General practice performance in referral for suspected cancer: influence of number of cases and case-mix on publically reported data

Background: Publicly available data show variation in GPs use of urgent suspected cancer referral pathways. We investigated if this could be due to small numbers of cancer cases and random case-mix, rather than true variation in performance.
Methods: We analysed individual practice urgent suspected cancer referral (USC) detection (number of practice’s cancer detected via USC) and conversion rates (number of practice’s USC referrals which are cancer) in routinely collected data on cancer referrals from GP practices in all of England (over four years) and North-east Scotland (over seven years). We explored the effect of pooling data. We then modelled the effects of adding random case-mix to practice variation.
Results: Correlations between practice detection rate and conversion rate became less positive when data were aggregated over several years. Adding random case-mix to between-practice variation indicated that the median proportion of poorly performing practices correctly identified after 25 cancer cases were examined was 20% (IQR 17 to 24) and after 100 cases was 44% (IQR 40 to 47).
Conclusion: Much apparent variation in GPs’ use of suspected cancer referral pathways can be attributed to random case-mix. The methods currently used to assess the quality of GP suspected cancer referral performance, and to compare individual practices, are misleading. These should no longer be used and more appropriate and robust methods should be developed.