Background: Saliva is increasingly promoted as an alternative diagnostic bio-sample to blood; however its role in respiratory disease requires elucidation. Our aim was to investigate whether C-reactive protein (CRP), procalcitonin (PCT) and neutrophil elastase (NE) could be measured in unstimulated whole saliva, and to explore differences between COPD patients and controls with normal lung function. We also determined the relationship between these salivary biomarkers and self-reported COPD-relevant metrics.
Methods: Salivary CRP, PCT and NE levels were measured at each of 3 visits over a 14-day period alongside spirometry and a daily self-assessment dairy in 143 subjects: 20 never-smokers and 25 smokers with normal spirometry; 98 COPD patients [GOLD Stage I, 16; Stage II, 32; Stage III, 39; Stage IV, 11]. Twenty-two randomly selected subjects provided simultaneous blood samples.
Results: Levels of each salivary biomarker could distinguish between the above cohorts. Significant differences remained for salivary CRP and NE (p < 0.05) following adjustment for age, gender, sampling time, gum disease and total co-morbidities; but not for BMI except for salivary NE, which remained higher in smokers compared to non-smokers and stable COPD subjects (p < 0.001). Patients with acute COPD exacerbations had a median increase in all 3 salivary biomarkers (p < 0.001); CRP: median 5.74 ng/ml, [interquartile range (IQR) 2.86-12.25], PCT 0.38 ng/ml, [IQR 0.22-0.94], and NE 539 ng/ml, [IQR 112.25-1264]. In COPD patients, only salivary CRP and PCT levels correlated with breathing scores (r = 0.14, p < 0.02; r = 0.13, p < 0.03 respectively) and sputum features but not with activities of daily living. Salivary CRP and PCT concentrations strongly correlated with serum counterparts [r = 0.82, (95% CI: 0.72-0.87), p < 0.001 by Spearman's; and r = 0.53, (95% CI: 0.33-0.69), p < 0.006 respectively]; salivary NE did not.
Conclusions: CRP, PCT and NE were reliably and reproducibly measured in saliva, providing clinically-relevant information on health status in COPD; additionally NE distinguished smoking status. All 3 salivary biomarkers increased during COPD exacerbations, with CRP and PCT correlating well with patient-derived clinical metrics. These results provide the conceptual basis for further development of saliva as a viable bio-sample in COPD monitoring and exacerbation management.