Abstract: Background Behrend et al (1998) showed serum samples stored at 37C had increased canine free thyroxine (fT4) results when measured by radioimmunoassay. Since then, chemiluminescence (CLIA) and Bulk Acoustic Wave (BAW) assay methods have been developed, however it is unknown if similar increases will be seen when samples are exposed to heat.
Due to concerns over fT4 stability, previously frozen samples used for research purposes should also be evaluated for post-thaw quality.
Hypotheses 1. fT4 concentration is not different between samples analyzed fresh on-site and the same sample after a freeze-thaw cycle. 2. fT4 will increase with warm-temperature exposed samples compared to controls when analyzed by CLIA and BAW technology.
Animals Eleven dogs receiving thyroid evaluation at a teaching hospital.
Methods Phase 1: An experimental study measuring fT4 by CLIA (IMMULITE 2000, Siemens Medical Solutions, USA) on a fresh serum sample and thawed serum samples. Phase 2: A prospective placebo-controlled evaluation of the effects of heat exposure. Samples were divided into two; a control sample kept at 4C overnight, and a test sample exposed to 37C. Both were tested for fT4 using both CLIA and BAW (TRUFORMA, Zomedica Inc, USA) analyzers.
Results The fT4 on thawed samples was significantly different compared to pre-freezing (Figure 1) (p=0.0006), with a mean sample difference of 0.69ng/dL. Heat-exposed samples were variably and unpredictably affected compared to controls (Figure 2).
Conclusions Frozen storage and heat exposure affect fT4 results. Minimizing these pre-analytic variables is critical for accurate results and correct interpretation of thyroid status.
