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Blood test shifts HCWs from TB-positive to negative
Employees have more faith in TB blood test
Switching from the skin test to the blood test can completely change the profile of your tuberculosis screening program, as many TB-positive employees suddenly learn that they are not infected.
Although that change may be unsettling to employees, they ultimately gain greater confidence in their TB test results and appreciate the swifter screening process, says Jackie Kinard, RN, APN, COHN-S, occupational health specialist at Sierra Health Services in Las Vegas.
Sierra Health began using the QuantiFERON-TB Gold test, manufactured by Cellestis, among its 1,600 employees in 2005. The health system, which includes a same-day surgery center, hospice, home health, and clinics, retested 218 employees who had previous positive skin tests. Only 44 of those employees (19.7%) tested positive on the blood test. Many of the previously positive employees were foreign-born and had a history of BCG vaccination.
It was hard for some of those employees to accept the change in status. "People questioned the results," acknowledged Kinard. But after educational sessions on QuantiFERON, they understood the difference between the blood test and the skin test, she says.
QuantiFERON is a whole blood assay test that measures the amount of interferon-gamma produced after the blood is incubated for six to 13 hours with TB antigens. The blood test is more specific than the skin test and does not react to BCG or other mycobacteria.
Those who tested positive with QuantiFERON at Sierra Health were more likely to accept treatment for latent TB infection. Of 121 employees who tested positive and received follow-up evaluation from the Clark County Health District in the first full year of QuantiFERON testing, 28 opted to take the treatment. Previously, only one or two employees a year would accept treatment for latent TB infection, says Kinard.
Meanwhile, the new screening protocol is much easier to administer and saves hundreds of hours of employee time, says Kinard. Employees no longer have to return for their skin test to be read, and employee health doesn't have to keep track of the "two-step" tests that require two placements and two readings for new employees.
"We're saving a tremendous amount of money [in employee time and productivity], and we feel more comfortable with the results," she says.
TST costly in employee time
Although the blood test represents a significant advance in TB screening, there still are obstacles for hospitals to overcome as they adapt to the new technology.
Sierra Health contracts with a private lab that runs the tests. "We had to find a lab that was willing to do the test. I tried unsuccessfully for several months," says Kinard. Finally, the medical director of occupational health convinced a lab to purchase the necessary equipment and training. Kinard also contacted the office of the state TB controller, which changed the wording in the state regulations to allow the use of blood tests
as well as tuberculin skin tests for screening.
The cost per test, including the lab fees, runs about $50. By contrast, the purified protein derivative for the skin test costs only about $10. Yet the true cost of the tuberculin skin test includes the training of test placers and readers, employee time away from work to have the test placed and read, and administrative time coordinating the program.
The cost per health care worker tested can range from $41 to $362 with the skin test, according to a Centers for Disease Control and Prevention study of TST programs at four hospitals.1 The largest portion of that cost was for personnel, including the program coordinator, the test placers and readers, and data entry. The study also accounted for lost work time as employees left their posts for the skin test.
Kinard conducted her own cost-benefit analysis, including the personnel costs and the time spent following up with people who had not returned to have the test read. She estimated that the skin test cost about $70 per health care worker tested.
Some employees were squeamish about the blood draw, says Kinard. But they overcame that, and were pleased with the convenience, she says. "You're still getting a needlestick when you plant the derivative [in the skin test]," she says. "Certainly there's a needle involved. If you need a two-step, it's twice. If you fail to come back, it has to be redone. This is a one-time test."
What do reversions mean?
Health care workers may have more confidence in a positive result from the blood test, but what does a negative mean? Could it revert back to a positive?
More research is needed on the cutoff values that signify a positive result on the blood tests, says Madhukar Pai, MD, PhD, assistant professor of biostatistics and occupational health at McGill University in Montreal. He advises employee health professionals to look at the actual values.
"Be careful in looking at the negative to positive change and calling that a conversion," he says. "If someone went just above the cutoff, you would call that a conversion."
There also are unresolved questions about conversions from positive to negative with the blood tests, he says. Research is needed on the reproducibility of interferon gamma release assays in multiple tests and how the levels may change over time.2
"Maybe the body has successfully cleared the organism or maybe the drugs [for latent TB infection] have worked," suggests Pai. "Maybe it's just nonspecific changes in the test over time. We don't really know how to approach the problem of reversions."
The CDC advises labs to report the actual values on the QuantiFERON test. If the level is near the cutoff, you may want to repeat the test, says Jerry Mazurek, MD, a commander in the U.S. Public Health Service and medical officer and epidemiologist in the CDC's Division of TB Elimination.
The CDC is studying conversions with QuantiFERON and the stability of the test. "It's not a perfect test. Unexplained revisions occur," says Mazurek "Sometimes, you can explain the conversion from negative to positive by the fact that the test is near the cut point and it could represent a false-positive."
The QuantiFERON test was designed to maximize specificity, says Mark Boyle, senior vice president of sales and marketing with Cellestis. But it also has good sensitivity, and only about 8% of people screened with QuantiFERON have values near the cutoff, he says.
As long as someone still has latent TB infection — and has not cleared the disease — their values should still be positive, Boyle says. "If you've been treated, your skin test will more than likely stay positive forever. But most people would be QuantiFERON-negative," he says.
Meanwhile, Sierra Health plans to retest its positive employees to gain information about the consistency of the results, says Kinard. "Once we have two full years of testing completed, then we can look at [the results] more clearly," she says.
1. Lambert L, Rajbhandary S, Qualls N, et al. Costs of implementing and maintaining a tuberculin skin test program in hospitals and health departments. Infect Control Hosp Epidemiol 2003; 24:814-820.
2. Pai M, O'Brien R. Serial testing for tuberculosis: Can we make sense of T cell assay conversions and reversions? PLoS Medicine 2007; 4:e208. Available at: www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1891321.