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Br J Ophthalmol. 2005 Sep;89(9):1143-6.

The role of case mix in the relation of volume and outcome in phacoemulsification.

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  • 1Sunderland Eye Infirmary, Queen Alexandra Road, Sunderland, SR2 9HP, UK.



The authors previously demonstrated a decrease in complication rate with an increase in volume of cases performed by a surgeon. All studies of volume and outcome are potentially hampered by the issue of case mix, in that some lower volume surgeons may in fact do fewer cases because they have more complex patients. This study was designed to assess the influence of case mix on the volume-outcome relation in phacoemulsification surgery that had previously been demonstrated.


This study took place wholly in Sunderland Eye Infirmary. 667 cases from between 1996 and 2001 were randomly selected from the operative lists of the six surgeons involved in a previous study. The case complexity was assessed using a potential difficulty score (PDS) devised from preoperative data predictive of potential surgical difficulty. The PDS was validated by a retrospective analysis of a sample of 100 cases.


528 complete sets of notes were retrieved. The overall PDS scores ranged from 1 to 6. There was a difference between the proportions of patients with each PDS value (p=0.015) in the two groups, which suggested that the low volume surgeons were doing potentially more difficult cases. The median PDS for each volume group were the same (=1.0). Retrospective validation analysis of the PDS score revealed higher mean and median values in complicated cases compared to uncomplicated cases.


This follow up study re-emphasises the importance of case mix adjustment in comparative assessment of healthcare quality. These results may explain in part the trend previously demonstrated of lower complication rates for higher volume surgeons.

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