HYPOPHOSPHATEMIA
1. Definition
Hypophosphatemia = serum phosphate < 2.5 mg/dL (0.81 mmol/L)
|
Severity |
Serum Phosphate |
|
Mild |
2.0–2.5 mg/dL (<0.65-0.81 mmol/L) |
|
Moderate |
1.0–1.9 mg/dL (<0.32-0.65 mmol/L) |
|
Severe (life-threatening) |
< 1.0 mg/dL (<0.32 mmol/L) |
Symptoms usually appear when <1 mg/dL
2. Functions of phosphate
- ATP synthesis → cellular energy
- 2,3-DPG in RBCs → oxygen delivery
- Cell membrane integrity (phospholipids)
- Buffering system
- Muscle contraction (including diaphragm)
3. Regulation of Phosphate Homeostasis
Intestinal absorption
- Vitamin D–dependent
- Increased by calcitriol
Renal handling (MOST IMPORTANT)
- Proximal tubule reabsorption (Na-Pi cotransporters)
- Normal excretion: <10% filtered load
Hormonal regulation
|
Hormone |
Effect |
|
PTH |
↓ renal phosphate reabsorption |
|
FGF-23 |
↓ phosphate reabsorption + ↓ vitamin D |
|
Vitamin D |
↑ intestinal absorption |
|
Insulin |
↑ intracellular shift |
4. Etiology – CLASSIFICATION
A. Decreased Intestinal Absorption
|
Cause |
Mechanism |
|
Malnutrition, starvation |
Low intake |
|
Chronic alcoholism |
Poor intake + renal loss |
|
Vitamin D deficiency |
↓ absorption |
|
Antacids (Al, Mg, Ca) |
Phosphate binding |
|
Chronic diarrhea |
GI loss |
|
Bariatric surgery |
Malabsorption |
B. Increased Renal Phosphate Loss (COMMON IN ICU)
1. Hormonal
- Hyperparathyroidism
- Tumor-induced osteomalacia (↑ FGF-23)
2. Tubular dysfunction
- Fanconi syndrome
- Proximal RTA(Type II RTA)
- Post-ATN diuretic phase
3. Drugs
- Diuretics
- Aminoglycosides
- Cisplatin
- Tenofovir
- Amphotericin B
- Acetazolamide
C. Intracellular Shift (MOST COMMON CAUSE IN ICU)
|
Cause |
Mechanism |
|
Refeeding syndrome |
Insulin-mediated cellular uptake |
|
DKA treatment |
Insulin + glucose |
|
Respiratory alkalosis |
↑ glycolysis → phosphate uptake |
|
Sepsis |
Cytokine-mediated shift |
|
Acute leukemia treatment |
Cellular uptake |
Total body phosphate may be depleted even if initial serum level normal
5. Clinical Manifestations
A. Neuromuscular (MOST COMMON)
- Proximal myopathy
- Respiratory muscle weakness → ventilator failure
- Rhabdomyolysis(rare)—it will mask Hypophosphatemia by increasing phosphate
- Paresthesia
- Tremors
Weaning failure from ventilator – think hypophosphatemia
B. CNS
- Irritability,Confusion,Delirium,Seizures,Coma
C. Cardiovascular
- Decreased myocardial contractility
- Heart failure,Arrhythmias,Hypotension
D. Hematologic
|
Effect |
Mechanism |
|
Hemolysis |
↓ RBC ATP |
|
Leukocyte dysfunction |
↓ phagocytosis |
|
Thrombocytopenia |
↓ platelet aggregation |
6. Diagnosis & Evaluation
When should phosphate be checked?
- Patients at risk of refeeding syndrome, particularly before and during initiation of nutrition.
- DKA or HHS, as phosphate may fall during treatment.
- Patients receiving CRRT, because phosphate losses are common.
- On ICU admission when clinically indicated, especially in undifferentiated or critically ill patients at risk of electrolyte abnormalities.
- When hypophosphatemia is clinically suspected, e.g., unexplained muscle weakness, respiratory failure, or difficulty weaning from mechanical ventilation.
Associated labs
|
Test |
Reason |
|
Calcium |
Reciprocal relationship |
|
Magnesium |
Commonly low |
|
Potassium |
Refeeding |
|
Vitamin D |
Malabsorption |
|
PTH |
Renal loss |
|
ABG |
Respiratory alkalosis |
Renal vs Non-renal Loss
Fractional Excretion of Phosphate (FEPO₄)
- <5% → redistribution / poor intake
- >5–10% → renal wasting
7. Management
When to Treat?
|
Serum phosphate |
Approach |
|
2.0–2.5 mg/dL |
Usually no specific replacement if asymptomatic; correct the underlying cause |
|
1.0–2.0 mg/dL |
Oral/enteral phosphate if asymptomatic and gut is functional; consider IV replacement in critically ill patients, especially if symptomatic or enteral therapy is not feasible |
|
<1.0 mg/dL |
Treat promptly; IV phosphate is generally indicated, particularly when symptomatic or critically ill |
A. ORAL PHOSPHATE (Mild–Moderate)
- Sodium or potassium phosphate
- Dose: 30–60 mmol/day in divided doses(B.D or T.D.S)
- Side effects:Diarrhea,Hyperphosphatemia (rare)
B. INTRAVENOUS PHOSPHATE (CRITICAL CARE)
Indications
- Serum phosphate <1 mg/dL
- Malabsorption
- Symptomatic
Choice of preparation
|
Situation |
Use |
|
Hypokalemia |
Potassium phosphate(Each 1 mmol of phosphate contains ~1.5 mEq of potassium;) |
|
Hyperkalemia / renal failure |
Sodium phosphate |
IV Dosing
- Phosphorus content: 93mg (3mM)/mL
- Potassium content: 170mg (4.4 mEq)/mL
|
Serum Phosphate |
Dose |
|
<1 mg/dL |
0.32–0.64 mmol/kg IV |
|
1–2 mg/dL |
0.16–0.32 mmol/kg IV |
- Maximum single dose: 45 mmol
- Infusion rate: ≤7.5 mmol/hour(Rapid infusion may cause transient hyperphosphatemia (which leads to hypocalcemia))
- Dilute in 250–500 mL NS/D5W
Another commonly cited regimen is:
- <1 mg/dL: 0.6 mmol/kg IV over 6 h
- 1.0–1.7 mg/dL: 0.4 mmol/kg IV over 6 h
- 1.7–2.2 mg/dL: 0.2 mmol/kg IV over 6 h
Monitoring
- Serum phosphate 6–12 hourly
- Calcium (risk of hypocalcemia)
- Magnesium
- ECG monitoring
Complications of Phosphate Therapy
|
Complication |
Mechanism |
|
Hypocalcemia |
Calcium-phosphate precipitation |
|
Hypotension |
Rapid infusion |
|
Acute kidney injury |
Nephrocalcinosis |
|
Metastatic calcification |
Excess dosing |