External validation of the APPS, a new and simple outcome prediction score in patients with the acute respiratory distress syndrome

Background: A recently developed prediction score based on age, arterial oxygen partial pressure to fractional inspired oxygen ratio (PaO2/FiO(2)) and plateau pressure (abbreviated as 'APPS') was shown to accurately predict mortality in patients diagnosed with the acute respiratory distress syndrome (ARDS). After thorough temporal external validation of the APPS, we tested the spatial external validity in a cohort of ARDS patients recruited during 3 years in two hospitals in the Netherlands. Methods: Consecutive patients with moderate or severe ARDS according to the Berlin definition were included in this observational multicenter cohort study from the mixed medical-surgical ICUs of two university hospitals. The APPS was calculated per patient with the maximal airway pressure instead of the plateau pressure as all patients were ventilated in pressure-controlled mode. The predictive accuracy for hospital mortality was evaluated by calculating the area under the receiver operating characteristics curve (AUC-ROC). Additionally, the score was recalibrated and reassessed. Results: In total, 439 patients with moderate or severe ARDS were analyzed. All-cause hospital mortality was 43 %. The APPS predicted all-cause hospital mortality with moderate accuracy, with an AUC-ROC of 0.62 [95 % confidence interval (CI) 0.56-0.67]. Calibration was moderate using the original cutoff values (Hosmer-Lemeshow goodness of fit P < 0.001), and recalibration was performed for the cutoff value for age and plateau pressure. This resulted in good calibration (P = 1.0), but predictive accuracy did not improve (AUC-ROC 0.63, 95 % CI 0.58-0.68). Conclusions: The predictive accuracy for all-cause hospital mortality of the APPS was moderate, also after recalibration of the score, and thus the APPS does not seem to be fitted for that purpose. The APPS might serve as simple tool for stratification of mortality in patients with moderate or severe ARDS. Without recalibrations, the performance of the APPS was moderate and we should therefore hesitate to blindly apply the score to other cohorts of ARDS patients.