Investigating the impact of case-mix on general practice cancer diagnostic outcome indicators

Background The Cancer Services profiles report indicators of cancer diagnostic activity for all English general practices. A recent study reported that several indicators were dominated by chance, with some practice-level variation explained by the practice’s age-sex profile. Aim To assess the variation explained by patient-level case-mix and whether the practice age-sex profile adequately adjusts for this. Method Five indicators from Cancer Waiting Times (2016/17, 6050 practices) or Routes to Diagnosis (2015, 6355 practices) data were considered: Two Week Wait (TWW) conversion and detection rates, and emergency-, referred- and other-diagnosis proportions. Mixed-effect logistic regression adjusted for patient-level case-mix, using national cancer registration data on age, sex, deprivation, referral/cancer-type and, where possible, ethnicity and stage at diagnosis, with and without practice-level age-sex profile. Results Chance explained 60–85% of practice-level variation, with the combination of chance and patient-level case-mix explaining 75% (TWW conversion rate) to 89% (emergency diagnosis proportion). For TWW conversion rate, there was considerable overlap in the variance explained by practice- and patient-level factors. For the other indicators, practice- or patient-level factors were largely independent. Conclusion Chance is not synonymous with case-mix and is the dominant source of variation in single-year practice indicators. Therefore, we recommend the continued aggregation of data over multiple years. For most studied indicators, adjustment for the age-sex profile of the whole practice population is not a substitute for case-mix of individual cancer patients and so should not be used. Patient-level case-mix adjustment leads to a modest reordering of practices and so may not be a priority.