An ongoing dialogue on HIV/AIDS, infectious diseases,
November 10th, 2014
Common Curbsides: The Tuberculin Skin Test and IGRA That Don’t Agree
As always, names/some details changed to protect patient confidentiality, plus my annotations in brackets/italics.
Quick question [Need I even comment about the “quick question” phrase, and how this unintentionally devalues what ID docs do? OK, I’ve commented, and yes I’m hypersensitive] — one of my patients, a 38-year-old woman from Serbia, is about to start biologics for rheumatoid arthritis [you can see this is going to be a latent tuberculosis testing question from miles away].
She’s not sure if she got BCG as a child [yep, called that one], so I did both [uh-oh] a PPD and sent a quantiferon gold [note this is not the crazy alphabetizing of the branded interferon gamma release assay (IGRA) products, which for some reason is “QuantiFERON®-TB Gold“ or the other one, which just goes with all-caps, “T-SPOT.TB”, and throws a period in the middle — jeeze, marketers]. The PPD is positive at 20 mm, but the quantiferon is negative [of course — otherwise there’s no question]. Her CXR is negative.
My conclusion is that she does not have TB and therefore does not need preventive therapy prior to starting biologics [that’s notable — putting your money down!]. Is that the correct conclusion?
I’m sure this is a very easy question [ah, not so easy after all] as you must get it all the time [second part is most certainly true].
Thank you very much sir [so formal! or maybe he was in the military] for your time.
Archie [not his real name, but it was the name of a dog we had growing up, a long-haired dachshund]
As noted above, nope, not so easy. Let’s consider the variables:
- She’s from a part of the world with a higher rate of TB than we have here. So the prior probability of latent TB is greater than if she was US-born, though it’s not as high as this.
- She’s about to get “biologics”, which by convention means some sort of expensive immunosuppressive drug, probably a TNF-inhibitor. These are of course great potentiators of TB, some worse than others.
- She may have received BCG, which can give you a false-positive tuberculin skin test. But this effect typically wanes over time, and even if she did get BCG, it’s probably not very effective in preventing adult TB.
- The sensitivity of the skin test vs IGRA is all but impossible to determine — because there is no gold standard for latent tuberculosis.
To summarize, the positive skin test could be a true positive, or could be a false positive from the (maybe) BCG. And the negative IGRA could be a true negative, or could be a false negative because it’s not 100% sensitive.
So what to do? Here was my answer:
The problem is that there is no gold standard — either one could be right. You might consider sending a T-SPOT, since it could be a bit more sensitive than the QF. But since biologics are strong potentiators of active TB, in general I’d recommend preventive therapy for a couple of months before she starts it (then completing a full course) since she’s from a part of the world with more TB than we have here.
PS: we had a dog named Archie when I was a kid.
So my question for you TB specialists out there — since this is such a common question, why doesn’t managing it appear in all the various guidelines and textbooks? Or am I missing it somewhere?
And speaking of dogs, it’s been a while since I showed a picture of Louie.