Patient selection and laboratory data
The study cohort included 46 consecutive COVID-19 patients admitted to Sapporo Medical University Hospital through April 20, 2020. Blood samples collected from all 46 patients (total blood samples, 82) were analyzed. Of the 46 patients, 7 were treated with ventilation plus ECMO, and 2 were treated with ventilation. Case severity was classified as severe for patients treated with oxygen therapy and non-severe for those without. The median age was 54.5 years (22–84 years), and 21 participants were male (45.6%). Most blood count and biochemical parameters, including total protein (TP), alanine aminotransferase (AST), aspartate aminotransferase (ALT), lactate dehydrogenase (LDH), and C-reactive protein (CRP), differed significantly between severe and non-severe cases (Table 1). The severe group had more patients with hypertension than did the non-severe group (33% vs 9%, respectively; p < 0.05). Radiologically, nearly all patients had multi-lobular ground glass opacification (GGO) with a peripheral or posterior distribution. In severe cases, ARDS-like findings, mainly consolidation, were present in images. To identify any relationships between biochemical parameters and chest CT images, the images were taken on the same day that the blood samples were drawn. The CT scores were significantly higher among severe cases than non-severe cases (16.4 ± 6.28 vs 3.6 ± 3.6, respectively; p < 0.001).
Elevations in serum surfactant proteins A and D were seen early in the course of pneumonia
We observed that the serum SP-A and SP-D levels were significantly higher in severe cases than in non-severe cases (Figure 1). Additionally, SP-A was higher in non-severe cases than in healthy subjects (Figure 1).
Serum SP-A, SP-D, KL-6, LDH, CRP, and IL-6 levels were compared between non-severe cases and severe cases using ROC curves (Figure 2). Results of area under the curve (AUC) comparison of severe to non-severe cases were as follows: SP-A, 0.796; SP-D, 0.827; KL-6, 0.640; LDH, 0.970; CRP, 0.920; and IL-6, 0.879. Using ROC curves, the diagnostic cut-off levels were set at 94.9 ng/mL for SP-A, 116 ng/mL for SP-D, and 275 U/mL for KL-6. Of these cut-off levels, SP-A and SP-D were higher than the cut-off level for detecting ILDs [SP-A: 94.9 vs 45.0 ng/mL, SP-D: 116 vs 110 ng/mL, KL-6: 275 vs 500 U/mL][11, 25].
Serum SP-A and SP-D levels correlate with severity indicated by chest CT images
We examined the relationship between chest CT images and blood test parameters using the Spearman rank correlation coefficient, a nonparametric measurement to identify correlations between two sets of data. We observed that the serum levels of SP-A and SP-D correlated significantly with the CT score for each patient (Figure 3; Table 2). We observed that the serum levels of SP-A and SP-D correlated more closely with CT score than did that of KL-6.
Serum SP-A, SP-D, KL-6, LDH, CRP, and IL-6 concentrations were evaluated with respect to the disease severity indicated by chest HRCT images using ROC curves (Figure 4). The median CT score in all cases was 6, so we considered the group with a score above the median as positive. The AUCs for high vs. low CT scores were as follows: SP-A, 0.863; SP-D, 0.831; KL-6, 0.686; LDH, 0.888; CRP, 0.949; and IL-6, 0.896. The diagnostic cut-off levels using ROC curves were set at 66.1 ng/mL for SP-A, 48.5 ng/mL for SP-D, and 247 U/mL for KL-6. Of these cut-off levels, only SP-A was higher than the cut-off level for detecting ILDs [SP-A: 66.1 vs 45.0 ng/mL, SP-D: 48.5 vs 110 ng/mL, KL-6: 247 vs 500 U/mL][11, 25].
A typical case of COVID-19 progression demonstrating a correlation between serum SP-A and SP-D concentrations and disease severity
Figure 5 shows a typical severe case of COVID-19 for which serum levels of SP-A, SP-D and KL-6 were monitored regularly. After admission to our hospital, this patient exhibited worsening of symptoms and chest radiological findings (Figure 5C). Concomitant with this deteriorating condition, the serum levels of SP-A, SP-D, and other biomarkers increased (Figure 5A). The rate of increase was highest for SP-D (Figure 5B). During this period, serum KL-6 levels were slightly elevated below the cut-off level used for ILDs (500 U/mL).