An 8 year old child with a history of cough for 2 weeks +.  The GP tested the child by throat swab and PCR which was positive. A 7 day course of erythromycin was prescribed.  Reviewed then and patient complaining that the cough worsening.

Q & A :

1. The cough is worsening – does he need another antibiotic course?

No.  Antibiotic therapy in pertussis seldom ameliorates symptoms unless given within a few days of onset of the disease.  A second course of antibiotics achieves nothing and exposes the child to an increased risk of antibiotic side effects. 

1. Should I repeat the PCR test to show proof of cure?

No. Pertussis PCR is highly sensitive and may remain positive even in patients who have received treatment (though unlikely to be positive after 4 weeks of illness in untreated cases). A positive PCR won’t distinguish between dead or alive pertussis bacteria. Provided that a patient has been compliant, 1 week of erythromycin (or more thatn 3 weeks of illness) renders a patient non-infectious. 

Reference 

Pertussis CDNA National Guidelines for Public Health Units