An ELISA for elastase alpha 1-protease inhibitor complexes in human plasma and serum

An ELISA for neutrophil elastase (ELA) in complex with alpha 1-protease inhibitor (PI) (alpha 1-antitrypsin) was developed in microtitre plates and compared to the ELISA kit from MERCK (2-h version). Recovery of ELA-PI was good in both assays. The detection limits were 4.4 micrograms l-1 and 7.7 micrograms l-1 of the in-house and MERCK assay, respectively, while limits of quantitation were estimated to 7.7 micrograms l-1 (5.5-9.9 micrograms l-1) and 28.9 micrograms l-1 (14.6-44.3 micrograms l-1) for the two assays. Furthermore, as dilution curves of normal plasma were parallel with the calibration curve in the in-house assay over a wide range of dilutions, it is feasible to assay plasma in dilutions of only 1:6, resulting in a limit of quantitation of only 1.1 micrograms l-1. The total analytical coefficient of variation for samples measured in double determinations was 10.5%-12.5% in the in-house assay and 13.9%-14.6% in the MERCK assay. One-hundred-and-eight plasma samples covering a wide range of ELA-PI concentrations were analysed in both assays. A proportional difference between the two methods was detected, the mean ratio (in-house/MERCK) with 95% confidence limits was 1.115 (1.070-1.160). The cause of the difference was probably due to difference calibration of the assays. Until this problem is solved, method specific reference intervals are needed. A reference interval for the in-house method based on plasma samples from 123 healthy adults; median age 36 years (range: 19-65 years) was estimated to 16.5-48.5 micrograms l-1.