Acutely ill patients are commonly treated with intravenous fluids. In this study we will use a clustercross over design to compare the routine use of 0.9% saline for fluid therapy with Plasma Lyte® 148 in intensive care unit patients.The study hypothesis is that routinely using Plasma Lyte® 148 for fluid therapy instead of 0.9% saline will reduce the risk of developing acute kidney failure. Kidney failure which occurs in the setting of acute illness is associated with a high risk of death and may require treatment with costly kidney dialysis treatments. This study addresses an issue of major global public health significance because more than a million litres of 0.9% saline are currently administered to patients around the world daily. The study design will embed research within usual practice to ensure that the study’s findings are incorporated into routine care.