Variation in requests for imaging investigations by general practitioners: a multilevel analysis.

Objectives: To describe the variation in the numbers of imaging investigations requested by general practitioners (GPs) and to find likely explanations for this variation.

Methods: Cross-sectional survey of the use of eight imaging investigations by 229 GPs collaborating in 40 local GP groups from five regions in the Netherlands during 1997. A multivariable, multilevel regression analysis was used to link these data with survey data on professional characteristics such as knowledge about and attitude towards test ordering, and with data on contextual factors such as practice type or experience with feedback on test-ordering data.

Results: Data for 221 GPs (97%) were available. After adjustment for practice size and working time, the median number of imaging investigations ordered per GP was 148 (interquartile range 71 to 300), with large differences (up to twofold) between the regions (P<0.001). Overall, chest X-rays were the largest single category (median = 48 interquartile range 17 to 100). GPs working in a group practice requested, on average, 34% fewer investigations than their colleagues working in single-person practices (95% confidence interval 17 to 48%).

Conclusions: Only practice type was found to be associated with the number of imaging investigations requested, adjusted for practice size and working time factor. No further explanations were found for the inter-regional differences. Future studies on the ordering of imaging investigations by GPs should attempt to delineate contextual from regional factors.