29, 37, 28, 55
Saeed O et al. Ann Thorac Surg 2019 (graded stroke hazard below the threshold); Strong AE et al. Kidney360 2023 (renal hazard). Also in tension with the analytical evidence: Baud 2023 established point-of-care concordance only FROM 50 mg/dL upward, so the clinically interesting band is the one the assay was not validated in.
The threshold is a definition of an event for registry and comparison purposes. It was never established as a level below which free haemoglobin is biologically inert — Saeed 2019 found an adjusted HR of 7.6 for non-haemorrhagic stroke entirely WITHIN the 11 to 50 mg/dL band. Treat 50 mg/dL as the threshold for calling an event, not the threshold for paying attention. Note also the unit trap: 50 mg/dL equals 500 mg/L, and studies report in both.
The Extracorporeal Life Support Organization defines significant haemolysis as a plasma free haemoglobin over 50 mg/dL (equivalently 500 mg/L). The figure is reproduced across the field as the operative threshold. [VERIFICATION REQUIRED: the primary ELSO document and its exact wording were not retrieved in full; the threshold is carried here as it appears in this book's textbook sources.]
Haemolysis definition and laboratory monitoring