Serum alpha1-proteinase inhibitor concentrations in healthy dogs - method validation and determination of reference interval and intra-individual variation

Background A chronic loss of canine α1‐proteinase inhibitor (cα1‐PI) into the gastrointestinal (GI) tract could change the systemic proteinase‐proteinase inhibitor balance. Serum cα1‐PI concentrations have not been studied in dogs with well‐defined GI diseases. Objectives To further evaluate serum cα1‐PI concentrations in dogs with GI diseases, the objectives of this study were to (1) analytically validate a previously developed fecal cα1‐PI immunoassay to determine serum concentrations, (2) determine a population‐based reference interval (RI) and assess the clinical utility, (3) determine stability of serum cα1‐PI, (4) determine the intra‐individual variation in healthy dogs, and (5) determine the clinically relevant magnitude of change of serum cα1‐PI. Methods Prestudy validation of the 125I‐cα1‐PI immunoassay included linearity, spiking recovery, and intra‐ and inter‐assay precision. A RI was calculated with samples of healthy dogs. Stability at −20°C was tested on 36 samples. Intra‐individual variation was assessed using samples collected from 11 healthy dogs over a 12‐week period. Results The cα1‐PI radioimmunoassay (RIA) was linear, accurate, precise, and reproducible. Serum cα1‐PI decreased by 11% after one year at −20°C. Analytical, intra‐individual, inter‐individual, and total variation were 6.4, 9.9, 9.0, and 25.3%, respectively. The RI for serum cα1‐PI was 732–1802 mg/L (n = 87); there were no differences between sex and age groups. The index of individuality was 1.31. As analytical variation was > ½ inter‐individual variation, the minimum critical difference was not determined. Conclusions The results of this study provide the basis for further evaluating serum cα1‐PI in dogs with GI disease. Using a population‐based RI for serum cα1‐PI appears to be appropriate.