The adaptation of a point of care test for the quantitative determination of active neutrophil elastase in sputum

Introduction: Active neutrophil elastase (NE) is a biomarker of disease severity in respiratory disorders such as bronchiectasis; with its routine measurement, at the point of care (PoC), being of considerable interest to clinicians to assist in patient triaging. To address this need, ProAxsis Ltd developed NEATstik®, a lateral flow test for the rapid and qualitative measurement of active NE in sputum. However, the value of a quantitative test was highlighted by a recent study (Shoemark, A. et al ERJ 2019;53:1900303) who reported that a NEATstik® visual grade >6 had predictive value for various clinical outcomes in bronchiectasis. Aims and Objectives: To evaluate the feasibility of rendering NEATstik® more quantitative, through the combined use of a lateral flow reader device and calibration standards, prepared in sputum. Methods: Sputum was diluted x10 in sample dilution buffer, and spiked with active NE at 5000 ng/ml. This was followed by a x2 serial dilution to a spiked concentration of 78.125 ng/ml; prior to the transfer of 70 µl of each calibrant to a NEATstik® device. Test-line intensity was measured after 10 minutes, using a lateral flow reader device. Results: A clear, and reproducible, dose response relationship was observed, between spiked sample levels and lateral flow reader measurements; thereby, raising the possibility that ProAxsis’ NEATstik® device may be adapted for use in the quantitative determination of active NE in sputum. Conclusions: The development of a quantitative test may assist in the identification of key NE thresholds i.e. for subclinical infection/ exacerbations, and thus, improve the care of patients with respiratory disease.