Improving the NHS cervical screening laboratory performance indicators by making allowance for population age, risk and screening interval.
OBJECTIVE: One of the key performance measures in the monitoring of the NHS cervical screening programme is the targeting of laboratories with very high or low percentages (outside the 10th-90th percentile) of adequate smears that have moderate dyskaryosis or worse. These laboratories are assumed to include those laboratories that may have extremes of sensitivity and specificity. A clear limitation with this methodology is that laboratories do not examine smears from women with the same underlying risk, age distribution or screening interval and adjustment for these factors should considerably improve the method. METHODS: This paper describes a method that allows for these confounding variables and a new age-risk-interval adjusted moderate dyskaryosis or worse rate (ARI-adjusted mod+ rate) can be calculated. The adjusted rate is the rate of moderate or worse dyskaryotic smears that the laboratory would have detected had it been screening women with an English 'average' age-risk-interval. All laboratories can therefore be compared using this method. RESULTS: The methodology is illustrated using data from the NHSCSP South West Region. The particularly low percentage of moderate or worse smears detected by one or two laboratories can be shown to be due to a local screened population with a very low risk because of a high mean age, relatively short screening interval and census variables associated with a low risk, rather than any under-calling by the associated laboratories. CONCLUSIONS: The ARI-adjusted mod+ rate requires to be calculated for all laboratories in England if it is to be used as a primary performance indicator. Alternatively, it can be used to further examine laboratories that are deemed to be outliers using the current methodology.