New test for infection may supplant PPD

QuantiFERON starts multi-city trials

A new diagnostic test for TB infection now in trials is a strong contender to replace the PPD skin test, researchers say.

The new test, called QuantiFERON, promises to be much more precise than the PPD skin test, as well as more economical, say experts at the Centers for Disease Control and Prevention (CDC).

QuantiFERON, invented by Australian veterinarian P.R. Wood and manufactured by CLS Limited, last month began multi-city trials co-sponsored by the CDC and five university medical schools. What's especially promising is how, unlike the PPD skin test, QuantiFERON can tell whether someone is reacting to M. tuberculosis, an atypical mycobacteria, or a BCG vaccination, says Gerald Mazurek, MD, project officer for the study and an epidemiologist at the division for tuberculosis elimination at the CDC.

"The test is still a bit complicated right now, but so was the [first experiment with flight at] Kitty Hawk - and today, we're flying," says Mazurek. "This test has great potential and represents a big step forward. Someday, it may replace the PPD skin test."

Data from trial results are expected within less than a year.

The theoretical basis for the test is simple: First, the user measures the amount of gamma interferon produced by lymphocytes in response to an array of antigens; next, responses are compared. Whichever antigen produces the greatest response is presumed to be the one to which patients have been exposed and with which they are infected, Mazurek says.

^{How it works}

Specifically, the test is conducted like this:

A small quantity of blood is drawn and divided among wells of a test tube. Three drops of one of five different antigens are added to each well. The antigens include two controls - one of them a mitogen, which should provoke a positive reaction in everyone; and for the other control, normal saline, which shouldn't provoke any reaction.

The other three antigens include a purified protein derived from M. tuberculosis; a protein derived from M. avium; and a protein derived from M. bovis, the same mycobacterium from which BCG was originally prepared.

Specimens and antigens incubate in the wells for about five hours. The serum is drawn off; and then, using an enzyme-linked immunosorbent assay, or ELISA, the amount of gamma interferon produced by each combination is measured, and the results are compared. The antigen that provokes lymphocytes to produce the most gamma interferon is presumed to be the one with which the patient has been infected.

"If lymphocytes are sensitized and have seen the antigen before, they will mount a response," says Mazurek. "We measure the activity by the amount of gamma interferon that is produced." Gamma interferon is a key cytokine that controls and regulates cell-mediated immunity.

^{Some rough edges to file down}

Some rough edges remain in the test in its present form, says Mazurek. For one thing, investigators at the CDC aren't completely satisfied with the antigen from M. bovis and have decided to try out some recombinant antigens instead.

In addition, the test needs some streamlining to make it more user-friendly. "There are some pretty complicated calculations that must be done to make sure everything's running right," Mazurek says. "We're talking to the company about making it simpler."

QuantiFERON was originally developed for use on free-range cattle in Australia, Mazurek says. The animals must be screened for the presence of M. bovis and M. tuberculosis; if enough animals in a herd test positive for either bacteria, the entire herd generally is destroyed.

With Aussie cattle herds, the impetus for inventing a one-time serum test was especially compelling, says Mazurek. "Since the cattle roam the open range, it's hard enough getting to them the first time," he says. Three days later, the cows are even less approachable, since they now associate the approach of humans with sharp needles.

The obvious solution was to devise a test that called for only a single encounter.

The single-encounter feature of the test means it will be much more economical than the two-step, plant-and-read PPD test, Mazurek says. "The PPD test is a very crude screening device," he says. "We wind up skin-testing 80% to 90% more people than are really infected with TB. If we can reduce even by half the number of visits, clinic encounters, and nursing time expended, that in itself would be a great advantage."

But the one-step test will do even better than that, he adds. "Ninety percent of the people you test don't need to come back in the first place," he points out. "It's only that 10% who have infection who need to come back and have something done."

With a one-step test, the no-shows who need to be found can be determined upfront, allowing investigators to concentrate their efforts on a much smaller proportion of people, Mazurek says.

Trials are now under way at the University of California in San Diego and the University of California in San Francisco; at Johns Hopkins University in Baltimore; at Boston University in Boston; and at the TB Model Center at the New Jersey School of Medicine and Dentistry in Newark.

Three hundred people will be enrolled at each site, including some with culture-confirmed TB; some with suspected TB; contacts; immigrants from TB-endemic areas; and finally, some who are not at risk for TB but who merely need baseline screening for jobs or tests for school.

The QuantiFERON test has another feature investigators appreciate, says Mazurek. "In one or two tests, you can determine if what you're working with is a good antigen."

Reagents that work well with QuantiFERON may prove to be good replacements for what's currently being used for skin-testing, he says. Similarly, the test offers "a good way to look for other, more refined skin tests or to evaluate vaccines," Mazurek says.