A comparison of variability in Papanicolaou and liquid-based cytology inadequacy rates using Shewhart control charts

Objective: To use Shewhart control charts to compare variability in inadequacy rates from Papanicolaou (Pap) and liquid-based cytology (LBC). Design: Retrospective analysis of quality assurance data. Setting: Eleven Welsh cytology laboratories. Methods: Shewhart 'p' charts were plotted for proportions of slides reported as inadequate. Charts were compared for statistical control. Main outcome mesures: Evidence of statistical control in the processes. Results: Control charts allowed easy interpretation of patterns in the data. Variability in inadequacy rates was much lower for LBC than for Pap cytology. Conclusion: Monitoring inadequate rates with Shewhart charts provides more information than tabular monitoring reports, assisting in quality improvement. With respect to inadequacy rates, LBC is less variable than Pap cytology.