Serum free cortisol fraction in healthy and septic neonatal foals

Relative cortisol insufficiency occurs in septic foals and impacts survival. Serum free (biologically available) cortisol concentration might be a better indicator of physiologic cortisol status than serum total cortisol concentration in foals.

In septic foals, (1) low free cortisol concentration correlates with disease severity and survival and (2) predicts disease severity and outcome better than total cortisol concentration.

Fifty-one septic foals; 11 healthy foals; 6 healthy horses.

In this prospective clinical study, foals meeting criteria for sepsis at admission were enrolled. University-owned animals served as healthy controls. Basal and cosyntropin-stimulated total cortisol concentration and percent free cortisol (% free cortisol) were determined by chemiluminescent immunoassay and ultrafiltration/ligand-binding methods, respectively. Group data were compared by ANOVA, Mann-Whitney U-tests, and receiver operator characteristic curves. Significance was set at P < .05.

Basal % free cortisol was highest in healthy foals at birth (58 ± 8% mean ± SD), and was higher (P ≤ .004) in healthy foals of all ages (33 ± 6 to 58 ± 8%) than in adult horses (7 ± 3%). Cosyntropin-stimulated total and free cortisol concentrations were lower (P ≤ .03) in foals with shock (total = 6.2 ± 8.1 μg/dL; free = 3.5 ± 4.8 μg/dL versus total = 10.8 ± 6.0 μg/dL; free = 6.9 ± 3.3 μg/dL in foals without shock) and in nonsurvivors (total = 3.8 ± 6.9 μg/dL; free = 1.9 ± 3.9 μg/dL versus total = 9.1 ± 7.7 μg/dL; free = 5.5 ± 4.4 μg/dL in survivors). Free cortisol was no better than total cortisol at predicting disease severity or outcome in septic foals.

Serum free cortisol is impacted by age and illness in the horse. There is no advantage to measuring free over total cortisol in septic foals.