Dr. Peter McKenna:

My understanding is that different laboratories receive tests from different parts of the country. Consequently, some of the laboratories might receive tests from a disproportionately high number of women who attend colposcopy clinics and therefore one would expect there to be a disproportionate number of high grade lesions. Within the country one would also expect there to be some variations between urban and rural. There would be socioeconomic parity reasons as well. Issues such as smoking which are socioeconomic are also a factor. As it is, if it is split geographically, there will be variation.