Biological Variation of Thromboelastography Variables in Ten Healthy Female Horses

Biological variability (BV) has important applications in laboratory medicine. It can be a source of variation in measured analyte values and provide guidance on reference interval use. BV has three components: between-individual variation (CVg), caused by differences in mean values of a particular analyte among members of a group, within-individual variation (CVi), caused by fluctuations around an individual’s inherent homeostatic set point, and analytical variation (CVa). Thromboelastography (TEG), a type of viscoelastic coagulation analysis, is becoming increasingly common in veterinary referral centers. Despite increased popularity, the optimal method of results interpretation is not clear. While population-based reference intervals (PRI) are used for many analytes, reference change values (RCV) are more sensitive for disease detection for other analytes. The relative sensitivity of PRI or RCV in detecting significant deviations of reported analyte values can be derived from BV data, via incorporation of CVi and CVg into a simple formula to calculate index of individuality (IOI). This study measured BV of four TEG variables R, K, angle and MA in clinically healthy horses and, using calculated IOI, found that population-based reference intervals are appropriate when interpreting results from individual animals. Additionally, the ability to freeze a key reagent used in the TEG assay, tissue factor, was also investigated.