Optimal asthma management focuses on controlling symptoms and preventing exacerbations. While serum and sputum eosinophil counts have traditionally been used to gauge eosinophilic airway inflammation, these measures are invasive and may not always be practical. The fraction of exhaled nitric oxide (FeNO) has emerged as a promising, non-invasive biomarker that reflects airway eosinophilia and may guide timely adjustments in inhaled corticosteroid therapy, particularly in patients with uncontrolled asthma. However, studies examining the relationship between FeNO, other inflammatory markers, and lung function remain limited, especially in sub-Saharan Africa, where FeNO testing is uncommon. This study aimed to investigate how FeNO levels relate to serum eosinophils, spirometry results, and asthma symptom control. This observational study was conducted at the asthma clinic of a tertiary care hospital and included 82 patients with physician-confirmed asthma receiving routine care. Each participant completed the asthma control test (ACT), underwent FeNO measurement and spirometry according to ATS guidelines, and provided blood samples for serum eosinophil counts. Correlation analyses assessed associations between FeNO, ACT scores, serum eosinophils, and spirometry metrics. Logistic regression was used to evaluate the relationship between elevated FeNO (> 50 ppb) and abnormal FEV1 percentage predicted (< 80%), controlling for age, sex, and BMI. Among the study population, females represented 72%, and 40.2% were aged 60 years or older. The median FeNO was 42.0 (26.0–52.5) ppb, the median ACT score was 20.0 (18–23), and the median serum eosinophil count was 0.25 (0.90–0.38) × 10⁹/L. Patients with partially or poorly controlled asthma had notably higher FeNO levels compared to those with well-controlled disease (P < 0.001). Overall, 57% of participants had well-controlled asthma, while 42% were uncontrolled. FeNO correlated positively with serum eosinophil counts (r = 0.450, P < 0.001) and negatively with ACT scores (r = −0.648, P < 0.001) and FEV1 percentage predicted (r = −0.353, P = 0.001). High FeNO levels were associated with over five times greater odds of having an abnormal FEV1 percentage predicted. In this cohort of asthma patients receiving inhaled corticosteroids, FeNO levels closely reflected airway eosinophilia, symptom control, and lung function. Elevated FeNO was strongly linked to impaired FEV1, suggesting that routine point-of-care FeNO testing, combined with ACT assessment, could serve as an effective strategy to improve asthma management and enhance disease control.
