Serum Osteocalcin and CTX-I in Fit-to-Race Thoroughbred Racehorses: Reference Intervals and Associations with Demographic, Training, and Lameness-Related Factors

Musculoskeletal injury remains a major welfare and performance concern in Thoroughbred racing, and biomarkers for early risk stratification are of scientific interest. This multicentre observational cohort study characterised serum concentrations of two bone turnover markers, osteocalcin/BGLAP (OC) and C-terminal telopeptide of type I collagen (CTX-I), in 1359 fit-to-race Thoroughbred racehorses sampled across New South Wales, Victoria, and Western Australia. Biomarker distributions, population-wide percentile ranges, demographic and training-related associations, and short-term soundness outcomes were evaluated using nonparametric methods. Both markers were positively skewed, with population-wide percentile ranges (2.5-97.5 percentile) of 0.13-9.44 ng/mL for CTX-I and 0.02-7.25 ng/mL for OC; CTX-I varied substantially by jurisdiction, so this pooled range should not be interpreted as a clinical reference interval for individual horses. CTX-I varied significantly by age, jurisdiction, venue, and training surface, with markedly higher concentrations in New South Wales horses and lower concentrations associated with polytrack training, but did not predict lameness outcomes. OC concentrations were significantly higher in horses classified as lame at sampling and showed modest discriminatory ability for subsequent lameness in univariate analysis, with the strongest performance for persistent lameness at both 7 and 28 days; this association attenuated after adjustment for age, sex, state, and training surface. At an unadjusted operational threshold of approximately 1.24 ng/mL, OC achieved high negative predictive value for persistent lameness. These findings suggest OC may have scientific value as a potential rule-out screening marker for short-term soundness, warranting further investigation, while CTX-I appears more informative for characterising population-level skeletal turnover variation.