Summary The aim of this study was to determine whether bone biomarkers (osteocalcin, PICP, ICTP and CTX-I) could be used to identify 2- and 3-year-olds at increased risk of fracture in the subsequent flat racing season. It was concluded that these bone biomarkers cannot be used to identify 2- and 3-year-olds that sustain a fracture. Whether bone biomarkers have better predictive value in older horses or when measured serially in the same animal remains to be determined. Introduction Fractures continue to be a serious problem for the horse racing industry, as both a significant cause of fatality and a common cause of days lost from training (Dyson et al. 2008). Recent epidemiological studies in UK Thoroughbreds have reported a fracture incidence of around 1/100 horse months in both flat and National Hunt racehorses have been reported (Verheyen and Wood 2004; Ely et al. 2009). In flat racehorses the majority (80%) occur during training, rather than on the racecourse, with stress fractures the most common type (Verheyen and Wood 2004; Dyson et al. 2008). The ability to identify horses at increased risk of fracture before injury occurs would therefore be of considerable benefit as it would allow early interventions aimed at preventing injury; e.g. by modification of a horse’s training regimen. Measuring bone biomarkers in serum, early in the training season, may be useful for identifying 2-year-old Thoroughbreds at risk of developing dorsal metacarpal disease (Jackson et al. 2005). The aims of the present study were to determine: whether bone biomarkers measured early in the training season could be used to identify 2- and 3-year-old flat racing Thoroughbreds at increased risk of fracture in the subsequent season; and whether measuring bone biomarkers early in the 2-year-old training session could help identify horses at increased risk of fracture during their following 2 training seasons. 410 EQUINE VETERINARY JOURNAL Equine vet. J. (2009) 41 (4) 410-413 doi: 10.2746/042516409X416206 Short Communication Bone biomarkers and risk of fracture in two- and three-year- old Thoroughbreds B. F. JACKSON, P. K. DYSON, C. LONNELL † , K. L. P. VERHEYEN, D. U. PFEIFFER and J. S. PRICE* † Department of Veterinary Clinical Sciences, Hawkshead Lane, Hatfield, Hertfordshire AL9 7TA; and † Department of Veterinary Basic Sciences, The Royal Veterinary College, Royal College Street, London NW1 0TU, UK. Keywords: horse; Thoroughbred; biomarker; fracture; training; bone Materials and methods Horses and sampling Two-year-old study: Blood samples were collected from 529 yearling Thoroughbreds in late November/December shortly after arriving at 12 different training establishments in the north and south of England. Three-year-old study: Blood samples were collected from 326 3-year-old Thoroughbreds in 7 of the training establishments that took part in the 2-year-old study. The 3-year-olds were sampled in January, since many of these older animals were away from the training yards in December. Subset sampled as both 2- and 3-year-olds: 156 horses were sampled as both 2- and 3-year-olds and monitored over 2 training seasons. All samples were taken before exercise between 06.00 h and 08.00 h. Serum was separated within 90 min, and stored at -80ºC until biomarker measurements were undertaken. Fractures Fracture cases were identified by making regular visits to the training yards over the following 9 or 10 months. Horses that left their training yard permanently during the study were not followed up. Only cases of fracture that were confirmed by radiography and/or nuclear scintigraphy were included in the study. Fractures that occurred as a direct result of trauma (e.g. kick or fall) were excluded since these were not incurred as a direct result of exercise. Osteocalcin The concentration of osteocalcin, a specific marker of bone formation, was measured in serum samples using a competitive *Author to whom correspondence should be addressed. Present addresses: P. Dyson: Clevedale Veterinary Practice, Upleatham, Redcar, TS11 8AG, UK; C. Lonnell: Department of Clinical Science, Swedish University of Agricultural Sciences, Uppsala, Sweden. [Paper received for publication 13.01.09; Accepted 20.01.09]