(See also Overview of Disorders of Phosphate Concentration.)
Hypophosphatemia occurs in 2% of hospitalized patients but is more prevalent in certain populations (eg, it occurs in up to 10% of hospitalized patients with alcohol use disorder).
Hypophosphatemia has numerous causes but clinically significant acute hypophosphatemia occurs in relatively few clinical settings, including the following:
Acute severe hypophosphatemia with serum phosphate < 1 mg/dL (< 0.32 mmol/L) is most often caused by transcellular shifts of phosphate often superimposed on chronic phosphate depletion.
Chronic hypophosphatemia usually is the result of decreased renal phosphate reabsorption. Causes include the following:
Increased parathyroid hormone levels, as in primary and secondary hyperparathyroidism
Long-term diuretic use
Severe chronic hypophosphatemia usually results from a prolonged negative phosphate balance. Causes include
Patients with advanced chronic kidney disease (especially those on dialysis) often take phosphate binders with meals to reduce absorption of dietary phosphate. The prolonged use of these binders can cause hypophosphatemia, particularly when combined with greatly decreased dietary intake of phosphate.
Although hypophosphatemia usually is asymptomatic, anorexia, muscle weakness, and osteomalacia can occur in severe chronic depletion. Serious neuromuscular disturbances may occur, including progressive encephalopathy, seizures, coma, and death. The muscle weakness of profound hypophosphatemia may be accompanied by rhabdomyolysis, especially in acute alcoholism.
Hematologic disturbances of profound hypophosphatemia include hemolytic anemia, decreased release of oxygen from hemoglobin, and impaired leukocyte and platelet function.
Hypophosphatemia is diagnosed by a serum phosphate concentration < 2.5 mg/dL (< 0.81 mmol/L). Most causes of hypophosphatemia (eg, diabetic ketoacidosis, burns, refeeding) are readily apparent. Testing to diagnose the cause is done when clinically indicated (eg, suggestive liver test results or signs of cirrhosis in patients with suspected alcohol use disorder).
Treatment of the underlying disorder and oral phosphate replacement are usually adequate in asymptomatic patients, even when the serum concentration is very low. Phosphate can be given in doses up to about 1 g orally 3 times a day in tablets containing sodium phosphate or potassium phosphate. Oral sodium phosphate or potassium phosphate may be poorly tolerated because of diarrhea. Ingestion of 1 L of low-fat or skim milk provides 1 g of phosphate and may be more acceptable. Removal of the cause of hypophosphatemia may include stopping phosphate-binding antacids or diuretics or correcting hypomagnesemia.
Parenteral phosphate is usually given IV. It should be administered in any of the following circumstances:
When serum phosphate is < 1 mg/dL (< 0.32 mmol/L)
Rhabdomyolysis, hemolysis, or central nervous system symptoms are present
Oral replacement is not feasible due to underlying disorder
IV administration of potassium phosphate (as buffered mix of K2HPO4 and KH2PO4) is relatively safe when renal function is well preserved. Parenteral potassium phosphate contains 93 mg (3 mmol) phosphorus and 170 mg (4.4 mEq) potassium per mL. The usual dose is 0.5 mmol phosphorus/kg (0.17 mL/kg) IV over 6 hours. Patients with alcohol use disorder may require ≥ 1 g/day during total parenteral nutrition; supplemental phosphate is stopped when oral intake is resumed.
If patients have impaired renal function or serum potassium > 4 mEq/L (> 4 mmol/L), sodium phosphate preparations generally should be used; these preparations also contain 3 mmol/mL of phosphorus and are thus given at the same dose.
Serum calcium and phosphate concentrations should be monitored during therapy, particularly when phosphate is given IV or to patients with impaired renal function. In most cases, no more than 7 mg/kg (about 500 mg for a 70-kg adult) of phosphate should be given over 6 hours. Close monitoring is done, and more rapid rates of phosphate administration should be avoided to prevent hypocalcemia, hyperphosphatemia, and metastatic calcification due to excessive calcium phosphate product.
Acute hypophosphatemia most often occurs in patients with of alcohol use disorder, burns, or starvation.
Acute severe hypophosphatemia can cause serious neuromuscular disturbances, rhabdomyolysis, seizures, coma, and death.
Chronic hypophosphatemia may be due to hormonal disorders (eg, hyperparathyroidism, Cushing syndrome, hypothyroidism), chronic diuretic use, or use of aluminum-containing antacids by patients with chronic kidney disease.
Hypophosphatemia is usually asymptomatic, but severe depletion can cause anorexia, muscle weakness, and osteomalacia.
Treat the underlying disorder, but some patients require oral, or rarely, IV phosphate replacement.
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