A two-month-old boy is sent to the emergency department by his pediatrician for cough and an abnormal CBC with lymphocytosis. He is up-to-date with immunizations. His older brother, who is four years old, is not up-to-date since the pediatrician suspended his immunizations due to a developing neurologic condition. The brother has also had a febrile illness and has been coughing for more than three weeks. You observe the two-month-old coughing and see a period of peri-oral cyanosis. What method of confirmatory testing has the best combined sensitivity and specificity for diagnosis?

-Blood cultures
-Direct fluorescent antibody on nasal swabs
-Polymerase chain reaction of nasopharyngeal secretions
-Sputum cultures