Review of biological variation and its applications in interpretation of equine clinical pathology results.

Biological variation in laboratory results refers to physiological fluctuations that occur around a homeostatic setpoint (HSP) for various laboratory measurands. Assessment of biological variation includes determining individual variation (CV), group variation (CV), and analytical variation (CV). Reference change value (RCV) is an objective tool for an evidence-based approach to interpret data by assessing the significance of consecutive results in an individual for the diagnosis, prognosis, and monitoring of disease. The individualised reference intervals (iRI) are derived from the determination of the HSP for an individual and the RCV. Evaluation of dispersion (DI) of results around any numerical result, reference interval limit, or recommended cut-off value can help define grey zones in clinical data interpretation. The Index of Individuality (II), which compares the amount of biological variation in a group (CV) relative to the amount of biological variation in an individual (CV), is useful to determine if 95% population-based or iRI would be more valuable to detect a significant change in a measurand. The critical number of specimens provides confidence in a defined probability needed for the estimation of the HSP for laboratory measurands. Biological variation data may help support or disprove empirically derived or consensus-derived interpretation recommendations. Biological variation data has the potential to improve the diagnostic accuracy of clinical laboratory testing. Examples of these applications are included.