Ionized magnesium and calcium concentration and their ratio in equine plasma samples as determined by a regulatory laboratory compared to a clinical reference laboratory.

Magnesium sulfate (MgSO4 ) was administered to calm competition horses. We evaluated the impact of regulatory requirements for the handling of blood samples on plasma ionized magnesium (iMg), ionized calcium (iCa), the iMg to iCa ratio, and pH. We hypothesized that iCa, iMg. and iMg/iCa would be similar among storage and collection methods. Four blood samples were collected from each of 50 horses on the same day: Group 1 - collection in a heparinized syringe and processed within hours in a clinical laboratory; Group 2 - collection into a plasma separator tube (PST) centrifuged just prior to analysis, and plasma processed as in (1); Group 3 - collection into a PST, refrigerated, shipped via overnight carrier to the United States Equestrian Federation (USEF) Equine Drug Testing and Research laboratory, centrifuged just prior to analysis, and plasma processed; and Group 4 - as in Group 3, but stored frozen at -80°C for 90 days, thawed, and plasma processed as in Group 3. Results for iMg/iCa are unit-less, adjusted iMg for potential influence of plasma protein and iCa, and highly correlated with iMg pH (r = -.933; P < 0.01). Samples processed immediately in a clinical reference laboratory had the greatest iMg/iCa. Both iMg/iCa and pH predictably decreased after freezing (P < 0.001). These data suggest that the iMg/iCa mirrors alterations in iMg regardless of storage and collection methods. This understanding can facilitate the development of a regulatory threshold for the control of the nefarious use of magnesium sulfate in competing horses, and an understanding of potential changes to iMg/iCa with storage of B samples.