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Failure of pH to predict ionized calcium percentage in healthy horses
1Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Ohio State University, Columbus 43210.
Insights
Direct measurement of serum ionized calcium is recommended for assessing calcium status in horses. While individual horse serum pH showed correlations, pooled data revealed no significant relationship between pH and ionized calcium percentage.
Area of Science:
- Veterinary Medicine
- Clinical Pathology
- Equine Physiology
Background:
- Accurate assessment of calcium status is crucial for equine health.
- Ionized calcium, the biologically active form, is influenced by various factors including pH.
- Understanding the relationship between serum pH and ionized calcium in horses is important for diagnostic interpretation.
Purpose of the Study:
- To determine the relationship between serum ionized calcium percentage and serum pH in healthy horses.
- To evaluate the reliability of total calcium measurements versus ionized calcium measurements.
- To provide recommendations for accurate calcium status evaluation in horses.
Main Methods:
- Anaerobic determination of blood, serum, and plasma total calcium and ionized calcium concentrations using a calcium-specific electrode in 39 healthy horses.
- In vitro manipulation of serum pH to assess the correlation between serum ionized calcium percentage and pH.
- Repeated-measures multiple regression analysis to examine the relationship between serum ionized calcium percentage and serum pH.
Main Results:
- Mean serum ionized calcium concentration was 6.6 ± 0.3 mg/dl (58.2 ± 3.4%).
- No significant correlation was found between serum ionized calcium percentage and serum pH in pooled data.
- While individual horse data showed correlations, heterogeneity in serum constituents likely obscured a universal relationship.
Conclusions:
- Direct measurement of serum ionized calcium concentration is the most reliable method for evaluating calcium status in horses.
- The influence of undefined biochemical factors in serum can affect the equilibrium of calcium binding, impacting the pH-ionized calcium relationship.
- Standard clinical chemistry panels may not fully capture the dynamic interplay affecting calcium bioavailability in horses.
Abstract:
Blood, serum, and plasma total calcium concentrations and plasma and serum ionized calcium concentrations were anaerobically determined by use of a calcium-specific electrode for samples obtained from 39 healthy horses. Mean (+/- SD) serum ionized calcium concentration was 6.6 +/- 0.3 mg/dl (1.6 +/- 0.1 mmol/L) and the mean serum ionized calcium percentage was 58.2 +/- 3.4%. Serum ionized calcium percentage was not significantly correlated with serum pH. Plasma ionized calcium percentage was weakly correlated with plasma pH (r = -0.480; P less than or equal to 0.05). Ionized calcium concentration was determined in serum samples manipulated in vitro by additions of 1 to 80 microliters of 0.1N hydrochloric acid or sodium hydroxide to yield 6 to 10 pH values between 6.8 and 8.2. In all horses, the relationship between serum ionized calcium percentage and serum pH at these pH values was then examined by use of a repeated-measures multiple regression analysis. Correlations between serum ionized calcium percentage and adjusted serum pH value for each horse were highly significant (P less than or equal to 0.05); however, analysis of pooled data from all horses indicated that a statistically significant relationship between serum pH and ionized calcium percentage did not exist. Lack of a significant relationship between these variables was most likely attributable to heterogeneity of variance of ionized calcium percentage among horses, reflecting variation in undefined biochemical constituents of serum that affect the equilibrium of calcium binding. When it is essential to evaluate the calcium status of a horse, direct measurement of serum ionized calcium concentration is recommended.