Diagnostic testing and discharge coding for whooping cough in a children's hospital

Objective:  To evaluate the diagnostic pathways for whooping cough in a large urban paediatric hospital to inform assessment of the relative merits of notification and hospitalization data for measuring pertussis disease burden in Australian children.

Methods:  All laboratory requests for Bordetella pertussis (BP) culture or serology between 30 June 1997 and 30 June 1999 were reviewed and cross-checked against discharge diagnoses with International Classification of Disease (ICD) codes A37.0, 033.0 (whooping cough due to BP) or 37.9, 033.9 (whooping cough due to unspecified organisms). Culture-positive (CP) cases were defined as a positive culture or polymerase chain reaction for BP. Culture-negative (CN) cases either fulfilled the current Australian clinical case definition (≥14 days of cough with one or more of paroxysms, whoop, post-tussive vomiting), or had a cough illness with either positive BP serology or documented contact with an individual coughing for >14 days. In infants <6-months-old, a coughing illness with apnoea and negative investigations for other causes was also accepted. Culture positive and CN cases were cross-referenced with notification data.

Results:  During the study period, laboratory tests for BP were performed in 677 children, of whom 230 were hospitalized and 71 (31%) had an eligible ICD code at discharge; 29 were CP, 40 CN, and two (3%) were misclassified. A further 14 CP children were not admitted. Although 61 hospitalized cases (88%) fulfilled notification criteria, including 32 (80%) of CN cases, only 26 (90%) of CP and eight (20%) of CN cases were notified.

Conclusions:  Notifications substantially under-enumerate hospitalized infant cases, especially those without positive laboratory tests. Hospital discharge data add significantly to surveillance for pertussis, particularly in infancy where most severe cases occur.