What is the likely next step in diagnosis following a suspicion of pertussis in a child presenting with typical symptoms?

When a child presents with typical symptoms of pertussis, such as a paroxysmal cough, inspiratory whoop, and post-tussive vomiting, the next appropriate step in diagnosis is to perform a Pertussis PCR testing. This polymerase chain reaction (PCR) test is highly sensitive and specific for detecting the genetic material of Bordetella pertussis, the bacterium that causes pertussis.

PCR testing allows for the detection of the bacteria early in the course of the illness, even before the characteristic cough develops or after antibiotic treatment has started. This early detection can be crucial for initiating treatment and implementing necessary public health measures to prevent the spread of pertussis.

While other diagnostic tools like blood cultures or complete blood counts may provide some information about a child's overall health or the presence of other infections, they are not specific or sensitive enough to confirm a diagnosis of pertussis. A chest X-ray may help assess complications or rule out other respiratory conditions, but it does not directly support the diagnosis of pertussis. Thus, Pertussis PCR testing is the most appropriate next step following the suspicion of the disease in a child exhibiting hallmark signs.