Stable Levels of Thiol-Oxidised Plasma Albumin, a Biomarker of Oxidative Stress, Is Correlated with Enhanced Performance in Australian Thoroughbred Racehorses

Thoroughbred race performance is influenced by genetics, training, recovery, and management, but decisions about race readiness are often subjective and lack validated physiological markers. This study evaluated thiol-oxidised albumin, a blood-based indicator of oxidative stress, as a potential biomarker of pre-race fitness in Australian Thoroughbreds. Seventy-five clinically healthy racehorses from seven stables were monitored across 216 competitive events (150 races, 66 trials). Blood samples were collected from the jugular at baseline, 48 h pre-race, and for each day up to 8 days post-race to examine levels of thiol-oxidised using the OxiDx test. At baseline, thiol-oxidised albumin levels were stable, but 24% of horses exhibited oxidative stress before racing, rising to 53% after three consecutive races. Thiol-oxidised albumin levels also remained increased for at least 8 days post-race. Statistical analyses were conducted in GraphPad Prism 10.6 using paired t-tests or repeated-measures one-way ANOVA with Dunnett's post hoc test, alongside a generalised estimating equation model to assess associations with ordinal variables; significance was set at <i>p</i> < 0.05. Reference change values were calculated to determine thresholds for biologically meaningful changes in thiol-oxidised albumin levels. Increased pre-race oxidative stress was significantly associated with poorer finishing positions and lower trainer-perceived performance but was unrelated to horse age or sex. Horses without oxidative stress were more likely to finish in the top three. These findings demonstrate that oxidative stress prior to racing is common and adversely affects performance. Thiol-oxidised albumin represents a promising objective biomarker to guide training and racing strategies, with implications for both performance and equine welfare.