Fractionation of canine serum calcium, using a micropartition system

OBJECTIVE: To determine usefulness of a micropartition system for calcium fractionation of canine serum, and to establish reference values for protein-bound, complexed, and ionized calcium fractions in clinically normal dogs. DESIGN: Performance characteristics of a micropartition system were evaluated, using serum from clinically normal dogs. This micropartition system was then used to determine a reference range for calcium fractions. ANIMALS: 13 clinically normal dogs. PROCEDURE: Dog serum was placed in the micropartition system, and spun for 20 minutes at 1,300 x g. Total calcium concentration, ionized calcium concentration, and pH were measured in whole serum, and total calcium concentration was measured in the ultrafiltrate. The protein-bound fraction was calculated by subtracting total calcium of the ultrafiltrate from total calcium of whole serum. The ionized calcium measurement of whole serum was subtracted from the total calcium measurement of the ultrafiltrate, determining the complexed calcium fraction. RESULTS: During validation of the ability of the micropartition system to separate calcium fractions, no significant amount of serum calcium was adsorbed by the plastic micropartition system or membrane. The micropartition membrane separated the protein-bound calcium fraction (retentate) from the ultrafiltrate, which contained ionized and complexed fractions of calcium. Concentrations of protein-bound, ionized, and complexed calcium from clinically normal dogs were determined to be 3.40 +/- 0.63, 5.49 +/- 0.17, and 1.01 +/- 0.30 mg/dl, representing 34, 56, and 10% of the total calcium concentration, respectively. CONCLUSIONS: This method is a rapid, repeatable means to completely fractionate serum calcium, and most importantly provides accurate assessment of the protein-bound and complexed calcium fractions. CLINICAL RELEVANCE: Complete assessment of calcium fractions may increase sensitivity for detection of disease processes that affect calcium metabolism.