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

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Abstract

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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