Background: Sepsis-associated acute kidney injury (AKI) is a severe condition associated with unfavorable outcomes in critically ill patients, not least because of its delayed diagnosis and management due to limitations in the current standard of care. We aimed to evaluate the performance of sphingotest penKid for the early diagnosis of AKI in patients with sepsis or septic shock. Methods: Plasma Proenkephalin A 119-159 (penKid) was measured in 120 healthy subjects and 529 critically ill sepsis patients. Subgroup analyses were performed for patients with and without a history of heart failure or hypertension. The clinical performance for the diagnosis of AKI within 48 hours (AKI48h) was calculated using the upper limit of the penKid reference range. To improve clinical decision-making, interpretation bands and likelihood ratios for optimized rule-in and rule-out performance were established. Results: Of the 529 patients with sepsis or septic shock, 328 were male (62%), and the median age was 66 (interquartile range 56-75) years. Two hundred thirty-four (44%) patients were diagnosed with AKI48h, and those patients presented increased penKid levels on intensive care unit admission compared with patients without AKI48h, with an area under the curve of 0.87 (95% confidence interval [CI] 0.84-0.90, p<0.0001). A penKid cut-off of 89 pmol/L, corresponding to the 97.5th percentile in the healthy reference population, resulted in 72% sensitivity (95% CI 66-77%), 83% specificity (95% CI 78-87%), 77% positive predictive value (95% CI 71-82%), 79% negative predictive value (95% CI 74-83%), a positive likelihood ratio of 4.2, and a negative likelihood ratio of 0.34 to diagnose AKI48h. An improved performance was obtained when patients with a history of chronic heart failure and/or hypertension were excluded. PenKid cutoff values of 54 pmol/L (>92% sensitivity) and 105 pmol/L (>92% specificity) were derived to establish actionable interpretation bands for diagnostic rule-in and rule-out. Conclusions: The sphingotest penKid assay can facilitate an early diagnosis of AKI up to 48 hours before the KDIGO criteria are met. Based on the present results, the assay was registered as an aid in the early diagnosis of AKI in patients with sepsis or septic shock.

PFL has received consulting fees from Adrenomed, Ferring, and Lascco, and is a clinical development advisor at Inotrem. PP serves as a consultant and has received consulting fees from Adrenomed and SphingoTec. JS, OH and FU are employed by SphingoTec GmbH, the manufacturer of the sphingotest penKid assay. The other authors report no conflicts of interest.

AdrenOSS-1 (ClinicalTrials.gov identifier NCT02393781) was funded by SphingoTec GmbH, Neuendorfstr. 15a, 16761 Hennigsdorf, Germany. This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement 666328.

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Ethics committee/IRB "Comite d'Ethique Hospitalo-Facultaire" of Universite Catholique de Louvain, Faculte de Medecine, Cliniques Universitaires Saint-Luc gave ethical approval for this work on April 13th, 2015 (no. 2015/16MAR/111).

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