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A comparison of variability in Papanicolaou and liquid-based cytology inadequacy rates using Shewhart control charts
1National Public Health Service for Wales, Temple of Peace and Health, Cardiff, UK. rosemary.fox@nphs.wales.nhs.uk
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 Measures:
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.
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