Validation and clinical utility of a novel immunoradiometric assay exclusively for biologically active whole parathyroid hormone in the horse.

The research presents a validation study for a new method to measure parathyroid hormone (PTH) in horses. The new assay, targeted at the active, intact form of PTH, is compared with an older method, with findings pointing to it as a potentially more accurate tool to diagnose disorders related to mineral metabolism in horses.

Context and Objective

• The parathyroid hormone (PTH) is essential in managing mineral metabolism in mammals. Previously, the ‘intact-PTH’ (I-PTH) assay was the standard for measuring this hormone in horses’ plasma.
• However, it has been found that it also detects an inactive form of the hormone found in horses with chronic kidney disease (uraemic patients). This affects its accuracy, necessitating the development of a more precise method: the ‘whole PTH’ (W-PTH) assay, also known as ‘cyclase-activating PTH’.
• The study aimed to validate this new assay by comparing its performance with the older method in normal horses, young foals, horses with nutritional secondary hyperparathyroidism (N2HPT), and horses with chronic renal failure (CRF).

Methods

• Measurements of both W-PTH and I-PTH were taken from plasma samples of the different groups of horses.
• To validate the assays, the researchers repeated measurements and performed dilutional paralellism, a process of serially diluting a solution to provide multiple samples for testing.
• To stimulate changes in PTH level, the researchers induced changes in blood ionized calcium levels using administrations of the chemicals EDTA and CaCl2.

Results

• The W-PTH assay performed comparably to the I-PTH assay in terms of accuracy, sensitivity, specificity, and response to changes in calcium levels.
• Interestingly, the concentration of W-PTH in healthy horses and foals was higher than that of I-PTH. This discrepancy was maintained even during acute hypo- and hypercalcaemia situations, with W-PTH concentrations remaining higher.
• In horses with N2HPT, both W-PTH and I-PTH concentrations increased. However, for horses with CRF, both values were very low, with no observable increase in I-PTH.

Conclusion

• The W-PTH assay has been validated as a suitable tool for measuring equine PTH.
• The use of the W-PTH assay could enhance the diagnosis of conditions related to mineral metabolism in horses.