Influence of multiple factors on hematologic reference intervals in horses residing in livery yards in Spain

The hemogram is a routine analysis for equine veterinary practitioners in the assessment of patient clinical status. Reference intervals (RIs) of hematologic constituents vary according to different horse populations and are often described for a particular breed or horse type. The aims of this study were to determine RIs for hematologic constituents in a mixed-breed horse population residing in livery yards in central Spain and evaluate the associations between estimated RIs and multiple phenotypic and management characteristics. A total of 122 healthy horses from different breeds in central Spain were included in the study. RIs were calculated following the American Society for Veterinary Clinical Pathology guidelines. Significant associations between red blood cell (RBC) counts, packed cell volumes (PCVs), hemoglobin (HGB) concentrations, white blood cell (WBC) counts, and phenotypic and management features were evaluated using a novel multiple linear regression model analysis. Reference intervals were 5.8-10.0 × 10¹² /L for RBCs, 97-164 g/L for HGB, 0.27-0.46 L/L for PCVs, 37.1-53.6 fL for MCVs, 3.8-10.8 × 10⁹ /L for WBCs, and 76.1-377.9 × 10⁹ /L for platelet counts. The season, discipline, and housing when and where the horses were sampled were factors significantly associated with WBC counts and/or red cell values (HGB, RBC, and PCV). Hematologic RIs for these horses were comparable to the RIs of warm-blooded horses and influenced by husbandry. These location-specific RIs should allow veterinary practitioners to make better-informed decisions for their patients residing in livery yards.