"Outbreaks" show pertussis diagnosis not easy
■ The CDC seeks a better test to differentiate pertussis from other respiratory illnesses.
By Susan J. Landers — Posted Sept. 17, 2007
Washington -- Pertussis can be difficult to diagnose quickly and accurately. This reality may be behind the incorrect attribution of three recent outbreaks of respiratory illness to the highly contagious disease, noted the Centers for Disease Control and Prevention.
Subsequent investigations of outbreaks in New Hampshire, Massachusetts and Tennessee -- two of which occurred in hospitals -- revealed negative or at least equivocal lab results for pertussis, according to a report in the Aug. 24 Morbidity and Mortality Weekly Report.
Diagnosing pertussis is complicated by its symptoms -- which, especially early on, mimic common viral infections -- and the lack of rapid, sensitive lab tests, noted the CDC.
At Dartmouth-Hitchcock Medical Center in Lebanon, N.H., the site of one outbreak, 978 employees were tested, treated with antibiotics and furloughed; 4,524 employees were vaccinated; and an additional 1,311 family members and friends suspected of being infected were given antibiotics.
Although several pertussis detection tests are available, the polymerase chain reaction test is the most popular, said Matt Zahn, MD, assistant professor of pediatrics and pediatric infectious diseases at Kosair Children's Hospital in Louisville, Ky. He is also the medical director of the Louisville Public Health Dept.
"The advantage is you can get an answer quickly," he said. The disadvantage is that it can produce many false-positive results.
Although the problem of false-positives is more likely in the public health domain where more people with less specific symptoms are tested in suspected outbreaks, most actual cases of the disease probably occur on a sporadic basis and are identified in individual physicians' offices, said Tom Clark, MD, MPH, an epidemiologist at the CDC, and the leader of the team that drafted the MMWR report.
Physicians should maintain a heightened suspicion, he noted. "For your average doctor in a clinic with a patient who has a cough and a positive PCR, you treat them as if they had pertussis."
What physicians can do to avoid problems is to understand what types of tests are administered at the lab they use, said Dr. Clark. "We use a two-target PCR to improve the specificity of testing. The more common, single specificity test, is more prone to contamination."
Another issue: The PCR is more accurate early in the disease's course. However, physicians may not suspect pertussis until a patient has been coughing for a few weeks. At that point, the test is less reliable.
Although the misdiagnoses do call into question reports that pertussis cases have tripled in recent years, this increase is still probably very real, added Dr. Clark.
"There is a lot of awareness on the part of physicians that ... it should be considered in people with prolonged coughs," he said.
Other factors that make this rise in cases believable, said Dr. Clark, are waning immunity in teens and young adults, recommendation that this age group receive a pertussis booster, and increased use of PCR testing.
But the CDC had noted the difficulty in accurately diagnosing pertussis before the recent misdiagnoses and had begun a large, multicenter trial to develop a standardized PCR test that could be used by all labs as well as a blood test that could be marketed.
Meanwhile, vaccination is a key strategy, the CDC says. Its Advisory Committee on Immunization Practices recommends that people ages 11 to 64 receive the Tdap vaccine, estimated at 85% to 92% effective in preventing pertussis.