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 1-proteinase inhibitor (c1-PI) into the gastrointestinal (GI) tract could change the systemic proteinase-proteinase inhibitor balance. Serum c1-PI concentrations have not been studied in dogs with well-defined GI diseases. Objectives To further evaluate serum c1-PI concentrations in dogs with GI diseases, the objectives of this study were to (1) analytically validate a previously developed fecal c1-PI immunoassay to determine serum concentrations, (2) determine a population-based reference interval (RI) and assess the clinical utility, (3) determine stability of serum c1-PI, (4) determine the intra-individual variation in healthy dogs, and (5) determine the clinically relevant magnitude of change of serum c1-PI. Methods Prestudy validation of the 125I-c1-PI immunoassay included linearity, spiking recovery, and intra- and inter-assay precision. A RI was calculated with samples of healthy dogs. Stability at -20 degrees 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 c1-PI radioimmunoassay (RIA) was linear, accurate, precise, and reproducible. Serum c1-PI decreased by 11% after oneyear at -20 degrees C. Analytical, intra-individual, inter-individual, and total variation were 6.4, 9.9, 9.0, and 25.3%, respectively. The RI for serum c1-PI was 732-1802mg/L (n=87); there were no differences between sex and age groups. The index of individuality was 1.31. As analytical variation was > 1/2 inter-individual variation, the minimum critical difference was not determined. Conclusions The results of this study provide the basis for further evaluating serum c1-PI in dogs with GI disease. Using a population-based RI for serum c1-PI appears to be appropriate.