The present invention is directed to the use of (1) mucin concentration in sputum; (2) individual airway mucins MUC5AC and MUC5B ratio (MUC5AC/MUC5B) in sputum; and (3) combination of both measurements (Kesimer MUCQuant index) that can be used as an objective biomarker for differential diagnosis of smoking status and chronic bronchitis (CB), disease severity of CB (mild, moderate, and severe), exacerbation status, monitoring progression of CB, and guiding of therapies for CB. In addition, the ratio of amount of IgGFc-binding protein (FCGBP) and the amount of bactericidal/permeability-increasing fold-containing family member A1 (BPIFA1 (or SPLUNC1)) present in a sputum sample from a subject may be used in combination with the Kesimer MUCQuant index (Kesimer MUCQuant Plus index) for differential diagnosis of smoking status and CB, disease severity of CB, exacerbation status, p monitoring progression of CB, and guiding of therapies for CB.