The impact of the introduction of liquid based cytology on the variation in the proportion of inadequate samples between GP practices

BACKGROUND: Historically there has been a wide variation in the proportion of inadequate smears between general practices. Cervical screening in the UK is undergoing a fundamental change by moving from conventional to liquid based cytology (LBC). The main driver for this change has been a predicted reduction in the proportions of inadequate samples. This study investigates the effect of LBC on the variation in the proportion of inadequate samples between general practices using Shewhart's theory of variation and control charts. METHODS: Routinely collected cervical cytology data was obtained for all general practices in two localities in South Staffordshire for periods before and after the introduction of liquid based cytology. Control charts of the proportion of inadequate smears were plotted for the practices stratified by laboratory. A standardised measure of variation for all of the practices in each laboratory and each time period was also calculated. RESULTS: Following the introduction of liquid based cytology the overall proportion of inadequate samples in the two localities fell from 11.8 to 1.3% (p < 0.05). This fall was associated with a reduction in the average variation between the GP practices in the two localities from 1.6 to 1.0 standard deviations. There has also been a reduction in the number of practices showing special cause variation from eight to one following the introduction of liquid based cytology. CONCLUSION: A reduction in the proportion of inadequate samples has been realised in these localities. The reduction in the overall proportion of inadequate samples has also been accompanied by a reduction in variation between GP practices.