Acute lung injury (ALI), and its more severe counterpart the acute respiratory distress syndrome (ARDS), are syndromes of acute respiratory failure associated with pulmonary oedema caused by increased permeability of the alveolar–capillary membrane. Many clinical scenarios are recognised as being associated with a high incidence of ALI, including the archetypal direct pulmonary and blood borne insults of pneumonia and severe sepsis, respectively. The internationally accepted diagnostic criteria1 are non-specific to the point of including patients with relatively mild hypoxia and patients with lung pathology that may be different from the classical diffuse alveolar damage.2 ALI is not uncommon but it is challenging to study, partly because the patients are heterogenous in the causes and severity of their illness. Furthermore, patients die with rather than from respiratory failure in the majority of cases.3 These issues partly account for the fact that only one intervention has been shown to affect the survival of patients with ALI. The National Heart, Lung and Blood Institute (NHLBI) ARDS Network ARMA study,4 arguably the most important trial in respiratory medicine in the last 20 years, demonstrated an approximately 10% survival advantage in favour of a ventilation strategy that limited tidal volume (6 ml/kg predicted body weight) and plateau pressure (⩽30 cm H₂O) compared with “standard” ventilatory parameters (12 ml/kg and ⩽50 cm H₂O).

A biomarker is a clinical parameter that is measured with a view to providing information about a disease process, in this case ALI (box 1). Apart from informing the diagnostic process, …