Fractionation of calcium and magnesium in equine serum.
OBJECTIVE
To establish reference values for protein-bound, ionized, and weak-acid complexed fractions of calcium and magnesium in equine serum and determine stability of ionized calcium (iCa) and ionized magnesium (iMg) in serum samples kept under various storage conditions.


METHODS
28 clinically normal horses.


METHODS
Total calcium (tCa) and magnesium (tMg) in equine serum were fractionated by use of a micropartition system that allows separation of protein-bound calcium (pCa) and magnesium (pMg) and ultrafiltrable calcium (microCa) and magnesium (microMg) fractions. Serum concentrations of iCa and iMg were measured in the ultrafiltrate by use of selective electrodes. Serum concentration of complexed calcium (cCa) or magnesium (cMg) was calculated by subtracting iCa or iMg from microCa or microMg, respectively.


RESULTS
Mean +/-SE serum tCa concentration was 3.26 +/- 0.06 mmol/L. Calcium fractions were as follows: pCa, 1.55 +/- 0.03 mmol/L (47.4 +/- 0.9%); iCa, 1.58 +/- 0.03 mmol/L (48.5 +/- 0.7%); and cCa, 0.13 +/- 0.02 mmol/L (4.1 +/- 0.9%). Serum tMg concentration was 0.99 +/- 0.04 mmol/L. Magnesium fractions were as follows: pMg, 0.33 +/- 0.04 mmol/L (33.3 +/- 4.2%); iMg, 0.57 +/- 0.02 mmol/L (57.6 +/- 1.7%); and cMg, 0.09 +/- 0.02 mmol/L (9.1 +/- 1.9%). Refrigeration (4 degrees C) did not affect iCa values, whereas iMg declined by 8% after 120 hours. Neither iCa nor iMg was affected by freezing (-20 degrees C).


CONCLUSIONS
In equine serum, iMg is less stable than iCa; thus, when serum samples are not going to be analyzed promptly, freezing may be preferable to refrigeration for storage.
Lopez
Ignacio
I
Department of Medicina y Cirugia Animal, Universidad de Cordoba, Campus Universitario Rabanales, Ctra Madrid-Cadiz km 396, 14014 Cordoba, Spain.
Estepa
Jose C
JC
Mendoza
Francisco J
FJ
Mayer-Valor
Rafael
R
Aguilera-Tejero
Escolastico
E
D000818:Animals; D002118:Calcium / Q000097:blood*; D005260:Female; D006736:Horses / Q000097:blood*; D008274:Magnesium / Q000097:blood*; D008297:Male; D012016:Reference Values; D014462:Ultrafiltration / Q000662:veterinary
D008274:Magnesium; D002118:Calcium