Phosphate (PO₄) is a trivalent anion that is most notable for the energy-rich bonds it forms in adenosine triphosphate (ATP). Phosphate is also intimately in involved red blood cell function (2,3-diphosphoglycerate), platelet function, immune function, and the central nervous system (CNS). Phosphate homeostasis is closely linked with that of calcium, which together are controlled by parathyroid hormone, calcitonin, and vitamin D.

Patients in the intensive care unit (ICU) can have either hyperphosphatemia or hypophosphatemia. Postoperative hypophosphatemia is classically linked with liver resection and living-donor hepatectomies for liver transplantation. Classic teaching on this subject highlights the utilization of total body phosphate stores for the regeneration of the liver; however, a recent study concluded the presence of hyperphosphaturia points toward a renal origin of phosphate loss. Low serum phosphate has also been reported after other surgeries, such as colorectal surgery, renal transplantation, abdominal aortic aneurysm repair, and open heart surgery. If not prevented or treated, hypophosphatemia will frequently develop after major surgery on postoperative day one and reach a nadir on postoperative day two.