Respirology | APSR Respiratory Medicine Journal

Positive T-SPOT Blood Test Brings Result Of Latent Tuberculosis

Dr. Keith Roach

DEAR DR. ROACH: I had a tuberculosis (TB) skin test for my new job in May, and it was negative. They did a blood test in July. I have a question about a positive T-SPOT blood test for TB. They did a chest X-ray, and it shows nothing wrong. Is there any treatment necessary? I haven't been told of any, but from what I read, it could be latent TB, which means it's just silent but can show up at any time. Does this mean that I came into contact with TB and my body just fought it off, or could this be a false positive? Any help would be great. — A.M.ANSWER: The T-SPOT, like the QuantiFERON-TB Gold, is a blood test to look for tuberculosis infection. Tuberculosis is an infection known for millennia. "Active" tuberculosis and "latent" tuberculosis are different, and the blood tests will be positive in both active and latent TB. Active tuberculosis commonly causes cough, fatigue and weight loss (centuries ago, it was called "consumption"). A chest X-ray is abnormal in virtually all cases of active tuberculosis: Since yours is negative (and assuming you have no symptoms), it's very likely you have latent tuberculosis. Both blood tests (these work by measuring the response of T cells to tuberculosis proteins) are more accurate than the TB skin test. Blood tests have the advantage of only coming in once, as opposed to the skin test where you need to have the test read by a clinician in 48 to 72 hours. The blood test is more than 95% specific, meaning it's extremely likely that you have latent infection and extremely unlikely to be a false positive. One disadvantage is that the blood test is much more expensive than the skin test.Latent infection means the tuberculosis bacteria are still alive inside you, but kept in check by your immune system. Unfortunately, due to illness, stress or just getting older, sometimes latent infections can escape the immune system and become active infections. Roughly 8% of people with latent TB will develop active TB during their lifetimes, but it's most common within a few years of acquiring the latent infection.People of any age who recently acquired latent TB (meaning their previous tests were negative, and now they're positive, called "converters") are at high enough risk for active TB that they are recommended to receive treatment. This is intended to kill all the latent TB bacteria and reduce the likelihood of developing active TB later. Treatment is highly effective, with most studies showing only about a 1% lifetime chance of developing active tuberculosis after completing treatment for latent TB. There are both one-drug and two-drug regimens for latent TB, with two-drug regimens lasting three months, and one-drug regimens lasting four to nine months. The most commonly used regimen now is probably rifamycin for four months, which is both very effective and minimally toxic.People who have a positive test without a recent negative test (called "reactors") are also usually treated, but there are some conditions where the risks and benefits of treatment need to be carefully weighed to make an individualized recommendation. But if someone ordered the test, they should also have made the decision that it's worthwhile to treat you if the result is positive.* * *Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to [email protected] or send mail to 628 Virginia Dr., Orlando, FL 32803.(c) 2022 North America Syndicate Inc.All Rights Reserved

Who Should Receive Preventive Treatment For TB? Individuals Of All Ages With Positive Skin Or Blood Test

Preventive treatment for tuberculosis (TB) can stop latent TB infections from developing into deadly TB disease. Despite TB infection being fully treatable, there is no global consensus as to which subgroups of individuals exposed to TB should be prioritized for preventive treatment, nor whether the benefits of this treatment vary based on factors such as age or confirmed infection.

A new study led by a Boston University School of Public Health (BUSPH) researcher provides clarity to this issue, finding that exposed individuals with confirmed TB infection -- i.E. A positive skin or blood test -- should receive priority treatment in settings with a low prevalence of the disease, regardless of their age.

However, in high-burden settings, all exposed individuals should be considered for preventative treatment, even without a confirmed infection, according to the findings published in The Lancet Respiratory Medicine.

This strategy can help end the tuberculosis epidemic and support global public health efforts to reduce TB mortality by 95 percent by 2035 (from 2015 estimates). In 2022, there were more than 10 million cases of active TB worldwide, resulting in 1.5 million deaths.

"Tuberculosis affects tens of millions of people every year and has long-term lasting effects, even after people recover," says study lead and corresponding author Dr. Leonardo Martinez, assistant professor of epidemiology at BUSPH. "Finding ways to optimize prevention is really important to tackle the epidemic."

For the study, Dr. Martinez and colleagues conducted a comprehensive review and analysis to identify new cases of TB among people who were in close contact with individuals diagnosed with the disease, and compared the effectiveness of preventive treatment in these exposed individuals based on age, infection status, and burden of TB in their settings.

Among 439,644 participants, the team found that preventive TB treatment was 49 percent effective among the 2,496 individuals who developed TB, but particularly among individuals with a positive skin or blood test (for which the effectiveness was 80 percent).

Notably, the researchers found that preventive TB treatment was not effective in most individuals who did not show evidence of infection, except for children under 5.

For those who did have a positive skin or blood test, the effectiveness of the treatment was comparable among individuals of all age groups -- adults, children ages 5-17, and children under 5 -- and the treatment was more effective among individuals in high-burden settings than low-burden settings.

The team also estimated the number of individuals needed to receive treatment (NNT) in order to prevent one person from developing TB disease. Regardless of infection status, the NNT was lower in high-burden settings (29 to 43 people) versus low-burden settings (213 to 455 people). Despite the fact that individuals with negative blood or skin tests do not seem to benefit from preventive treatments, the researchers say the overall low NNT may justify prioritizing this treatment to all exposed contacts in areas where testing for TB infection is inaccessible.

"While it is critical to find and treat people who are spreading TB in the community, the threat of global TB will never end until people with latent TB receive treatment," says study coauthor Dr. C. Robert Horsburgh, professor of global health. "The results of this study show just how effective such treatment can be."

Latent Tuberculosis — Which Is Not Contagious — Affects 80% Of Tested Brockton Migrants

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