# CRP under 130 mg/L rules out the diagnosis of Legionella pneumophila serogroup 1 (URINELLA Study)

## TL;DR

A CRP level under 130 mg/L can reliably rule out Legionella pneumophila serogroup 1 infection in most pneumonia cases.

## Contribution

Identified a CRP threshold of 130 mg/L with 100% negative predictive value for Legionella Urinary-Antigen-Test in immunocompetent patients.

## Key findings

- CRP threshold of 131.9 mg/L achieved 100% sensitivity and NPV for negative L-UAT results.
- External validation confirmed high NPV (99.9%) and AUC of 89.8% for the CRP threshold.
- Excluding immunosuppressed patients improved sensitivity and NPV to 100%.

## Abstract

In case of pneumonia, some biological findings are suggestive for Legionnaire’s disease (LD) including C-reactive protein (CRP). A low level of CRP is predictive for negative Legionella Urinary-Antigen-Test (L-UAT).

Observational retrospective study in Nord-Franche‐Comté Hospital with external validation in Besançon University Hospital, France which included all adults with L-UAT performed during January 2018 to December 2022. The objective was to determine CRP optimal threshold to predict a L-UAT negative result.

URINELLA included 5051 patients (83 with positive L-UAT). CRP optimal threshold was 131.9 mg/L, with a negative predictive value (NPV) at 100%, sensitivity at 100% and specificity at 58.0%. The AUC of the ROC-Curve was at 88.7% (95% CI, 86.3–91.1). External validation in Besançon Hospital patients showed an AUC at 89.8% (95% CI, 85.5–94.1) and NPV, sensitivity and specificity was respectively 99.9%, 97.6% and 59.1% for a CRP threshold at 131.9 mg/L; after exclusion of immunosuppressed patients, index sensitivity and NPV reached also 100%.

In case of pneumonia suspicion with a CRP level under 130 mg/L (independently of the severity) L-UAT is useless in immunocompetent patients with a NPV at 100%. We must remain cautious in patients with symptoms onset less than 48 h before CRP dosage.

The online version contains supplementary material available at 10.1007/s10096-024-04814-x.