Hypophosphatemia

  • Definition: Phosphate <2.5 mg/dl, signs and symptoms at <1 mg/dl
  • Etiology
    • Results in decreased ATP formation, decreased 2,3-DPG, resulting in decreased O2 release in hypoxemic tissues
    • Decreased Net intestinal Absorption
      • Phosphate Binders
      • Alcoholism/Malabsorption
      • Burns
      • Vitamin D deficiency
  • Impaired phosphate absorption in the gut and increased phosphate excretion - Antacids - Chronic Diarrhea
    • Transcellular shift of phosphate from the ECF to ICF
      • DKA
      • Respiratory alkalosis
      • Refeeding Syndrome
    • Increased Renal Excretion
      • Urinary Wasting (Fanconi Syndrome, Tumor-induced Osteomalacia)
      • Diuretics (loops, acetazolamide, thiazides)
      • Osmotic diuresis
      • Auto-diuresis
      • Post-ATN or post-obstructive polyuria
      • Hypothermia
      • CRRT
      • Meds
        • Aminoglycosides, IV iron, Tenofovir
        • Imatinib, VEGF inhibitors, temsirolimus
      • Primary Hyperparathyroidism
        • Hungry Bone Syndrome - Osteogenic Osteomalacia
    • Pseudo-hypophosphatemia
      • Hyperbilirubinemia
      • Acute leukemia
      • Mannitol
      • Paraproteinemia
  • Symptoms
    • Seizures, paresthesias, tremors
    • Confusion, Central pontine myelinolysis
    • Impaired heart contractility, heart failure
    • Rhabdomyolysis
    • Insulin resistance, Hemolysis
    • Respiratory muscle weakness
      • Difficulty to wean off ventilator
    • Neurologic symptoms: paresthesia, dysarthria, confusion, seizures, coma
  • Labs
    • Rarely, Hemolytic anemia, platelet dysfunction
  • Complications
    • Rhabdomyolysis
    • HF
    • Impaired diaphragmatic function
  • Diagnosis: Fraction or 24-hour Urine Phosphate - Urine excretion >100 mg or >5% fractional: Excessive renal loss
  • Hyperparathyroidism, VitD - Urine excretion <100 mg or <5% fractional: Everything else
    • Next: Serum Calcium
    • Then: Aminoaciduria, glycosuria
  • Treatment
    • Mild 2.0-2.5: Nothing
    • 1) Moderate to severe renal failure:
      • Caution, give 50% less
    • 2) Abnormal Calcium
      • Hypercalcemia: May risk calciphylaxis
      • Hypocalcemia: IV may worsen hypocalcemia
    • 3) Not Severe
      • Moderate: 1.0-2.4:
        • Asymptomatic: No treatment required
        • Symptomatic: Oral sodium phosphate/potassium phosphate/PHOS-NAK
          • Phosphate >1.5: 8 mM phosphate q6hr-q8hr x 24hs for total of 24-32mM
          • Phosphate <1.5: 16 mM phosphate q6hr x 24hs for total of 64mM
          • Follow lytes (Ca/Mg/Phos)
    • 4) Severe < 1.0, Symptoms, Malnutrition, lack enteral access:
      • IV Sodium phosphate or IV Potassium phosphate
        • Phosphate >1.5: 15mM over 4 hours
        • Phosphate <1.5: 30mM over 4 hours
      • Follow lytes (Ca/Mg/Phos) - Dipyridamole if low GFR ± Vitamin D