Funding informationVerein zur Förderung der Forschung im Pferdesport e.V.; Laboklin Background: Blood is collected for hematologic and biochemical analyses when racehorses perform poorly. However, racing affects most analyte levels; therefore, the timing of blood sampling can affect analyte levels and interpretations.Objective: This study aimed to determine if the blood variable levels returned to pre-racing levels 2 and 3 days post-racing.Methods: Blood was sampled from 17 healthy racehorses pre-and post-racing. The variables measured from plasma were albumin, cholesterol, creatinine, calcium (Ca), phosphorus (P), magnesium (Mg), sodium (Na), potassium (K), creatine phosphokinase (CK), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), gamma-glutamyl transferase (GGT), and cortisol. Hematocrit (HCT), hemoglobin (HGB), white blood cell (WBC), red blood cell (RBC), granulocyte, and lymphocyte counts were analyzed from blood collected in EDTA-coated vials.Results: Calcium was lower 3 days post-racing compared with 2 days pre-racing (P < 0.01), P and GGT were higher 2 and 3 days post-racing compared with those at the pre-racing timepoints (P ≤ 0.01), and RBC, HCT, and HGB were higher 2 days post-racing compared with those at the pre-racing and 3-day post-racing time points (P < 0.01, all). Conclusions:A few blood biochemical and hematologic variables were significantly altered 2 and 3 days post-racing. The level of these changes did not affect the clinicopathologic interpretation of the values. K E Y W O R D SBiochemistry, exercise, hematology, performance, sampling, variation Many studies have reported that the blood variable levels in Thoroughbred and Standardbred horses are affected by racing. 1-8 Blood

Introduction Blood is often collected from performing poorly horses to measure blood variables that may indicate a reason. However, intense exercise affects many blood variables and therefore time of sampling can affect the interpretation of results. This study examined the behaviour of blood variables after racing to determine the magnitude of the changes and to determine when it is better to collect samples. Methods Blood was sampled in 17 Standardbred racehorses 3 and 2 days before as well as 2 and 3 days after racing. GGT, albumin, cholesterol, creatinine, Ca, P, Mg, Na, K, CK, AST, LDH and cortisol were measured in plasma, and haematocrit, haemoglobin, number of erythrocytes, total white blood cells, granulocytes and lymphocytes were measured in EDTA blood. Parametric data was analyzed with ANOVA for repeated measures and Fisher test; non‐parametric data with Friedman test and Wilcoxon test. Results Plasma Ca concentration was lower on day 3 after racing than on day 2 before racing (P<0.01). P concentration and GGT activity were higher at days 2 and 3 after racing compared to before racing (P = 0.01). Erythrocytes, haematocrit and haemoglobin were higher at day 2 after racing than before and 3 days after racing (P<0.01). The other variables were not affected. Conclusions Some slight but significant changes in blood variables have to be taken in account when interpreting biochemical and haematologic variables for poor performance diagnosis in Standardbreds. It seems preferable to take blood samples on the third day after racing than on the second day. Ethical Animal Research Horse owners gave consent for these procedures within a veterinary monitoring system. Sources of funding: Verein zur Förderung der Forschung im Pferdesport and Laboklin, Germany. Competing interests: none.