Adjustment for case mix in comparisons of cesarean delivery rates: university versus community hospitals in Vermont.

Objective: Our objective was to determine whether case mix model adjustment would help to explain differences in cesarean delivery rates between community and university hospitals. We also wished to define a patient population in which the cesarean delivery rate would be more reflective of individual practice patterns than of obstetric or medical risk.

Study Design: Established risk factors for cesarean delivery were identified by retrospective chart review at two community hospitals (designated A and B) and a university hospital. Each delivery was assigned exclusively to 1 of 6 risk categories: (1) multiple gestation, (2) fetal malpresentation, (3) delivery at <36 weeks' gestation, (4) not suitable for trial of labor, and (5) term delivery (> or =36 weeks' gestation) with medical complications, and (6) term delivery (> or =36 weeks' gestation) without medical complications. Parity and history of cesarean delivery further subdivided these categories into a total of 18 unique subgroups. Case mix was defined as the distribution of patients into each subgroup. Patients assigned to the categories of multiple gestation, fetal malpresentation, delivery at <36 weeks' gestation, and not eligible for trial of labor were considered to compose the group at high risk for cesarean delivery. The remaining patients composed the group at low risk for cesarean delivery. Observed cesarean delivery rates were calculated for each cell of the case mix grid within individual hospitals. Total, primary, and repeat cesarean delivery rates were determined for each hospital. The cesarean delivery rates for the low-risk populations were calculated. Data were evaluated both by chi(2) test and by direct standardization analysis with the university hospital case mix used as the standard population.

Results: A total of 5705 delivery reports were reviewed (university hospital, n = 4538; hospital A, n = 531; hospital B, n = 636). The cesarean delivery rates were significantly different between hospitals (university hospital, 16. 9%; hospital A, 13.6%; hospital B, 12.0%; P =.002). The distributions of patients in the high-risk group were also significantly different between hospitals (university hospital, 16. 8%; hospital A, 5.8%; hospital B, 8.8%; P = .001). The percentage of medically complicated cases in the low risk for cesarean group was significantly higher at the university hospital (university hospital, 16.9%; hospital A, 8.8%; hospital B, 9.8%; P =.001). However, no statistical differences were detected between hospitals in either the observed cesarean delivery rates or the standardized rates for the low-risk groups.

Conclusion: The case mix model provides a more accurate method of comparing cesarean delivery rates between community and university hospitals. The low-risk group of patients discriminated in this model represents a population in which the cesarean delivery rate may be more reflective of individual practice patterns than of maternal or fetal risks.