Abstract
BackgroundHistorically 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.MethodsRoutinely 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.ResultsFollowing 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.ConclusionA 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.
Highlights
There has been a wide variation in the proportion of inadequate smears between general practices
Following the introduction of liquid based cytology (LBC) the overall proportion of inadequate samples fell to 1.3% (p < 0.05) (1.5% for Laboratory 1 and 0.6% for Laboratory 2)
A systematic review commissioned before the decision to roll out LBC found an overall proportion of inadequate samples of 0.8%, which is comparable to the 1.3% in this study [7]
Summary
There has been a wide variation in the proportion of inadequate smears between general practices. 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. Inadequate smears are a source of avoidable distress to women, and a potential waste of resources in general practices, clinics and cytology laboratories [2]. The resulting P-charts show the impact of sample size on the control limits, and secondly, the resulting control limits are easy on the eye because they appear like a funnel. Such plots have been advocated in health care [6]
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