Employee PPD testing effective, cost-efficient

Even two-step testing is worth added effort

Two new studies — one of physicians, the other of hospital employees — have shown that purified protein derivative (PPD) tuberculin skin testing is an effective and cost-efficient means of preventing tuberculosis (TB). However, getting employees to take the tests may become more difficult as TB rates go down, says one of the study’s authors.

The Centers for Disease Control and Prevention has recommended two-step testing of new health care employees since 1994 as a means of eliminating false conversions that result from the boosting phenomenon. However, the costs and benefits of two-step testing have not been studied.

In a retrospective review of health care workers hired between 1993 and 1994 at St. Clare’s Hospital in New York City, researchers analyzed the records of 262 employees who were given two-step testing at the time of employment. Of that group, 107 or 21% had positive results on the first skin test. After a second test one week later, 15 or 9.7% of the remaining 155 patients became positive. These employees who converted on the second test were more likely to be male, foreign-born, or to have the bacille Calmette-Guerin (BCG) vaccination. By identifying these 15 people — and thereby being able to exclude them as false converters — the hospital was able to prevent a nearly 50% increase in its annual conversion rate, says Kent Sepkowitz, MD, lead author and associate chairman of medicine at Memorial Sloan-Kettering Cancer Center in New York City.1

"We had a medium conversion rate that we brought down considerably — from 4.7% to 3.2%. That is a huge difference, and these are Occupational Safety and Health Administration (OSHA) reportable events, so they are monitored, as they should be, quite carefully," he says.

Two-step testing is seen as essential to effective TB control because without it, it is difficult to establish reliable baseline tests for employees, particularly for those who are elderly or come from countries where BCG is administered. Both of those groups are more likely to have negative results on their first skin test because of a weakened immune system or BCG vaccination. They test positive on the second test only after they have been "primed" by the first test. These false conversions confound the actual rate of exposure at a facility.

"The reason two-step testings save the facility money and angst is you have a real read on your conversion rate, and it is always going to be lower," Sepkowitz explains. "Any system has nonspecific boosting that is misclassified as true conversion, when in fact it is a pseudo conversion."

While some studies have shown that two-step testing is not cost-effective in some settings, Sepkowitz says it makes sense for facilities that have high rates of elderly workers or BCG-vaccinated employees. "In facilities where employees haven’t received BCG or are quite young, I think one could argue it doesn’t pay off," he adds.

However, he points out a study of employees at Sacramento (CA) Kaiser-Permanente Medical Center more than 10 years ago in which only six of 1,521 employees were identified as false converters. And yet, the authors concluded that the cost of two-step testing at that facility was justifiable even with its low prevalence of TB.

In a second article, published recently in the Archives of Internal Medicine, researchers studied the cost-effectiveness of skin testing physicians and found that annual testing costs only $29,000 per life-year saved and $39,000 per case of TB prevented.2

The authors used the decision analysis to determine the cost-effectiveness of TB skin testing over the lifetime of physicians now in medical school. The analysis assumed a modest rate of infection with TB — .4% a year and a 10% lifetime risk of developing active TB in people who are infected.

The study was unable to determine which physicians might benefit more from skin testing, but the authors suggest that those who treat high-risk populations and those who spend internal medicine residency in high-risk areas are more prone to conversions.

While the study shows that skin testing for physicians is a more effective prevention method than particulate respirators (estimated to cost as much as $7 million to $18 million per case of TB prevented), getting physicians to agree to skin testing is a major challenge.

As Sepkowitz bluntly puts it, "Docs never want to do it."

And facilities have a harder time making them do it. Many facilities have tied employee paychecks to compliance with skin testing, but for physicians who have facility privileges but are not paid by the facility, there are fewer options.

Indeed, as TB rates continue to drop each year, facilities may find it harder to justify regular skin testing, Sepkowitz says.

"People are sufficiently selfishly concerned they will get TB that they usually will undergo testing," he says. "But I think that enthusiasm is starting to wane among health care workers because the threat seems less acute."

On the other hand, health experts point out that the new TB standard from OSHA, which is expected to propose annual PPD testing for health care workers, will make it harder for facilities to become lax in compliance.

References

1. Sepkowitz K, Feldman J, Louther J, et al. Benefit of two-step PPD testing of new employees at a New York City hospital. Am J Infect Control 1997; 25:283-286.

2. Nettleman M, Geerdes H, Roy M. The cost-effectiveness of preventing tuberculosis in physicians using tuberculin skin testing or a hypothetical vaccine. Arch Intern Med 1997; 157:1,121-1,127.