BACKGROUND: Extreme leukocytosis in pertussis is a rare condition, and without effective interventions to reduce white blood cell counts, the mortality rate can approach 100%. The clinical characteristics of these patients and the application of exchange transfusion (ET) in their management are not yet clear. METHODS: This retrospective study examines the clinical characteristics and impact of ET in infant pertussis with extreme leukocytosis. RESULTS: We have included six infant pertussis patients with extreme leukocytosis, all of whom were female and underwent ET. Two patients survived, while four died. The surviving patients were relatively older at disease onset compared to those who died, and all three unvaccinated patients died. All patients required admission to the pediatric intensive care unit, presenting with fever, whooping cough, cyanosis, severe pneumonia, and respiratory failure. Pulmonary consolidation, cardiovascular failure, and pulmonary hypertension (PH) were also common, especially among those who died. Hypoglycemia and seizures were rare. Acute-phase proteins, such as C-reactive protein and procalcitonin, were elevated to varying degrees. ET effectively reduced peripheral blood leukocytes
however, a significant increase in leukocytes was observed 1-2 days after the first ET in the deceased patients. CONCLUSION: Extreme hyperleukocytosis is more commonly observed in young female children with pertussis. Younger age, unvaccinated status, and the presence of concurrent heart failure and PH may be associated with a poor prognosis. ET can effectively reduce peripheral blood leukocytes, but a rapid leukocyte rebound post-ET may be indicative of impending death.