This study aims to determine changes in kidney function during and after critical illness, comparing conventional creatinine based methods with the gold standard to accurately establish the presence of new or worsened chronic kidney disease. In addition, investigators will assess the confounding effect of muscle wasting on the conventional assessment of kidney function and investigate the information that measures of kidney function may contribute to the assessment of musculoskeletal health after critical illness.

More people than ever are surviving life-threatening illnesses such as major trauma. However, until now doctors and researchers have focused more on improving short term survival than on the serious, long-term complications experienced by survivors of critical illness. In response, the National Institute for Health and Care Excellence (NICE) and patient-clinician partnerships such as the James Lind Alliance, have now prioritised research into the diagnosis, follow-up and treatment of critical care survivors.

Development of chronic kidney disease and persistent muscle weakness are two commonly encountered complications which significantly impact long-term health and wellbeing after critical illness. Worsening of kidney function strongly predisposes to development of heart disease, premature death or need for long-term dialysis. Similarly, the muscle wasting experienced by almost all survivors of critical illness can result in persistent, life changing limitations to daily living, inability to work and decreased quality of life. Importantly, the human and economic consequences of critical illness may be particularly profound in major trauma victims who are often young and previously healthy. In this project, investigators will aim to simultaneously measure changes in kidney function and muscle mass after critical illness allowing researchers to understand how these processes interact in affecting longer-term patient outcomes.

The investigators will recruit 62 patients, 31 admitted to intensive care after major trauma and 31 admitted for other reasons. Complementary methods will be used to accurately monitor muscle mass and kidney function. Six months after discharge from hospital, patient's ability to manage their daily activities and quality of life will be assessed alongside measurements of muscle mass, strength and kidney function. The study will be performed at the Royal London Hospital, an internationally renowned centre for critical care and trauma research.

Inclusion Criteria:

• Major trauma cohort: Patients ≥18y admitted to ICU and anticipated to be mechanically ventilated for ≥48 hours with a primary admission diagnosis of major trauma.
• Non-trauma cohort: Patients ≥18y admitted to ICU and anticipated to be mechanically ventilated for ≥48 hours without a primary admission diagnosis of major trauma.

Exclusion Criteria:

• Death or discharge from hospital considered highly likely by treating physician within 7 days of ICU admission.
• Any of the following conditions: major traumatic brain injury (Abbreviated Injury Scale head injury score ≥ 5), spinal cord injury with paralysis, lower limb amputation, end stage renal disease or disseminated cancer, lack of independence with activities of daily living or non-ambulatory status prior to admission. (Rationale - exclusion of factors where type of injury or comorbid disease will overwhelming determine functional or renal outcomes.)

Adult patients admitted to critical care.