Establishing normal ranges for induced sputum cell counts in Singapore

<b>Background:</b> Induced sputum is a non-invasive method for quantifying airway inflammatory indices for asthma management. Regional characteristics such as prevalence of atopy or air pollution may modulate induced sputum cellularity. Therefore, locally derived normal reference ranges are needed and these have not been established in Singapore. <b>Objective:</b> To determine the cellular composition of induced sputum among healthy adults in Singapore, and evaluate whether blood eosinophils and fractional exhaled nitric oxide (FENO) correlate with sputum eosinophils. <b>Method:</b> 50 healthy adults aged ≥21 years with no history of chronic lung disease, no nasal or chest symptoms, no respiratory tract infection in the past 4 weeks and FEV1/FVC≥0.7 were recruited. Induced sputum, blood and FENO were obtained. <b>Results:</b> 22 males and 28 females participated in this study. Adequate sputum samples were obtained from 32 of the 50 participants (64%). Mean age (SD) was 28 (9) years, 56.3% were females, and 18 (56.3%) had IgE&gt;100IU/ml. The mean, median (10th-90th percentile) % cell counts are: macrophages 25.6%, 19.1% (0-73.2%), neutrophils 52.9%, 62.1% (8.4-93.9%), eosinophils 6.8%, 2.6% (0.5-19.9%) and lymphocytes 11.2%, 4.9% (0-21.8%). FENO significantly correlated with sputum eosinophils (rs=0.386, p=0.03), but no correlation was found between blood and sputum eosinophils (rs=0.044, p=0.81). <b>Conclusion:</b> Healthy volunteers in Singapore demonstrate a higher range of sputum neutrophils (90th centile: 93.9%) and sputum eosinophils (90th centile: 19.9%) than reported for other populations (90th centiles: neutrophils around 60%, eosinophils &lt;2-3%), which should be considered when interpreting induced sputum results to titrate asthma treatment.