Faith in Quick Test Leads to Epidemic That Wasn’t
NY Times 2007 Article articulates the problems associated with PCR test, looking back on the episode, epidemiologists and infectious disease specialists say the problem was that they placed too much faith in a quick and highly sensitive molecular test that led them astray.
“It’s a problem; we know it’s a problem,” Dr. Perl said. “My guess is that what happened at Dartmouth is going to become more common.”
Many of the new molecular tests are quick but technically demanding, and each laboratory may do them in its own way. These tests, called “home brews,” are not commercially available, and there are no good estimates of their error rates. But their very sensitivity makes false positives likely, and when hundreds or thousands of people are tested, as occurred at Dartmouth, false positives can make it seem like there is an epidemic.
“You’re in a little bit of no man’s land,” with the new molecular tests, said Dr. Mark Perkins, an infectious disease specialist and chief scientific officer at the Foundation for Innovative New Diagnostics, a nonprofit foundation supported by the Bill and Melinda Gates Foundation.
At Dartmouth the decision was to use a test, P.C.R.
At Dartmouth, when the first suspect pertussis cases emerged and the P.C.R. test showed pertussis, doctors believed it. The results seem completely consistent with the patients’ symptoms.
“Because we had cases we thought were pertussis and because we had vulnerable patients at the hospital, we lowered our threshold,” she said. Anyone who had a cough got a P.C.R. test, and so did anyone with a runny nose who worked with high-risk patients like infants.
“That’s how we ended up with 134 suspect cases,” Dr. Kirkland said. And that, she added, was why 1,445 health care workers ended up taking antibiotics and 4,524 health care workers at the hospital, or 72 percent of all the health care workers there, were immunized against whooping cough in a matter of days.
The Dartmouth doctors sent samples from 27 patients they thought had pertussis to the state health departments and the Centers for Disease Control. There, scientists tried to grow the bacteria, a process that can take weeks. Finally, they had their answer: There was no pertussis in any of the samples.
They could only get suitable blood samples from 39 patients — the others had gotten the vaccine which itself elicits pertussis antibodies. But when the Centers for Disease Control tested those 39 samples, its scientists reported that only one showed increases in antibody levels indicative of pertussis.
The disease center did additional tests too, including molecular tests to look for features of the pertussis bacteria. Its scientists also did additional P.C.R. tests on samples from 116 of the 134 people who were thought to have whooping cough. Only one P.C.R. was positive, but other tests did not show that that person was infected with pertussis bacteria.
“It was going on for months,” Dr. Kirkland said. But in the end, the conclusion was clear: There was no pertussis epidemic.
“The big message is that every lab is vulnerable to having false positives,” Dr. Petti said. “No single test result is absolute and that is even more important with a test result based on P.C.R.”