Baseline characteristics and patient outcome
No statistically significant differences were found in age, gender or BMI among the mild, moderate and severe ARDS groups. Pneumonia, sepsis and pancreatitis were the most common aetiologies of ARDS. As measured by the APACHE II score (P=0.016), SOFA score (P=0.027), and PaO2/FiO2 (P=0.000), the severity of critical illness on the day of enrolment worsened from mild to severe ARDS, as shown in Tables 1 and 2. The 100-day mortality rate was 60.2% (121/201) in patients with ARDS. Compared with survivors, non-survivors had significantly older age and higher APACHE II and SOFA scores. Additionally, survivors had higher BMIs and PaO2/FiO2 ratios than non-survivors.
Correlations of the lymphocyte/neutrophil ratio with disease severity and outcome
Severe ARDS patients compared with those in the mild group, the frequencies of lymphocyte cells were decreased (P=0.025). Moreover, the lymphocyte/neutrophil ratio progressively decreased with increasing ARDS severity from mild to moderate and severe ARDS (P=0.001). Moreover, in non-survivors, a significantly lower lymphocyte/neutrophil ratio was found compared with that of the survivors (P=0.008) (Table 2), and the frequencies of lymphocyte cells in non-survivors were lower than those in survivors (P=0.025) (Table 2). However, although the frequencies of leukocytes and neutrophil cells in non-survivors were higher than those in survivors (P=0.012,0.016) (Table 2), there were no significant differences among the three groups in terms of the frequencies of leukocytes and neutrophil cells (Table 1).
Alterations in inflammatory biomarkers, immunoglobulins, complement components, circulating T-lymphocyte cells, B-lymphocyte cells and NK cells in ARDS
CRP progressively increased with increasing ARDS severity from mild to moderate and severe ARDS (P=0.011).PCT levels were higher in patients with severe ARDS than in patients with mild ARDS (P=0.002).
Interestingly, the lymphocyte count decreased as the severity of ARDS increased from mild to severe (P=0.025) (Table 1). Furthermore, compared with survivors, non-survivors had older age (P=0.004), higher leukocyte counts and neutrophil counts (P=0.012,0.016),and lower BMI, lymphocyte counts and lymphocyte/neutrophil ratios (P=0.027,0.025,and 0.008) (Table 2). The CRP and PCT levels were similar in the two groups (Table 2).
The peripheral blood immunoglobulin IgE and complement C3 levels in patients with mild ARDS were significantly higher than those in patients with severe ARDS (P=0.023,0.019).Moreover, non-survivors had lower immunoglobulin IgE and complement C3 levels than did survivors (P=0.009,0.018) (Table 2). However, peripheral blood complement C4 levels in patients with mild ARDS were significantly lower than those in patients with severe ARDS (P=0.026) (Table 1), but the complement C4 levels were similar between the survivors and non-survivors (Table 2).
The level of peripheral blood B-lymphocyte cells were significantly lower in non-survivors than in survivors (P=0.009), and the level of peripheral blood CD8+ cells was significantly lower in non-survivors than survivors (P=0.024). However, the levels of both peripheral blood B-lymphocyte cells and CD8+ cells were similar in the three groups (Table 1). In addition, the proportions of CD3+ cells, CD4+ cells, and NK cells and the CD4+/CD8+ ratio in the peripheral blood showed no significant differences in three groups of ARDS patients stratified by oxygenation index or in the survivor and non-survivor groups.
Correlations of lymphocytes, the lymphocyte/neutrophil ratio, immunoglobulin IgE levels, complement C3 levels, T-CD8+ lymphocyte levels and B-lymphocyte levels with disease severity and outcome
In all ARDS patients, significant moderate negative correlations were found between the lymphocyte/neutrophil ratio and age (r=-0.153,P=0.030), the SOFA score (r=-0.140, P=0.038), and the APACHE II score (r=-0.177, P=0.012). In addition, we noted a moderate positive correlation between the lymphocyte/neutrophil ratio and PaO2/FiO2 ratio (r=0.143, P=0.023). Moreover, significant mild positive correlations were found between the lymphocyte count and BMI (r=0.145, P=0.041) and the lymphocyte count and the PaO2/FiO2 ratio (r=0.110, P=0.121), the immunoglobulin IgE level and the PaO2/FiO2 ratio (r=0.288, P=0.036), the C3 level and BMI (r=0.342, P=0.026), the T-CD8+ lymphocyte counts and lymphocyte counts (r=0.755, P=0.001), B-lymphocyte cell counts and BMI (r=0.588, P=0.013), B-lymphocyte cell counts and lymphocyte counts (r=0.582, P=0.014).
The area under the ROC curve (AUC) for the lymphocyte/neutrophil ratio in combination with the lymphocyte count for the prediction of 100-day survival in ARDS patients was 0.723 (95% CI 0.656 to 0.784), which was larger than both the AUC for the lymphocyte/neutrophil ratio (0.721, 95% CI 0.653 to 0.782, P=0.8601) and the lymphocyte/neutrophil ratio in combination with the PaO2/FiO2 ratio (0.719, 95% CI 0.651 to 0.780, P=0.7734). The AUC for the PaO2/FiO2 was 0.625 (95% CI 0.554 to 0.692), the AUC for BMI was 0.593 (95% CI 0.521 to 0.661) and the AUC for the lymphocyte count was 0.592 (95% CI 0.520 to 0.660), all of which were significantly lower than the AUC for the lymphocyte/neutrophil ratio (P=0.0062, 0.0001, and 0.0154, respectively). The AUC for the lymphocyte/neutrophil ratio in combination with the lymphocyte count was 0.723 (95% CI 0.656 to 0.784), which was significantly higher than those for the PaO2/FiO2 ratio alone (P=0.0060), BMI alone (P=0.0001), and lymphocyte count alone (P=0.0067) for predicting survival in patients with ARDS. The AUC for the lymphocyte/neutrophil ratio in combination with the PaO2/FiO2 ratio was 0.719 (95% CI 0.651 to 0.780), which was significantly higher than those for the PaO2/FiO2 ratio alone (P=0.0014), BMI alone (P=0.0001), and lymphocytes alone (P=0.0162) for predicting survival in patients with ARDS.
A cut-off value of the lymphocyte/neutrocyte ratio of >0.0537 was used to predict the survival of ARDS patients, with a sensitivity of 83.8%, specificity of 80.2%, positive likelihood ratio of 4.23 and negative likelihood ratio of 0.20. Moreover, using a leukocyte count cut-off of >0.415 (109/L) for predicting survival in patients with ARDS, the sensitivity and specificity were 87.5% and 81.0%, respectively, and the positive and negative likelihood ratios were 4.61 and 0.15, respectively.
Predictors of 28-day and 100-day mortality in patients with ARDS
Table 3 shows that the age (per log10 years) (OR=1.269, P=0.019), BMI<24 (OR=1.665, P=0.015), SOFA score (OR=1.287, P=0.002), leukocyte count<0.415 (109/L) (OR=1.671, P=0.042), and lymphocyte/neutrophil ratio (OR=2.132, P=0.009) were the independent predictors of 100-day mortality in patients with ARDS. Moreover, ARDS patients with a lymphocyte/neutrophil ratio <0.0537 had a higher 28-day mortality rate than those with a lymphocyte/neutrophil ratio >0.0537 (P=0.0283, Figure 3A). The 28-day and 100-day mortality rates were significantly lower in the under-40 years old and 40-60 years old age groups than in the over-60 years old age group (P=0.0064, 0.0057, Figure 3B, C). The 100-day mortality rate was significantly higher in the over-80 years old age group than in the under-40 years old age group, the 40-60 years old age group and the 60-80 years old age group (P=0.0029, Figure 3D).