Background : The involvement of neutrophil‐mediated inflammation may play an important role in the pathogenesis of acute respitory syncytial virus bronchiolitis. However, no measurable marker is sensitive enough to assess neutrophil‐mediated inflammation in the airways. Released neutrophil elastase (NE) in intraluminal airways has been reported to induce pulmonary inflammation. The aim of this study was to determine whether the amount of urinary trypsin inhibitor (UTI) in serum, a degenerate induced by NE, reflects the degree of airway inflammation in children with respiratory syncytial viral (RSV) bronchiolitis and whether the severity of inflammation is evaluated. The pre‐α‐/inter‐α‐trypsin inhibitor is assumed to be precursors of the UTI. When NE degrades these inhibitors, UTI is liberated.

Methods : Serum UTI concentrations in infants admitted with RSV bronchiolitis, other viral infections, bacterial pneumonia and control subjects were measured by means of one‐step sandwich‐type enzyme immunoassay.

Results : Serum UTI concentrations in 25 patients on admission were significantly higher than the 15 infantile control values (mean ± SEM, 22.126 ± 2.317 and 6.701 ± 0.719 U/mL, respectively; P < 0.0001). The elevated levels returned to baseline values with improvement in the respiratory symptoms. Higher levels of serum UTI with RSV infection were consistently associated with clinical symptoms and artificial ventilation. Serum NE concentrations of patients were elevated in some patients but not significantly different from controls in the patients who showed only upper respiratory symptoms with RSV infections.

Conclusion : The findings strongly suggested that neutrophil‐mediated events are involved in the pathogenesis of RSV bronchiolitis, and the monitoring of UTI concentrations might be useful for evaluating the neutrophil‐mediated airway inflammation.