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.

Analysis of inadequate cervical smears using Shewhart control charts.

BACKGROUND: Inadequate cervical smears cannot be analysed, can cause distress to women, are a financial burden to the NHS and may lead to further unnecessary procedures being undertaken. Furthermore, the proportion of inadequate smears is known to vary widely amongst providers. This study investigates this variation using Shewhart's theory of variation and control charts, and suggests strategies for addressing this. METHODS: Cervical cytology data, from six laboratories, serving 100 general practices in a former UK Health Authority area were obtained for the years 2000 and 2001. Control charts of the proportion of inadequate smears were plotted for all general practices, for the six laboratories and for the practices stratified by laboratory. The relationship between proportion of inadequate smears and the proportion of negative, borderline, mild, moderate or severe dyskaryosis as well as the positive predictive value of a smear in each laboratory was also investigated. RESULTS: There was wide variation in the proportion of inadequate smears with 23% of practices showing evidence of special cause variation and four of the six laboratories showing evidence of special cause variation.There was no evidence of a clinically important association between high rates of inadequate smears and better detection of dyskaryosis (R2 = 0.082). CONCLUSIONS: The proportion of inadequate smears is influenced by two distinct sources of variation - general practices and cytology laboratories, which are classified by the control chart methodology as either being consistent with common or special cause variation. This guidance from the control chart methodology appears to be useful in delivering the aim of continual improvement.