A new study suggests that people who test positive with latent immunologic tuberculosis (TB) skin or blood tests rarely develop the disease because the infecting organism, Mycobacterium tuberculosis, is likely dead, wiped out naturally by people’s immune systems. This likely explains why standard tuberculosis tests show a positive result despite the bacteria’s absence, since those tests look for an immune response and not live bacteria, the authors said.
Notably, there are two types of tests used to diagnose TB: a blood test and a skin test. Your results from either test won’t reveal whether you have latent or active TB. Instead, they’re used to determine if you should be treated and with what type of medication.
While active TB disease is treated with a combination of medications for six to nine months, latent TB is usually treated as well to prevent future active disease. According to University of Pennsylvania, persons with LTBI (latent tuberculosis infection) do not have active TB disease, but may develop it in the future, a process called TB reactivation.
“The National Institutes of Health and other nonprofit organisations spend millions of dollars on studies of the latent state because of the assumption that TB infection is life-long, held in check by the immune system,” said study co-author Paul H Edelstein from the University of Pennsylvania.
“However, based on our analysis, we believe that it is rarely life-long, and in 90 per cent or more of infected people, there is no possibility of TB development even with severe immunosuppression,” Edelstein said.
In their review published in the British Medical Journal (BMJ), the team pointed to several previous studies to demonstrate the natural history of TB immunoreactivity in people given preventive treatment, and of active TB in immunoreactive people with various forms of severe immunosuppression, like patients with HIV and those who have received an organ transplant.
One study showed that the treatment of people with TB immunoreactivity for one year lowered the incidence of active TB by 60 to 70 per cent over the next nine years.
In another study of patients co-infected with HIV and TB — who are at a higher risk for active TB — researchers showed that between 89 per cent and 97.5 per cent of those with positive TB immunoreactivity tests remained free of TB from their remote infection over a five-year period.
And in the third study of stem cell transplant patients, none of the 29 participants from the US cohort with TB immunoreactivity developed TB after a total of 89 person-years of observation.
“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,” Edelstein said.
“We need to put more effort into controlling active TB and to determine how to detect the 10 per cent of people who actually do have a life-long infection,” he added.