The expected reduction in the proportion of inadequate samples following the introduction of LBC has initially been realised in these localities. A systematic review commissioned before the decision (by the National Screening Committee) to roll out LBC found an overall proportion of inadequate samples of 0.8% (95% CI 0.1% to 5.5%), which is comparable to the 1.3% in this study (95% CI 1.2% to 1.5%) .
Our findings are contrary to a recent systematic review of primary studies comparing conventional and LBC techniques that found a median difference in the percentage of unsatisfactory slides of only 0.17% (IQR 0.98% to -0.37%) . This difference seems mainly due to a higher proportion of inadequate samples for conventional cytology in practice compared to the primary studies ; it remains to be seen if the low proportion of inadequate samples for LBC can be maintained over the longer term.
The introduction of LBC in these two localities has also been associated with a reduction in the variation in the proportion of inadequate samples between GP practices. Because of the relatively low number of samples taken after the introduction of LBC, some practices with zero inadequate samples can still fall above the lower control limit. However, this affects only one practice served by each laboratory so any effect would be minimal. There is further evidence that the introduction of LBC has resulted in a reduction in variation with only one practice now displaying special cause variation compared to the eight practices previously.
It seems therefore that the practices in these two localities are now displaying a more consistent process. This is perhaps not surprising as the introduction of the new technique was accompanied by training for all primary care staff involved in the process as part of local clinical governance arrangements. Training was carried in a centralised fashion by the PCT Cervical Screening Co-ordinators using educational material provided by SurePath®. Attendance at a training session was mandatory prior to any primary care practitioner (GP or nurse) undertaking cervical sample-taking with LBC.
Our original study noted a wide variation in the proportion of inadequate samples between all 156 English laboratories . More recently a study has compared the variability of inadequacy rates in eleven Welsh laboratories following the introduction of LBC using longitudinal control charts . They reported a reduction in variability between the laboratories. The results that we present here indicate this reduction may be mirrored at a practice level, suggesting that an improvement has been made in the overall process.
There are a number of limitations in our study. As it is a before/after comparison changes in other factors, such as the demographics of the population screened may have contributed to the results observed. We have demonstrated a large absolute and relative drop in the proportion of inadequate samples, both overall and for each laboratory. However, due to limitations in the data we were unable to adjust this by practice or by age. Nevertheless, the size of the change suggests that confounding factors are unlikely to explain it entirely. We are also unable to estimate how much of the reduction in both the proportion of inadequate samples and the variation between practices is due to LBC as a technique or due to other factors such as mandatory training.
The proportion of inadequate samples are not the only factor to consider when measuring quality improvement in cervical screening. Diagnostic accuracy is also important. In our original study we found no relationship between the proportion of smears reported as inadequate and the positive predictive value of a smear in data from English laboratories . We have not had the opportunity to measure any change in the detection of abnormalities in the current data, though recent reports from two large medium quality studies (based on the criteria used by Davey et al ) have indicated that LBC has been associated with an increase in diagnostic accuracy [10, 11].