Hypophosphatemia
Adult-Gerontology Acute Care Practice Guidelines
Definition
A.See Box 5.6.
B.Serum phosphorus less than 3.5 mg/dL.
C.Moderate hypophosphatemia less than 2.5 mg/dL.
D.Severe hypophosphatemia less than 1 mg/dL.
Incidence
A.Incidence in general population is typically asymptomatic and estimated to be around 1% to 5%.
The incidence rises sharply in patients with diabetic ketoacidosis, sepsis, or history of alcoholism.
Pathogenesis
A.Homeostasis of phosphate is maintained through gastrointestinal (GI) absorption and renal excretion.
B.Imbalances in either of these mechanisms can result in hypophosphatemia.
Predisposing Factors
A.Eating disorders.
B.Alcoholism.
C.Tumors.
D.Vitamin D deficiency.
E.Refeeding syndrome.
F.Malabsorption.
Subjective Data
A.Common complaints/symptoms.
1.Depend on magnitude of hypophosphatemia: Moderate and severe before symptomatic.
2.Muscle weakness.
a.Diaphragmatic weakness and difficulty with ventilation or weaning mechanical ventilation.
b.Impaired myocardial contractility.
3.Neurological dysfunction.
a.Irritability ranging to seizures or coma.
b.Paresthesias.
4.Hematologic dysfunction, including hemolysis and platelet dysfunction.
Physical Examination
A.Respiratory examination.
B.Musculoskeletal examination.
C.Neurological examination.
D.Gastrointestinal examination.
Diagnostic Tests
A.Serum phosphorus.
B.Total calcium and ionized calcium.
Differential Diagnosis
A.Decreased intestinal absorption.
1.Malabsorption and chronic diarrhea.
2.Antacid abuse, excessive calcium supplement use, or overdose of phosphate binders.
3.Vitamin D deficiency.
4.Alcoholism.
5.Malnutrition, starvation, or anorexia.
B.Increased urinary losses.
1.Primary hyperparathyroidism.
2.Fanconi syndrome.
3.Osmotic diuresis.
a.Post-obstructive or resolving acute tubular necrosis (ATN) diuresis.
b.Glucosuria.
4.Acetazolamide.
5.Rickets (X-linked or vitamin D dependent).
6.Oncogenic osteomalacia.
7.Enuresis/polyuria.
a.Postoperative, especially immediately after kidney transplant.
b.Extracellular volume expansion.
C.Redistribution (shift to intracellular space).
1.Respiratory alkalosis.
2.Diabetic ketoacidosis/treatment of hyperglycemia.
3.Refeeding syndrome (shift of phosphorus intracellularly in response to carbohydrate load).
a.Malnourished patients are at high risk with total parenteral nutrition.
4.Alcohol withdrawal.
5.Severe burns.
6.Leukemic blast crisis.
Evaluation and Management Plan
A.Step 1—If acute, severe, or symptomatic, replace phosphate intravenously.
1.Give intravenous repletion for symptomatic patients with moderate or severe hypophosphatemia or those who cannot tolerate or receive oral supplementation.
a.Potassium phosphate or sodium phosphate may be given. Sodium phosphate is recommended unless the patient also has hypokalemia.
b.The initial dose is 0.16 to 0.25 mmol/kg, not to exceed 0.5 mmol/kg.
i.Give dose over a minimum of 4 to 6 hours.
ii.Reduce dose by 50% in patients with renal dysfunction.
B.Step 2—Assess for ongoing losses or further phosphorus replacement needs.
1.Give intravenous replacement to patients who cannot take or absorb oral phosphorus or patients with ongoing symptoms.
2.Use oral supplements in patients with asymptomatic mild hypophosphatemia.
a.May cause or worsen diarrhea.
b.Gastrointestinal absorption is variable.
c.Oral formulations contain various amounts of phosphorus, sodium, and potassium.
d.Common regimen is 250 mg of elemental phosphorus four times daily.
3.If oral supplements fail to maintain phosphorous levels, supplement with intravenous phosphorus.
C.Step 3—Identify and treat cause.
D.Step 4—Treat other electrolyte and acid–base disorders.
Follow-Up
A.Follow-up should occur with the provider who treats the underlying condition causing the electrolyte imbalance.
Consultation/Referral
A.Depends on underlying condition causing hypophosphatemia.
B.Endocrinology should be consulted if the diagnosis is related to hyperparathyroidism.
C.Gastroenterology should be consulted if there is a malabsorption condition.
D.Nephrology should be consulted in renal phosphate wasting.
E.Psychiatry should be consulted for eating disorders.
Special/Geriatric Considerations
A.Elderly patients can develop osteomalacia and be prone to bone pain and fractures.
Bibliography
Kraft, M. D., Btaiche, I. F., Sacks, G. S., & Kudsk, K. A. (2005, August). Treatment of electrolyte disorders in adult patients in the intensive care unit. American Journal of Health System Pharmacy, 62(16), 1663–1682. doi:10.2146/ajhp040300
Moe, S. M., & Daoud, J. R. (2014). Disorders of mineral metabolism: calcium, phosphorous, and magnesium. In D. S. Gipson, M. A. Perazella, & M. Tonelli (Eds.), National Kidney Foundation’s primer on kidney diseases (6th ed., pp. 100–112). Philadelphia, PA: Elsevier Saunders.
Yu, A. S. L., & Stubbs, J. R. (2019, February 12). Evaluation and treatment of hypophosphatemia. In A. Q. Lam (Ed.), UpToDate. Retrieved from https://www.uptodate.com/contents/evaluation-and-treatment-of-hypophosphatemia