Our previous findings were derived from a single hospital's intensive care unit. We have recently confirmed this finding across many intensive care units, which will help show that the test can be used in 'real life'. The aim of this study is to take the new test to the next step and determine whether it can improve the care of patients by reducing the amount of unnecessary antibiotics prescribed. This will be done using a 'randomised controlled trial', the best tool for scientifically testing a new diagnostic test. To do this we shall identify patients with suspected VAP, all of whom will have a lung sample - half of the patients will receive 'usual care' for suspected VAP, the other half will have the new test performed on their lung fluid. If the new test suggests no lung infection, the doctors will be asked to consider not giving antibiotics. We shall test how much antibiotic is given to each group.