Serum is the recommended sample to use for measurement of cortisol and T4 using Immulite technology. However, cortisol is more stable in EDTA plasma than serum. The use of EDTA plasma can significantly affect results obtained by Immulite, and addition of magnesium (Mg) can potentially overcome the effects of EDTA (Kemppainenet al. J Vet Diagnost Invest, in press). Our objective was to compare the effect of EDTA with and without addition of Mg on measurement of cortisol and T4 by Immulite in canine samples.
Paired canine EDTA-plasma and serum samples leftover after use for indicated diagnostic testing were obtained. The EDTA concentration in each plasma sample was unknown and likely varied depending on the degree of underfilling of the tube. Cortisol and T4 were measured in the EDTA-plasma and serum samples with and without Mg added using a previously validated, solid-phase, automated, chemiluminescent enzyme immunoassay (Immulite 1000). For statistical analysis, we used a one-way repeated measures ANOVA to compare the measured concentrations. A post-hoc test using Dunnett’s method was performed with the concentrations in serum without added Mg as the control group. To detect differences in concentrations between plasma with and without Mg, data were analyzed using a paired t-test. Significance was set at the P < 0.05 level.
Blood samples from 15 dogs were included. Cortisol and T4 values were significantly different between plasma and serum samples (both without Mg). For cortisol and T4, concentrations measured in EDTA plasma were 34% and 45% higher than in serum samples, respectively (P < 0.001 for both). The addition of Mg to plasma significantly decreased the measured cortisol and T4 concentrations (P < 0.001 and P = 0.016, respectively). After addition of Mg, the cortisol concentrations measured in EDTA plasma were no longer significantly different from those measured in serum; however, for T4, the concentrations measured in EDTA plasma remained significantly different from serum.
Use of EDTA plasma significantly increases the measured concentration of cortisol and T4 obtained by Immulite. Addition of Mg to plasma samples can overcome the effects of EDTA when measuring cortisol, but not T4. Thus, EDTA plasma can be used to maximize cortisol stability if Mg is added to the sample before assay via Immulite.
Joezy Griffin Endowed Professor and Alumni Professor
Auburn University College of Veterinary Medicine
Thursday, June 14
10:15 AM – 10:30 AM
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