Randomisation will be performed by a designated researcher trained in the use of the biomarker kit. The randomisation will be performed using a web-based randomisation service hosted by NCTU. Patients will be randomised to 'biomarker-guided recommendation for antibiotics' or 'routine use of antibiotics' in a 1:1 ratio with stratification by site. Permuted blocks of variable length will be used and not disclosed to investigators. On enrolment the research team will inform the researcher that a BAL is to be performed allowing the researcher to randomise the patient. Each patient will be assigned a unique identification number that the technician will enter into the randomisation system. The patient will then be allocated to either study arm in accordance to the randomisation schedule. The online randomisation service will instruct the researcher to either 'ANALYSE THE SAMPLE ON ARRIVAL' (ie. the 'biomarker-guided recommendation on antibiotics' group) or 'DO NOT ANALYSE THE SAMPLE ON ARRIVAL' (ie. 'Routine use of antibiotics' group). The randomisation service will automatically confirm randomisation details to the NCTU by email and the researcher will also send the BALF biomarker result to the NCTU using the unique identifier.