A child presented with complaints of cough with a characteristic inspiratory whoop. He was not immunised. Sample for investigation is:

Characteristic inspiratory whoop in a child suggests whooping cough. Culture of nasopharyngeal secretions remains the gold standard of diagnosis, although DNA detection by polymerase chain reaction (PCR) has replaced culture because of increased sensitivity and quicker results. The best specimen is collected by nasopharyngeal aspiration. An alternative to the aspirate is a Dacron or rayon nasopharyngeal swab and inoculation of culture plates should be immediate or an appropriate transpo medium (e.g., Regan-Lowe charcoal medium) should be used. Ref: Halperin S.A. (2012). Chapter 148. Peussis and Other Bordetella Infections. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.