Measurement of plasma colloid osmotic pressure in neonatal foals under intensive care: comparison of direct and indirect methods and the association of COP with selected clinical and clinicopathologic variables

Objectives: To describe and compare admission colloid osmotic pressure (COP) measurement using both direct and indirect methods in neonatal foals under intensive care, and to evaluate for associations between COP and clinical/clinicopathologic parameters. Design: Prospective study. Setting: Intensive care unit at a veterinary medical teaching hospital. Animals: Twenty-six critically ill neonatal foals were studied. A control group consisted of 9 clinically healthy neonatal foals. Interventions: Clinicopathologic data were collected at the time of admission. COP was measured directly using a colloid osmometer. Indirect COP was calculated using equations by both Landis-Pappenheimer (L-P) and Thomas and Brown. Measurements and main results: Measured admission COP values were 17.1 +/- 4.3 and 17.7 +/- 2.4 mmHg in critically ill and control foals, respectively, and these values were not significantly different. Critically ill foals with blood lactate concentrations >3 mmol/L had lower COP values than those with lactate less than or equal to 3 mmol/L. There was close agreement between indirect COP values calculated using the L-P equation and direct COP values measured in control foals (mean error = 0.0 +/- 1.3 mmHg; R-2 = 0.87). However, indirect values were not as predictive of direct COP in critically ill foals (mean error = 0.8 +/- 3.8 mmHg; R-2 = 0.64). As COP values increased, the indirect method tended to overestimate COP, whereas at lower values it slightly underestimated COP. Conclusion: While the L-P equation was a close approximation of direct COP in healthy foals, direct measurements of oncotic pressure cannot be replaced for monitoring of critically ill foals. Critically ill foals with higher lactate concentrations had lower COP values, suggesting a possible relationship between COP and lactate.