Sample stability and heparin interference in ionized calcium and ionized magnesium measurements in horses using the Stat Profile Prime Plus co‐oximetry electrolyte analyzer

Abstract

Background The determination of iCa and iMg is important in veterinary medicine, but their immediate determination in whole blood is not always possible. Their stability in other sample types and the existence of interferences must be evaluated before its use. Objectives We aimed to analyze the effects of storage time on the stability of iCa, iMg, and other analytes in whole blood, plasma, and serum samples in horses and assess the interference of heparin in these measurements. Methods Whole blood, heparin‐plasma, and serum samples from 10 horses were stored at 4°C and analyzed 1, 2, 3, 4, 5, 6, 7, 8, 24, 48, and 168 hours after sample collection using the Stat Profile Prime Plus Vet equipment (Nova Biomedical, Waltham, MA, USA). Results were analyzed by ANOVA or mixed‐effect models. Results The concentration of iCa, iMg, total calcium (tCa), total magnesium (tMg), and the ratios iCa/tCa and iMg/tMg did not differ up to 168 hours when compared to the initial time. Total Ca, iMg, and tMg were not significantly different among sample types, but iCa concentrations were slightly but significantly lower in plasma. Freezing at −20°C did not affect iCa, iMg, tCa, and tMg. The pH increased in serum and plasma after 8 hours, and a mild negative correlation existed between plasma iCa concentration and pH. A negative correlation was observed also between the ratios iCa/tCa or iMg/tMg and pH in plasma and serum. A significant decrease in iCa and iMg was detected when comparing homemade syringes at high heparin concentration (~200–300 U heparin/mL) and commercial lithium‐heparin tubes (20–30 U/mL). Conclusions Samples stored at 4°C can be used to determine iCa and iMg concentrations up to 7 days after collection. Other metabolites are stable for up to 8 hours; heparin interference should be taken into account if using homemade heparin syringes.

The Laboratory and Clinical Perspectives of Magnesium Imbalance

“…This is suspected if the sample exhibits concurrent hyperkalemia, hypocalcemia, hypozincemia, and low alkaline phosphatase levels [ 6 ]. The main issue during specimen handling, transportation, and separation before analysis is sample hemolysis, as this can lead to the release of intracellular Mg 2+ , hence falsely raising Mg 2+ results, which can mask true hypomagnesemia [ 10 , 11 ]. Therefore, forceful blood withdrawal, vigorous blood mixing in the tube, delayed specimen transportation, and centrifugation should be avoided to minimize erroneous results [ 11 ].…”

“…The main issue during specimen handling, transportation, and separation before analysis is sample hemolysis, as this can lead to the release of intracellular Mg 2+ , hence falsely raising Mg 2+ results, which can mask true hypomagnesemia [ 10 , 11 ]. Therefore, forceful blood withdrawal, vigorous blood mixing in the tube, delayed specimen transportation, and centrifugation should be avoided to minimize erroneous results [ 11 ]. To prevent hemolysis, serum or plasma must be separated from blood cells immediately.…”