This brief discussion is about the PPD test, how it’s done, and most importantly how it is read. This is a skin test for tuberculosis. While not the best test for tuberculosis, it certainly has wide appeal because it is so simple to do, and so simple to read, especially in a clinic setting.
As usual, don’t try to interpret the results of your PPD without a doctor or nurse involved in it. This is mostly for your own background edification and understanding.
What is Tuberclosis?
Tuberculosis of course is an infectious disease. It is indeed a bacterial disease, but it is a very slow growing bacterial disease. It classically affects the lungs, but of course can involve just about any tissue in the body.
We’ve pretty much eradicated tuberculosis from this country. But that unfortunately is not the case in the rest of the world. Still a very big killer.
It is such a problem in the rest the world that many parts of the world immunize against it.
If you are immunized against it or have had it, your PPD will be positive.
There are exceptions to this. And this is where the interpretation of the PPD becomes an important, but tricky thing.
If you have had tuberculosis or have it currently and are so immunologically dysfunctional enough, you may not even mount a positive PPD — and actually have a full blown case of TB. So the interpretation must bear in mind the patient’s health.
So let’s say you are fairly healthy. You get a PPD. You’ve never been immunized against tuberculosis because you’re not from one of those countries in the world that does this routinely, and you get a positive. What is a positive.
The best reading of the test is at the two day mark. Almost as good is the three day reading. But, the two day reading is still the ideal.
First of all, what is being read is not the amount of redness or inflammation at the site two or three days after receiving it. Rather, it is the amount of induration that is being read if your doctor or nurse is on the ball.
5 mm is all that is required if the patient has HIV or is an organ transplant patient or is immunocompromised.
10 mm is what is required for the recent immigrant (so this is what the specially licensed civil surgeon screening people for American citizenship must use, in case this applies to a family member or friend you’re bringing into this country and you’re wondering why it was different from other screening you had previously seen); IV drug abusers, & hospital personnel.
15 mm induration is required for everyone else.
If it’s positive, a chest x-ray must be done. Treatment must be considered, and is almost always empirically done. A lot more thought must go into this in terms of who needs to be treated, who may have been immunized in the past, and so forth.
Stay well. And get your PPD at least every other year, unless you work in a hospital or restaurant, in which case you should get one yearly.