Does Referral Bias Impact Outcomes of Surgery for Degenerative Mitral Valve Disease?

Geographic origin is cited as a possible factor influencing outcomes of operation for repair or replacement of degenerative mitral valve (MV) disease. Our study aimed to identify the potential impact of referral bias on clinical outcomes of MV surgery.

We analyzed clinical and echocardiographic information of 2353 patients undergoing primary or secondary MV surgery for degenerative MV disease. Patients were grouped as local (in-state), regional (5 surrounding states), or national referrals.

The number of patients (local, 827; regional, 809; national, 717) and median follow-up time (9.1 years) were similar between geographic groups. More comorbidities were found in the local patient group. Overall operative risk was 0.7% and was greater in local and regional patients compared with national patients (0.7% and 1.1% vs 0.1%, P = .05). Valve repair was performed in 97% of isolated MV surgeries, and repair rate was similar in the 3 geographic groups. The 3 groups had similar incidences of major morbidity, but local and regional groups had higher 30-day readmissions. In univariate analysis, survival was improved in national and regional patients compared with local patients; however in multivariable analysis this difference was no longer significant.

There were important variations in baseline demographic and clinical characteristics between referral groups; local and regional patients presented with more comorbid conditions compared with national referrals. Aside from a small difference in perioperative mortality, early outcomes were generally similar. Late survival, however, was superior in national patients, and this referral bias is explained by fewer associated medical illnesses.