Patients admitted to intensive care units (ICU) are at high risk of developing secondary infections, and this is in part due to dysfunction or failure of their 'germ killing' functions (the immune system). Our group has recently identified three signatures of immune system failure which can be readily detected on a blood sample, and importantly, appear to predict the chances of developing secondary infection. Such a test would have major benefits for the management of patients in intensive care if it can be translated into a test usable in everyday clinical practice. This study aims to validate our original findings in a cohort of patients from multiple ICUs, using a test which will be suitable for everyday clinical practice, and thus take the next step towards developing a market-ready test.

Study hypothesis:

Measurement of neutrophil CD88, monocyte HLA-DR and percentage Tregs will accurately predict the risk of nosocomial infection.

Inclusion Criteria:

• Age >16 (>18 in England)
• Requiring level 3 care (i.e. requiring invasive support of respiratory system alone, or two or more other organ systems (haemofiltration, inotropes/vasopressors)
• Predicted to remain in ICU for at least 48 hours,

Exclusion Criteria:

• Not expected to survive for a further 24 hours
• Known or suspected ICU-acquired infection at time of screening (non-ICU acquired nosocomial infection - i.e. non-ICU healthcare associated infection is NOT and exclusion)
• Known inborn errors of immune function
• Immunosuppression (corticosteroids up to 400mg hydrocortisone equivalent daily dose permitted)
• HIV infection
• Pregnancy
• Previously enrolled in the study

Critical Care patients who are expected to remain in ICU for at least 48 hours, and require the external support of one or more organs (invasive ventilation, inotropes/vasopressors or renal replacement therapy) and who are not expected to die within 48 hours of study entry.