Background: The acute phase proteins (APP) serum amyloid A (SAA), haptoglobin, and fibrinogen are valuable blood bi-omarkers in equine inflammatory diseases, but knowledge of factors influencing their concentrations in blood and peritonealfluid (PF) of horses with colic is needed.

Objectives: The objective of this study was to investigate the influence of demographics (age, sex, breed), disease process (sim-ple obstruction, strangulating obstruction, inflammatory), disease location, disease duration, hypovolemia, and admission hospi-tal on concentrations of APP, lactate and white blood cell counts (WBC) in horses with colic admitted to 2 referral hospitals.

Animals: The study included 367 horses with colic admitted at 2 referral hospitals.

Methods: Prospective multicenter observational study of clinical data, as well as blood and PF biomarkers. Associationsbetween biomarker concentrations and clinical variables were analyzed using multivariate linear regression analysis.

Results: Increasing pre-admission duration of colic was associated with increased concentrations of APP in blood andPF. Blood concentrations of SAA and fibrinogen were associated with disease process (inflammatory, strangulations, simpleobstructions) in more colic duration groups (5–12 and >24 hours) than any of the other biomarkers. No relevant associationsbetween demographic factors, hospital, or hydration status and the measured biomarkers were found.

Conclusions and Clinical Importance: In horses with colic, concentrations of APP are associated mainly with diseaseprocess and duration of colic and may thus be used for assessment of disease independently of demographic or geographicfactors. Serum amyloid A may be a diagnostic marker for use in colic differential diagnosis, but further evaluation is needed.

Key words: Fibrinogen; Haptoglobin; Peritoneal fluid; Serum amyloid A.