Effect of Collection and Preprocessing Methods on Neutrophil Elastase Plasma Concentrations

Objectives: Elevated plasma levels of the elastase α1-proteinase inhibitor complex (E-α1PI) have been proposed as a marker of bacterial infection and neutrophil activation. Liberation of elastase from neutrophils after collection of blood may cause falsely elevated results. Collection methods have not been validated for critically ill neonates and children. We evaluated the influence of preanalytical methods on E-α1PI results including the recommended collection into EDTA tubes. Design and methods: First, we compared varying acceleration speeds and centrifugation times. Centrifugation at 1550 g for 3 min resulted in reliable preparation of leukocyte free plasma. Second, we evaluated all collection tubes under consideration for absorption of E-α1PI. Finally, 12 sets of samples from healthy adults and 42 sets obtained from critically ill neonates and children were distributed into the various sampling tubes. Samples were centrifuged within 15 min of collection and analyzed with a new turbidimetric assay adapted to routine laboratory analyzers. Results: One of the two tubes containing a plasma-cell separation gel absorbed 22.1% of the E-α1PI content. In the remaining tubes without absorption of E-α1PI no differences were observed for samples from healthy adult patients. However, in samples from critically ill neonates or children, significantly higher results were obtained for plain Li-heparin tubes (mean = 183 μg/L), EDTA tubes (mean = 93 μg/L), and citrate tubes (mean = 88.5 μg/L) than for the Li-hep tube with cell-plasma separation gel and no absorption of E-α1PI (mean = 62.4 μg/L, p < 0.01). Conclusion: Contrary to healthy adults, E-α1PI results in plasma samples from critically ill neonates and children depend on the type of collection tube.