Diagnostic value of sputum cell count in severe asthma

Background In severe asthma, blood eosinophil count (BEC) is used as standard biomarker for assessment of eosinophilic phenotype and eligibility for biologic therapies. However, patients with type-2 inflammation may have normal blood eosinophil levels e.g. due to corticosteroid therapy or spontaneous temporal variability.

Aim The aim of this study was to investigate whether induced sputum cell count may improve identification of patients with eosinophilic phenotype compared to BEC and whether it improves prediction of response to biologic therapies.

Methods We prospectively analyzed patients with difficult-to-treat and/ or severe asthma who were referenced to our center for optimization of therapy. Sputum cell count was obtained before and- if initiated – 3-6 months after start of anti-IL4R or anti-IL5/R therapy. We defined response to biologic treatment as at least one positive criterion without significant decline in any other criterion: 1) OCS dose reduction of at least 50%, 2) FEV₁ increase of at least 150 ml, 3) ACT-Score increase of at least 3 points. Non-response to biologic treatment was defined as no significant improvement in any of the criteria or significant decline in at least one of the criteria.

Results 59 patients were included, hereof 19 were not treated with antibody later, 24 received anti-IL5(R) and 16 with anti-IL4(R) therapies. 55 patients produced an adequate sputum sample at baseline. Hereof 23 (41.8%) with sputum eosinophilia≥2% had BEC<300/µl; 7 (12.7%) with sputum eosinophilia had BEC<150/µl at baseline. Baseline sputum eosinophils categorized as≥4% or<4% and as≥10% or<10% were not associated with FEV₁ improvement during biologic treatment. 28 patients had sputum assessment during antibody therapy, hereof 22 were responders and 6 were non-responders. There were no significant differences in sputum cytology at baseline and follow-up between responders and non-responders to antibody therapy.

Conclusion Sputum cell count improved detection of eosinophilic phenotype compared to BEC in patients with severe asthma, but did not improve prediction of response to antibody therapies.

Publication History

Article published online:
09 March 2023