One Marker, Many Circuits: A Scoping Review of Lactate Across Donor, Perfusate and Recipient Phases in Solid Organ Transplantation

Objective: To map how lactate has been measured, interpreted and validated as a graft viability or outcome marker across donor, perfusate and recipient phases and organs. Background: Lactate is measured at every step of solid organ transplantation, but its meaning differs by phase, preservation method and organ. No review has mapped this evidence across the transplant continuum. Methods: Scoping review following JBI methodology and PRISMA-ScR. PubMed/MEDLINE, Embase, Web of Science and CENTRAL were searched from 2000, supplemented by journal-targeted searches, citation chasing and ClinicalTrials.gov. Human clinical and ex vivo studies reporting lactate against graft viability or outcome were charted and mapped by organ and phase. Results: 159 human studies were included. Liver transplantation accounted for 108, mainly in the recipient phase and machine perfusion; heart contributed 28, lung 12, kidney 7 and pancreas 1, and no intestine study was eligible. Proposed liver perfusion thresholds ranged from below 1.7 to 4.1 mmol/L. In the largest cohort, lactate clearance did not predict graft survival (AUC 0.50). Day-1 lactate contributed to a validated primary nonfunction model (AUROC 0.83). Confounders included stored red cells in circuits, absence of a clearing organ, donor type, measurement timing and drugs. Conclusions: Lactate is a useful but organ- and context-dependent signal, strongest as a recipient-phase marker after liver transplantation and weakest as a stand-alone ex situ viability criterion outside the liver. Phase-specific thresholds, standardized reporting and multi-marker validation are needed before lactate is used to discard organs.