Pertussis is quite frequent and severe among infants; therefore, rapid diagnosis and timely targeted therapy are essential. Although a molecular test for etiological diagnosis is now available, it may not be available everywhere, and therefore adjunctive diagnostic tests are still useful for presumptive diagnosis. We describe the use of procalcitonin (PCT) and lymphocyte count to discriminate among pertussis, bacterial and viral infections. Fourteen infants per group were studied. The decision tree, built considering all available variables, showed a major role of PCT in predicting the different groups. A PCT value equal to or greater than 0.75 ng/ml selected for bacterial infections. A PCT value lower than 0.75 ng/ml and a lymphocyte count equal to or greater than 10,400/mm(3) selected the subjects with pertussis, while a lymphocyte count lower than 10,400/mm(3) selected for viral etiology. PCT should be used in the diagnosis of infants suspected of having pertussis.
