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 (125) I-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.