Abstract
Background: Diagnostic carbon monoxide (CO) rebreathing measures total haemoglobinmass (tHb-mass), whereas repeated non-diagnostic exposure has been proposed as anerythropoietic intervention in endurance sport.Aim: To critically evaluate diagnostic validity, performance effects, safety, anti-dopingdetection, and current regulation.Material and methods: A structured narrative review used a targeted PubMed update,citation chaining, and official WADA/UCI documents. Thirty-three full-text-accessiblesources were selected purposively and synthesized thematically.Results: Standardized CO rebreathing can provide reproducible tHb-mass estimates, butprocedures are not automatically interchangeable. Acute CO impairs oxygen-dependentexercise. Repeated exposure increased tHb-mass in some small trials, yet effects on VO2maxand sport performance were inconsistent; the 2026 crossover trial reported haematologicalexpansion without performance improvement. Long-term safety is unquantified. WADA M1.4prohibits non-diagnostic equipment-mediated CO delivery, while UCI rules impose additionalcycling-specific conditions.Conclusions: Diagnostic measurement and performance-oriented exposure must beseparated. A tHb-mass increase is not proof of ergogenic benefit, safety evidence isinadequate, and non-diagnostic equipment-mediated use is prohibited