives his opinion in the BMJ.