Calculating the Lung Allocation Score Error for ECMO Patients

Abstract

The use of extracorporeal membranous oxygenation (ECMO) as a bridge to lung transplantation has increased dramatically. However, the Lung Allocation Score (LAS) does not use a dedicated ECMO variable; instead, ECMO is proxied by mechanical ventilation with 100% oxygen. We aimed to determine if this approximation accurately reflects the transplantation urgency for ECMO patients. Lung transplant candidates and recipients greater than 12 years of age between 2008-2014 were selected from the SRTR database. For both the wait-list (WL) and post-transplant (PT) time periods, predicted survival functions were calculated using the LAS model and compared to observed survival functions estimated by Kaplan-Meier. Differences between predicted and observed WL survival and PT survival were calculated for candidates and recipients, stratified by ECMO support at listing or transplant. The errors in WL and PT survival were then combined to estimate the LAS error for each group. During the study period, 16,259 candidates were listed; 236 were on ECMO at listing. A total of 12,169 patients were transplanted; 363 were on ECMO at transplant. Non-ECMO candidates survived a mean of 336 days from listing compared to a LAS prediction of 328 days, leading to a WL error of 9 days. ECMO candidates survived a mean of 199 days from listing compared to a LAS prediction of 123 days, leading to a WL error of 77 days. Non-ECMO recipients survived a mean of 333 days from transplant compared to a LAS prediction of 315 days, leading to a PT error of 17 days. ECMO recipients survived a mean of 296 days from transplant compared to a LAS prediction of 248 days, leading to a PT error of 48 days (Fig 1). ECMO was associated with an overestimation of the LAS by 10 points, compared to a non-ECMO underestimation of 0.04 points. The LAS ECMO correction underestimates both WL and PT one-year survival, leading to an overall upward bias in the LAS compared to non-ECMO candidates. This suggests that ECMO candidates have a lower transplantation urgency than implied by their LAS.