Carbon monoxide as a clinical marker of hemolysis

Carbon monoxide (CO)‐based tests have precisely measured hemolysis for over 40 years. End‐tidal CO was the primary marker in clinical hematology research, followed by carboxyhemoglobin. Quantification of CO reflects heme oxygenases degrading heme in a 1:1 stoichiometric ratio, making CO a direct marker of hemolysis. CO in alveolar air can be quantified using gas chromatography, whose high resolution allows detecting mild and moderate levels of hemolysis. CO can be elevated in active bleeding, resorbing hematoma, and smoking. Clinical acumen and other markers remain necessary to diagnose the cause of hemolysis. CO‐based tests constitute an opportunity for bench‐to‐bedside technology transfer.