Q&A: Time to retire tuberculin skin tests?

Andrea T. Cruz

Two tests are currently available to assist with the diagnosis of tuberculosis: tuberculin skin tests, or TSTs, and an interferon-gamma release assay, or an IGRA blood test.

Compared with TSTs, which have been used in clinical practice since the late 1800s, IGRAs have been around for only a relatively short time, but more data have become available to support their use in younger children.

Andrea T. Cruz, MD, MPH, an associate professor of pediatrics at Baylor College of Medicine, and colleagues recently authored a perspective published in Pediatrics in which they made a case to discontinue the widespread use of TSTs.

Cruz said in an interview that she and her colleagues want to reinforce the message that “there are better tests out there.”

“There are lab-based tests, which are less prone to erroneous interpretations,” she said. “We know that 20% of kids do not come back for skin test reading, and you avoid that issue altogether with IGRAs.”

Infectious Diseases in Children spoke with Cruz about the benefits and downsides to both tests. – by Katherine Bortz

Q: Why are TSTs so widely used if the ir interpretation can vary so much?

A: We have known for a long time that interpreters’ variation of measurement has been large. Before, we did not have an alternative test, so we accepted the limitations of the test we had.

We did move away from universal testing of children in the late 1990s toward targeted testing of children, or testing only if they have risk factors. That resulted in providers placing and reading a lot fewer skin tests. Because there are fewer tests being done, fewer positive tests and the fact that most providers at no point in their course received formal training on skin test interpretation, the provider pool is simply less comfortable with placement and interpretation of these tests compared with providers a couple of generations ago.

Q: Are IGRAs commonly used for pediatric patients?

A: They are becoming more commonly used. In the 2018 Red Book, the age for doing IGRAs dropped from age 5 years down to children aged 2 years. It has really opened up the use of these tests for toddlers.

When the test originally came out, there were actually very few data on young children, and most of the data were on older children. Frankly, blood draws are easier in older children. Because of this, the original national guidelines that came out recognized the absence of data in preschool-aged children and said that you should continue to use the skin test for that population.

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Now, we have abundant data that were collected since the first IGRAs were released in the early 2000s.

Q: What might be some of the issues related to switching to using IGRAs only for TB testing?

A: One of the questions that is always brought up is cost. When you look at the direct cost of sending the child to a lab compared with how much it costs to place and read a skin test with a provider, the cost is a lot cheaper for skin tests.

But what that doesn’t take into consideration is that parents will need to pull their child out of school or day care and themselves out of work for the skin test. It also means that they need to do the same again in 2 to 3 days for the test to be interpreted. When you factor in the cost of chasing down false-positive tests, the cost of unnecessary X-rays in these children and the lost time of work for families, then the overall costs likely favor using IGRAs.

It is also hard to get blood drawn from young children. They squirm, and they do not necessarily have good anatomical landmarks. The logistics could be an issue, especially in clinical settings where providers may not be as accustomed to venipuncture in children.

Q: Are there any reasons why TSTs may be preferable to providers over IGRAs?

A: There is a lot of comfort in what we know and what we are familiar with. For many physicians who have been in practice for a while, what we learned initially was all about TSTs.

We also need to recognize that most children who receive testing for TB infection do not receive this testing from infectious disease physicians. They receive testing — and very appropriately so — by their community pediatricians. I think one of the hurdles we need to face is how to increase the uptake and knowledge of IGRAs for our community partners who are the ones doing the bulk of the testing in kids.

Q: Are there any circumstance s in which you think TSTs should still be used?

A: Absolutely. There are times when you are not going to have lab resources available to you. One of these might be an investigation in a high school where you are going to be testing hundreds of children. It might be cost-prohibitive to perform IGRAs in that population.

There are also going to be children for whom you want to optimize the sensitivity of the test. If you have a child with Crohn’s disease, for example, who is about to be immunosuppressed, you might want to do both a skin test and an IGRA and offer preventive treatment if either of those tests are positive.

References:

Cruz AT, Reichman LB. Pediatrics. 2019;doi:10.1542/peds.2018-3327.

Goldstein EJC, et al. Clin Infect Dis. 2002;doi:10.1086/338149.

Disclosure: Cruz reports no relevant financial disclosures.