A Novel Diagnostic Scoring System to Differentiate between Legionella pneumophila Pneumonia and Streptococcus pneumoniae Pneumonia

: Objective We investigated a novel diagnostic scoring system to differentiate Legionella pneumophila pneumonia from Streptococcus pneumoniae pneumonia. Methods We retrospectively reviewed the clinical data of 62 patients with L. pneumophila pneumonia (L-group) and 70 patients with S. pneumoniae pneumonia (S-group). Results The serum sodium (Na) levels tended to be lower according to the severity [age, dehydration, respiratory failure, orientation disturbance, low blood pressure (A-DROP)] score in the L-group. On a multivariate analysis, we found that four factors were independent predictive markers for inclusion in the L-group: relative bradycardia [hazard ratio (HR) 5.177, 95% confidence interval (CI): 1.072-24.993, p=0.041], lactate dehydrogenase (LDH) levels ≥292 IU/L (HR 6.804, 95% CI: 1.629-28.416, p=0.009), C-reactive protein (CRP) levels ≥21 mg/dL (HR 28.073, 95% CI: 5.654-139.462, p<0.001), and Na levels ≤137 meq/L (HR 5.828, 95% CI: 1.411-24.065, p=0.015). Furthermore, a total score [ranging from 0 to 4, the sum of the points for each factor (0 or 1)] ≥3 points indicated a higher probability of inclusion in the L-group than in the S-group. The diagnostic accuracy of a total score of 3 had a sensitivity of 36.3%, specificity of 100%, and area under the curve of 0.682 (95% CI: 0.558-0.806, p=0.004), and that of a total score of 4 had a sensitivity 27.4%, specificity of 98.2%, and area under the curve (AUC) of 0.627 (95% CI: 0.501-0.754, p=0.045). The diagnostic accuracy had low sensitivity but high specificity. Conclusions We found four markers that might be useful for differentiating L-group from S-group and created a novel diagnostic scoring system.