Disease surveillance and referral bias in the veterinary medical database

The Veterinary Medical Database (VMDB) is a summary of veterinary medical records from North American veterinary schools, and is a potential source of disease surveillance information for companion animals. A retrospective record search from four U.S. university veterinary teaching hospitals was used to calculate crude disease rates. Our objectives were to evaluate the utility of the database for disease surveillance purposes by comparing the utility of two methodologies for creating disease categories, and to evaluate the database for evidence of referral bias. Summaries of the medical records from November 2006 to October 2007 for 9577 dogs and 4445 cats were retrieved from VMDB for all canines and felines treated at Kansas State University, Colorado State University, Purdue University and Ohio State University. Disease frequency, computed as apparent period-prevalence and as the percentage of veterinary visits, was compiled for 30 disease categories that were formulated by one of two methods. To assess the possible impact of referral bias, disease rates were compared between animals residing in zip codes within 5 miles of the hospitals (zone 1) and those animals living at more distant locations (zone 2). When compared to zone 1 animals, disease conditions commonly associated with primary veterinary care were reduced by 29-76% within zone 2, and selected diseases generally associated with more specialized care were increased from 46 to 80% among zone 2 animals. The major differences in disease prevalence seen between zones suggests that substantial referral bias may exist, and that adjustment on the basis of geographical proximity to the university teaching hospitals may be useful in reducing this type of selection bias in the VMDB, thereby improve the accuracy of prevalence estimates and enhancing the utility of this database for purposes of disease surveillance.