VAIL, COLO. – One of the most useful signs that a young infant with an afebrile coughing illness has pertussis is the combination of an elevated white blood cell count of 20,000 cells/mcL or more plus a lymphocyte count of at least 10,000 cells/mcL.
"This is a pediatric pearl. It’s really a poor man’s way of diagnosing pertussis. It’s an effect of the pertussis toxin spreading throughout the neonate’s body, causing a very high white cell count with absolute lymphocytosis," Dr. Ann-Christine Nyquist said at a conference on pediatric infectious diseases sponsored by the Children’s Hospital Colorado.
This clinical pearl is part of a highly useful algorithm put forth by the Global Pertussis Initiative in an effort to update and standardize the case definitions of pertussis. Existing case definitions were developed more than 40 years ago and have numerous shortcomings.
The group developed a three-pronged, age-based algorithm, reflecting an understanding that the key manifestations of pertussis are different in infants aged 0-3 months, children aged 4 months to 9 years, and adolescents or adults.
According to the algorithm, the presence of an elevated WBC count with absolute lymphocytosis in an infant up to 3 months old with an afebrile illness and a cough of less than 3 weeks’ duration that’s increasing in frequency and severity is "virtually diagnostic" of pertussis (Clin. Infect. Dis. 2012;54:1756-64).
Another key feature of pertussis – and this one applies across the age spectrum, from infants to adults – is that the coryza remains watery and doesn’t become purulent, unlike in most viral respiratory infections.
"That green snotty nose doesn’t usually happen when kids have pertussis," explained Dr. Nyquist, professor of pediatrics at the University of Colorado, Denver.
Similarly, in patients of all ages the pertussis cough, even as it worsens, does not become truly productive.
To help nail down the diagnosis of pertussis in infants, the key question to ask parents is, "Is there an adult or adolescent in your family who’s had the most severe cough in their life?" Most infants with pertussis will have had close exposure to an older family member with a prolonged afebrile coughing illness, Dr. Nyquist noted.
In the 4-month to 9-year-old age group, the cough becomes more paroxysmal. The key indicators of pertussis in this age group, according to the Global Pertussis Initiative algorithm, are worsening paroxysmal nonproductive cough of at least 7 days’ duration in an afebrile child with nonpurulent coryza.
This same triad – worsening paroxysmal nonproductive cough, afebrile illness, and nonpurulent coryza – also has high sensitivity and good specificity for the clinical diagnosis of pertussis in adolescents and adults. In addition, the algorithm highlights another useful clue to the diagnosis in patients in this age range: the occurrence of sweating episodes between coughing paroxysms.
In terms of laboratory diagnostics, real-time PCR and culture of nasopharyngeal mucus are most useful in the first 3 weeks after illness onset. Serology is a challenge because the results are influenced by the effects of vaccination; it shouldn’t be used to diagnose pertussis within 1 year following inoculation with any pertussis vaccine because it’s impossible to tell if a positive result represents a response to the vaccine or to infection.
Also, serology can’t distinguish between Bordetella pertussis and B. parapertussis infection. Most PCR tests can. It’s an important distinction because B. parapertussis turns out to be the pathogen in roughly 15% of cases of coughing illnesses similar to pertussis. B. parapertussis infection isn’t vaccine preventable, and its treatment hasn’t been well studied.
The expert consensus is that direct fluorescent antibody testing should be discouraged as a tool to diagnose pertussis because of its unreliable sensitivity and specificity. IgG anti–pertussis toxin ELISA testing is superior to IgA anti–pertussis toxin ELISA, which has a high false-negative rate, Dr. Nyquist observed.
She reported having no financial relationships with any commercial interests.