A Strong Endorsement for QuantiFERON

With a huge hat tip to "Drrc".

This article by Dr. Abby S. Van Voorhees, Smarter Meds Call for Smarter Screening is well worth reading. Dr. Voorhees is a working practitioner and she makes two points that are of interest to us.

• There are many drug treatments that weaken the immune system, increasing the chances of TB activation. As a matter of course she screens for TB all her patients that are starting most drug courses both before treatment and annually.
• The skin test presents real practical problems and therefore she has moved exclusively to QuantiFERON-TB Gold testing.

It is strong endorsements like these from the very front line that are going to rapidly increase the penetration of QuantiFERON into the TB Screening market.

It's worth reading the article but here are some highlights.

I now screen patients who I plan to start on any of the systemic medications, with the exception of acitretin, which does not suppress the immune system. These include adalimumab, alefecept, etanercept, infiximab, ustekinumab, cyclosporine and methotrexate. The screening is recommended both before treatment is initiated, as well as annually once it has commenced and for as long as treatment is continued.

She comments on some of the failings of the Skin Test

the PPD requires at least one extra visit and often two if they are going to their PMD. Too often, the next step is having my office nurse playing the “see if you can track down the PPD result game.” Of course, some primaries are very good at returning the PPD results, but others are notoriously remiss, at a high price in terms of staff time.

Then there is this all-too-familiar scenario: A patient with a placed PPD forgets to return to have it read, leaving us in the position of judging whether the patient’s own reading has any validity, a situation further complicated by the fact that placement of another PPD at too close an interval is another known cause of a falsely positive test.

and her endorsement of the QuantiFERON test

So it has been with delight that I’ve followed the development of the quantiferon gold assay story. If you’ve not been paying attention, this is something worth noting. This test has now become more generally available, which is a big boon for patients.

As with many new studies, the cost of the quantiferon gold test is generally greater than the PPD test. I therefore initially questioned whether this test, which appeared to be less cost-effective, should be more widely used and I first used it only as a second level test, just to verify a positive PPD.

Fewer False Positives

The quantiferon gold test is more specific for TB, so there are fewer false positive results, and since it does not cross react with Bacillus Calmette-Guérin (BCG) vaccination, it is very helpful — especially in those who had prior BCG vaccination. Many of my patients from various parts of the Asian continent clearly remember being told that their BCG vaccination would provide them with lifelong immunity. The dogma in the United States was that BCG vaccination would be “picked up” by a PPD test for only about 10 years and so there was much confusion when these previously vaccinated patients would have a positive PPD. Was the induration of the PPD therefore a result of a true exposure to TB or was it a result of a vaccination left over from a vaccination in childhood? Or did it truly represent a new exposure to TB? So initially the quantiferon gold test allowed me to distinguish between these two possibilities for many patients once they were shown to be PPD-positive. In fact several of my previously PPD- positive patients who had already undergone prophylaxis for latent TB requested the test and felt very vindicated to discover that their quantiferon gold test was indeed negative.

More Convenient

It also has become quite clear that patients prefer this test over the PPD. They like the convenience of being able to obtain it along with their routine blood work without necessitating a separate visit to their primary care doctor’s office.

An Alternative First-Line Approach

More recently however, the quantiferon gold test has been positioned as an alternative first-line approach for the screening of tuberculosis (rather than as a back-up if patients have a positive PPD) in the pulmonary literature. The enhanced specificity of the quantiferon gold test compared with the PPD, which has such a high false-positive rate due to its cross reactivity with other types of mycobacterium, the lack of human error in interpreting the test result by health care workers, and the lack of requirement for a 2-day follow-up visit all contribute to its being a truly cost-effective alternative to the PPD test.