Crude versus case-mix-adjusted control charts for safety monitoring in thyroid surgery
Antoine Duclos1, Nicolas Voirin, Sandrine Touzet
1Unité d'Étude des Pratiques Professionnelles, Hospices Civils de Lyon, Pôle d'Information Médicale Évaluation Recherche, 162 avenue Lacassagne, F-69424 Lyon Cedex 03, France. antoineduclos@yahoo.fr
Introduction:
Patient-safety monitoring based on health-outcome indicators can lead to misinterpretation of changes in case mix. This study aimed to compare the detection of indicator variations between crude and case-mix-adjusted control charts using data from thyroid surgeries.
Methods:
The study population included each patient who underwent thyroid surgery in a teaching hospital from January 2006 to May 2008. Patient safety was monitored according to two indicators, which are immediately recognisable postoperative complications: recurrent laryngeal nerve palsy and hypocalcaemia. Each indicator was plotted monthly on a p-control chart using exact limits. The weighted κ statistic was calculated to measure the agreement between crude and case-mix-adjusted control charts.
Results:
We evaluated the outcomes of 1405 thyroidectomies. The overall proportions of immediate recurrent laryngeal nerve palsy and hypocalcaemia were 7.4% and 20.5%, respectively. The proportion of agreement in the detection of indicator variations between the crude and case-mix-adjusted p-charts was 95% (95% CI 85% to 99%). The strength of the agreement was κ = 0.76 (95% CI 0.54 to 0.98). The single special cause of variation that occurred was only detected by the case-mix-adjusted p-chart.
Conclusions:
There was good agreement in the detection of indicator variations between crude and case-mix-adjusted p-charts. The joint use of crude and adjusted charts seems to be a reasonable approach to increase the accuracy of interpretation of variations in outcome indicators.
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