Phosphate is the most abundant intracellular anion and is essential for membrane structure, energy storage, and transport in all cells. In particular, phosphate is necessary to produce ATP, which provides energy for nearly all cell functions. Phosphate is an essential component of DNA and RNA. Phosphate is also necessary in red blood cells for production of 2,3-diphosphoglycerate (2,3-DPG), which facilitates release of oxygen from hemoglobin.
Approximately 85% of the body’s phosphorus is in bone as hydroxyapatite, while most of the remainder (15%) is present in soft tissue. Only 0.1% of phosphorus is present in extracellular fluid, and it is this fraction that is measured with a serum phosphorus level.
Reducing available phosphate may compromise any organ system, alone or in combination. The critical role phosphate plays in every cell, tissue, and organ explains the systemic nature of injury caused by phosphate deficiency.
Serum phosphate or phosphorus normally ranges from 2.5-4.5 mg/dL (0.81-1.45 mmol/L) in adults. Hypophosphatemia is defined as mild (2-2.5 mg/dL, or 0.65-0.81 mmol/L), moderate (1-2 mg/dL, or 0.32-0.65 mmol/L), or severe (< 1 mg/dL, or 0.32 mmol/L).
Mild to moderately severe hypophosphatemia is usually asymptomatic. Major clinical sequelae usually occur only in severe hypophosphatemia. If severe hypophosphatemia is present for longer than 2-3 days, serious complications can be seen, including rhabdomyolysis, respiratory failure, acute hemolytic anemia, and fatal arrhythmias.
It has also been shown to increase mortality by four-fold.
Approximately 5% of hospitalized patients have hypophosphatemia, mostly those patients with diabetic ketoacidosis, chronic obstructive pulmonary disease, malignancy, states of malnutrition, and sepsis.
As in the case of other intracellular ions (eg, potassium, magnesium), a decrease in the level of serum phosphate (hypophosphatemia) should be distinguished from a decrease in total body storage of phosphate (phosphate deficiency).