Evaluation of Legionella Diagnostic Prediction Score in patients with Chlamydia psittaci pneumonia

Legionella pneumonia is an important cause of community-acquired pneumonia (CAP). The Japanese Respiratory Society (JRS) pneumonia guideline 2024 proposed use of the Legionella Diagnostic Prediction Score for the management of CAP in adults. The committee for the JRS pneumonia guideline is required to verify the validity of the Legionella Diagnostic Prediction Score for the next revision. In addition, it is necessary to determine appropriate cutoff scores by examining all pneumonia cases. In the present study, we validated the usefulness of the Legionella Diagnostic Prediction Score using Chlamydia psittaci CAP. We analyzed 116 patients with L. pneumophila CAP and 72 patients with C. psittaci CAP. The median Legionella Diagnostic Prediction Score was significantly higher in the L. pneumophila CAP group than the C. psittaci CAP group (4 vs 2, p < 0.0001). When a total score ≥3 points was set as the cutoff level, the diagnostic sensitivity and specificity for presumptive diagnosis of L. pneumophila CAP were 90.5 % and 70.8 %, respectively. When the cutoff score was ≥4 points, the diagnostic sensitivity and specificity for presumptive diagnosis of L. pneumophila CAP were 76.7 % and 90.3 %, respectively. Our results demonstrated that the Legionella Diagnostic Prediction Score is capable of probabilistically distinguishing Legionella pneumonia from C. psittaci pneumonia. To set optimal indicators and cutoff values for the Legionella Diagnostic Prediction Score, the policy of the committee for the JRS pneumonia guideline is to continue testing for all pneumonia types.