Identifying if significant differences exist between the population of cats that are not registered with a veterinary practice and the population of registered cats is important in the design of future epidemiological studies. Due to practical reasons, studies within feline medicine are often conducted on populations of cats that visit veterinary practices, using randomly selected cats registered with the practice as a control group. Although it may be reasonable to assume that most, if not all, owners of sick cats will take their cat to a vet, whether or not the cat was previously registered with a veterinary practice, comparing these cats with a control group of "healthy" registered cats may introduce bias if these control cats are not representative of the general UK "owned" cat population. In addition, it is predicted that studies designed to estimate factors such as the prevalence of unneutered or unvaccinated cats would generate biased results by using a sample based on veterinary-registered cats. A study was therefore conducted to establish what percentage of "owned" UK cats are registered with a veterinary practice and to compare the characteristics of these cats and their owners with the characteristics of unregistered cats and their owners.

Owners of cats were contacted through dialing randomly selected telephone numbers of households listed on the UK electoral roll, which had not registered with the Telephone Preferential Service. Telephone questionnaires were completed by 300 cat owners for one cat in their care that was selected at random. Approximately 13% (40/300) of cats had not been registered with a veterinary practice since living at their current address. Univariable and multivariable logistic regression models were used to analyse the data. Unneutered cats and cats that were kept outdoors were significantly more likely (P<0.01) to be unregistered when compared with neutered cats and cats that were permitted inside the owners house. Cats that had not been vaccinated within the last 12 months were also significantly more likely to be unregistered than cats that had been vaccinated within the last year (P<0.01). These results indicate significant differences amongst the owned cat population and their registration status. It is therefore important that epidemiological studies use samples of cats that include unregistered cats as the characteristics of this group differ from cats registered with veterinary practices.