Unit Converter
Phosphorus (P)
Synonyms
- Phosphorus
- Inorganic phosphate (Pi)
- Serum phosphate
- Phosphate, inorganic
- PO₄³⁻
- Phosphate ion
Note: In clinical labs, “phosphorus” generally refers to inorganic phosphate.
Units of Measurement
- mmol/L
- µmol/L
- mg/dL
- mg/100 mL
- mg%
- mg/L
- µg/mL
Molecular Weight
(For elemental phosphorus): 30.97 g/mol
(For phosphate PO₄³⁻): 94.97 g/mol — but labs report as phosphorus, not phosphate.
Clinical measurement uses elemental phosphorus units.
Key Unit Conversions
(Based on elemental phosphorus, the standard for serum reporting)
mg/dL ↔ mmol/L
1 mg/dL=0.323 mmol/L1\ \text{mg/dL} = 0.323\ \text{mmol/L}1 mg/dL=0.323 mmol/L 1 mmol/L=3.10 mg/dL1\ \text{mmol/L} = 3.10\ \text{mg/dL}1 mmol/L=3.10 mg/dL
µg/mL ↔ mg/L
1 µg/mL=1 mg/L=0.1 mg/dL1\ \text{µg/mL} = 1\ \text{mg/L} = 0.1\ \text{mg/dL}1 µg/mL=1 mg/L=0.1 mg/dL
µmol/L
1 mmol/L=1000 µmol/L1\ \text{mmol/L} = 1000\ \text{µmol/L}1 mmol/L=1000 µmol/L
mg%
\text{mg%} = \text{mg/dL}
Description
Phosphorus is a major intracellular anion, critical for:
- ATP / energy metabolism
- Bone and teeth mineralization (as hydroxyapatite)
- Cell membranes (phospholipids)
- Acid–base buffering
- Nucleic acids (DNA/RNA)
About:
- 85% in bone
- 14% in soft tissue
- 1% in extracellular fluid
Regulated tightly by:
- PTH
- Vitamin D (1,25-OH₂D₃)
- FGF-23
- Kidney (major control)
Phosphate imbalance is common in:
- CKD
- Endocrine disorders
- Critical illness
- Diabetic ketoacidosis
- Malnutrition
Physiological Role
- Forms hydroxyapatite:
Ca10(PO4)6(OH)2\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2Ca10(PO4)6(OH)2 - ATP, ADP, AMP → energy metabolism
- 2,3-BPG affects oxygen delivery
- pH regulation via buffering
- Phosphorylation reactions
Clinical Significance
HIGH Phosphorus (Hyperphosphatemia)
Major Causes
1. Chronic Kidney Disease (CKD)
Most common cause → reduced renal excretion.
2. Hypoparathyroidism
PTH normally increases phosphate excretion → low PTH = high phosphorus.
3. Tumor Lysis Syndrome
Massive cell breakdown → phosphorus release.
4. Rhabdomyolysis
5. Metabolic Acidosis
6. Vitamin D Excess
Increases gut phosphorus absorption.
7. FGF-23 deficiency or resistance
Rare genetic causes.
Symptoms
Often asymptomatic; severe cases →
- Hypocalcemia symptoms: tetany, cramps
- Soft tissue calcification
- Vascular calcification in CKD
LOW Phosphorus (Hypophosphatemia)
Major Causes
1. Refeeding Syndrome
Shift of phosphate into cells → life-threatening.
2. Alcoholism / Malnutrition
3. Hyperparathyroidism
PTH increases phosphate loss.
4. Vitamin D Deficiency
↓ absorption.
5. DKA Treatment
Insulin drives phosphate into cells.
6. Respiratory Alkalosis
Shifts phosphate intracellularly.
7. Renal Phosphate-Wasting
- Fanconi syndrome
- X-linked hypophosphatemia (XLH)
- Tumor-induced osteomalacia (FGF-23 ↑)
Symptoms
- Weakness, myopathy
- Hemolysis
- Rhabdomyolysis
- Respiratory failure (diaphragm weakness)
- Confusion, seizures (severe)
Reference Intervals
Adults
- 0.80 – 1.45 mmol/L
(= 2.5 – 4.5 mg/dL)
Children (higher due to growth)
- 1.0 – 2.0 mmol/L
(= 3.0 – 6.0 mg/dL)
Elderly
Lower end of adult range.
Critical Levels
- < 0.3 mmol/L (<1 mg/dL) → severe symptomatic hypophosphatemia
- > 2.5 mmol/L (>7.5 mg/dL) → risk of hypocalcemia & calcification
Diagnostic Uses
1. Evaluation of CKD–Mineral Bone Disorder (CKD-MBD)
Hyperphosphatemia is key in CKD stages 3–5.
2. Parathyroid Disorders
Interpret with calcium, PTH, vitamin D.
3. Bone Metabolism Disorders
Important in osteoporosis, rickets, osteomalacia.
4. Diabetic Ketoacidosis Management
Monitor phosphate shifts during therapy.
5. Critical Care
Sepsis, respiratory alkalosis, shock → low phosphate.
6. Tumor Lysis Syndrome
Massive elevation requires urgent management.
7. Refeeding Syndrome Risk
Key indicator before starting nutrition.
Analytical Notes
- Avoid hemolysis (intracellular phosphorus → false elevation)
- Fasting not necessary
- Serum preferred; heparinized plasma acceptable
- Delayed processing elevates levels
- IV fluids may dilute serum phosphate
Clinical Pearls
- In CKD, high phosphorus increases mortality via vascular calcification.
- Hypophosphatemia causes respiratory failure (weak diaphragm).
- Always interpret phosphorus with calcium + PTH + vitamin D.
- Refeeding syndrome → check phosphorus every 6–12 hours initially.
- FGF-23–mediated disorders cause isolated low phosphate with normal calcium.
Interesti
ng Fact
The term “phosphate” in labs actually represents elemental phosphorus, even though the measured compound is phosphate - a historical convention that remains unchanged.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Electrolytes & Minerals
- KDIGO CKD–MBD 2017 Guidelines
- Endocrine Society - Calcium & Phosphate Disorders
- Mayo Clinic Laboratories - Phosphate
- ARUP Consult - Electrolyte Disorders
- MedlinePlus / NIH - Phosphorus Test