Pulmonary infections represent an increasing percentage of the world's total infections. The lack of a reference standard for the diagnosis of bacterial lower respiratory tract infections (LRTI) remains a challenge. We prospectively explored the association between clinical presentation, microbiology, and a blood-based 29 mRNA host-response test in patients with suspicion of LRTI.

The PAX study is a prospective, single-centre observational study involving 578 cases with suspected LRTI undergoing bronchoscopy with bronchoalveolar lavage (BAL). This entails an interim analysis of 366 cases. Blood was analyzed for 29 mRNA host-response using InSep?? (Inflammatix, California, USA). Strict definitions for very likely, possible, unlikely, and very unlikely bacterial infection were provided by the adjudicators.

The patients? average age was 62.4±15.3 years. Clinical presentation but not microbiological methods were significantly differentiated across the categories assessing probability of bacterial infection by mRNA signatures (p=<0.0001 and p=0.4862, respectively). Using conventional methods as a reference standard, the mRNA host-response score showed 36.3% sensitivity and 75.0% specificity, compared to clinical presentation that showed 71.0% sensitivity and 54.5% specificity for bacterial infection.

There is an association between clinical judgement and an inflammatory host-response test for bacterial infections. Complementary aspects for the likelihood of infection may be clinical presentation, microbiology, and mRNA host-response. The evaluation of the entire cohort will allow robust inferences after completion of analyses soon.