Sputum neutrophil counts in healthy subjects: relationship with age

A threshold of ~60% has commonly been used in asthma and COPD studies to define the presence of neutrophilic airway inflammation. This threshold is based on relatively young healthy subject datasets. However, age-related increases in sputum neutrophils have been observed previously. We used a healthy cohort, with a comparatively wider age range, to re-evaluate the age-related increase in sputum neutrophils, analysing changes by decade. We also studied the long-term repeatability of sputum neutrophil counts. Differential sputum cell count data for healthy subjects (n=121) was retrospectively analysed. Subjects with a repeated count (mean interval 4.8 years) were included in longitudinal analysis. There was a significant positive association between age and sputum neutrophil % (rho=0.24, p<0.01), with 51.2% of subjects having a sputum neutrophil count >60%. Sputum neutrophil counts increased with each decade until ~60 years where a plateau was observed. The baseline sputum neutrophil % increased significantly at repeated sampling (p=0.02), with excellent long-term repeatability (intraclass correlation coefficient=0.80). We confirm previous reports of an age-related increase in sputum neutrophil % in healthy individuals and identified a plateau which occurs at age ~60 years. There was an increase in sputum neutrophil % during longitudinal follow-up, indicating that age-related neutrophilia is a progressive phenomenon. These findings question the use of an unadjusted threshold, in relation to age, to identify the presence of neutrophilic airway inflammation.