Biological variation of routine haematology and biochemistry measurands in the horse.
Authors: Wright M E, Croser E L, Raidal S, Baral R M, Robinson W, Lievaart J, Freeman K P
Journal: Equine veterinary journal
Summary
# Editorial Summary Routine blood work interpretation in equine practice relies on population-based reference intervals, yet individual horses naturally exhibit considerable physiological variation that can mask or mimic genuine pathological changes. Wright and colleagues investigated the biological variation of 39 healthy privately owned horses, sampling them weekly over six weeks to quantify within-subject and between-subject variation across standard haematology and biochemistry measurands. Several clinically important parameters—including red blood cell count, total protein, globulins, albumin, gamma-glutamyl transferase and aspartate aminotransferase—demonstrated high individuality, meaning a result within the normal population range could represent a significant deviation for that individual animal; conversely, measurands such as mean corpuscular haemoglobin concentration, platelets, chloride, glucose and sodium showed low individuality and remain appropriately interpreted against population standards. The researchers derived reference change values for each parameter, enabling practitioners to establish each horse's personal baseline and recognise clinically relevant shifts even when absolute values remain technically "normal." This work underpins a more nuanced approach to equine clinical pathology: serial sampling with baseline establishment becomes particularly valuable for assessing chronic conditions or monitoring treatment response, whilst understanding which parameters warrant individualised interpretation can substantially improve diagnostic confidence and reduce unnecessary interventions or missed diagnoses in practice.
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Practical Takeaways
- •For nine common blood tests (protein, liver enzymes, red cell indices), establish baseline values for individual horses rather than relying solely on population reference ranges—this significantly improves clinical interpretation
- •Glucose, electrolytes (sodium, chloride), and platelet counts can confidently be interpreted against standard lab reference ranges without individualisation
- •Request multiple baseline samples 1–2 weeks apart for horses with chronic or recurring conditions to establish personal reference ranges for better monitoring
Key Findings
- •Nine measurands (RBC, MCH, MCV, total protein, globulins, albumin, GGT, AST) demonstrated high individuality warranting individualised reference intervals
- •Five measurands (MCHC, platelets, chloride, glucose, sodium) showed low individuality appropriate for traditional population-based reference intervals
- •Remaining measurands had intermediate individuality requiring interpretation with reference change values alongside population-based intervals
- •Serial sampling and biological variation analysis enable improved clinical decision-making in equine practice