Validation of an Enzyme Immunoassay for the Quantitative Determination of ACTH Precursors (Pro-Opiomelanocortin and Pro-Adrenocorticotropin) in Feline Plasma
Feline hyperadrenocorticism is a rare condition in cats; approximately 80% of cats have an autonomously functioning pituitary tumor. Diagnosis of FH is quite difficult. Clinical signs are non specific, except for the non systematic feline skin fragility syndrome. The specificity of all the tests validated in that species is questionable as well. An ACTH precursors (POMC/pro-ACTH) assay (OCTEIA POMC kit, Immunodiagnostic System, UK) has been validated in canine species (J Vet Intern Med 2005; 19:23-28). High ACTH precursors plasma concentrations have been measured in dogs with large corticotropic tumors whereas ACTH precursors are not detectable in healthy dogs or dogs with other pathological conditions (unpublished data). The aim of this study was to validate OCTEIA POMC assay in cats according to current recommendations for bioanalytical method validation in order to further evaluate its usefulness for diagnosis of feline pituitary-dependent hyperadrenocorticism (FPDH).
The calibration curve was prepared by spiking blank feline plasma with six different concentrations of human POMC/pro-ACTH. Blank plasma was collected from 10 healthy cats, 4 hours after IV administration of dexamethasone phosphate at 1mg/kg of body weight and pooled.
Sensitivity, defined as the concentration corresponding to the mean plus 2 standard deviations of replicate analyses of blank plasma, was 26 pmol/L. Accuracy, calculated with 3 known concentrations, measured five times each was 111%. Linearity, assessed by diluting a sample with blank feline plasma (1 in 1 to 1 in 16) was 104%. Intra-assay and inter-assay coefficient of variations determined on three samples, measured five times each, were 8%, 10%, 16% and 13%, 10%, 15% respectively.
This work validates the use of OCTEIA POMC kit on feline plasma samples. Its usefulness in FPDH is currently investigated with encouraging preliminary results.