A 15 year old female patient was presented to her pediatrician with a 2.5 week history of paroxysms of cough.  The paroxysms were worse in the evening and seemed to be getting progressively more severe.  The patient was compliant with recommended vaccinations; however the physician was concerned for whooping cough due to index cases reported in the community in recent weeks (despite the absence of a tell-tale “whoop”-like presentation).  The physician contacted the resident on-call covering the laboratory to ask what Pertussis testing would be most reliable at 2.5 weeks of clinical symptoms.

What should the resident recommend?