Simplified strong ion difference approach to acid-base balance in healthy foals

Objective: To determine the strong ion difference (SIDa ) and total nonvolatile weak buffers (ATOT ) in healthy foals during the first year of life and to compare reference biochemistry laboratory with analyzers available during emergency hours.

Design: Prospective study performed over 2 years.

Setting: University teaching hospital.

Animals: Two hundred thirty-six healthy foals distributed in 6 groups: A (21 days-2 months), B (2-3 months), C (3-6 months), D (6-9 months), E (9-12 months), and 33 neonatal foals (< 21 days old).

Interventions: Blood samples were obtained to determine L-lactate, sodium, potassium, chloride, and total plasma protein concentrations. In neonatal foals, samples were analyzed using 4 different devices. Reference intervals of SIDa and ATOT for each of the analyzers under comparison were established using mean ± 2 standard deviations. Age effect was evaluated using one-way ANOVA analysis. Linear regression in neonatal foals was employed to obtain a new equation to estimate ATOT from total plasma protein concentration.

Measurements and main results: A significant age effect was observed for ATOT and SIDa . In all foals younger than 6 months, ATOT values were lower than in older foals (P < 0.003). A clinically and statistically significant difference in SIDa was detected only in the neonatal period (P < 0.001). The equation to estimate ATOT from total plasma protein adjusted for neonatal foals is ATOT = 2.5 × total plasma protein concentration.

Conclusions: Reference intervals of ATOT should be considered different from adults during the first 6 months of life in horses. Regarding SIDa , values should be considered different only during first 21 days of life.