Diagnostic Usefulness of SP-D, CCL2/MCP-1, and IL-18 in Assessing Respiratory Function and Risk of Pulmonary Fibrosis in COVID-19 Patients

This study evaluated the diagnostic usefulness of surfactant protein D (SP-D), chemokine C-C motif ligand 2/monocyte chemoattractant protein-1 (CCL2/MCP-1), and interleukin 18 (IL-18) in patients with SARS-CoV-2 infection. The authors focused on lung failure assessment, lung parenchyma involvement, and the early assessment of the risk of developing pulmonary fibrosis at the onset of COVID-19. The study group included 87 patients with COVID-19 and 45 healthy subjects. Concentrations of all three markers were measured using the enzyme immunoassay method. The serum SP-D, CCL2/MCP-1, and IL-18 concentrations were significantly higher in the COVID-19 patients before admission to the hospital than those in the controls (p < 0.001 for all comparisons). Differences were only found in the IL-18 levels between the groups categorized according to disease severity (p < 0.001); levels were significantly higher in patients with critical disease severity compared to those with moderate disease severity (p < 0.001). IL-18 also showed a positive correlation with the disease severity (p = 0.025). SP-D and IL-18 levels varied depending on the amount of oxygen administration (p = 0.017 and p < 0.001, respectively). Among the blood gas parameters, SP-D levels were negatively associated with the partial pressure of arterial oxygen (PaO2) and oxygen saturation (O2Sat) (p = 0.026 and p = 0.048, respectively), IL-18 with O2Sat (p = 0.036), and CCL2/MCP-1 with PaO2 (p = 0.042). SP-D and IL-18 were significantly negatively correlated with oxygen therapy (p = 0.003 and p = 0.014, respectively). Conversely, CCL2/MCP-1 was significantly positively correlated with the pulmonary involvement severity (p = 0.017) and level of hyaluronic acid (HA), a marker of fibrosis (p = 0.042). We also observed a significant correlation between the IL-18 level and the pulmonary involvement severity (p < 0.001), HA (p < 0.001), and other markers of fibrosis. In summary, our study’s results indicate that SP-D, CCL2/MCP-1, and IL-18 may be used to assess lung function in the early stage of COVID-19 infection; additionally, SP-D may serve as an indicator of alveolar injury, while CCL2/MCP-1 and IL-18 may be markers of lung parenchyma involvement and potential predictors of the development of pulmonary fibrosis.