Phosphorus (P), Inorganic Phosphate (PO4), Inorganic Phosphorus
- This test is done in the serum of the patient.
- The heparinized plasma can be used.
- Separate the serum from blood as soon as possible, maximum within one hour.
- Fasting serum is preferred.
- The separated serum is stable at 4 °C for several days.
- The frozen sample is stable for several months.
- Avoid venous stasis.
- Hemolysis, icteric serum, and fluoride interfere with the chemical reaction.
- Be Careful about the phosphorus contamination of glassware.
- There is a diurnal variation with an increased level in the PM sample. So fasting (AM) sample is preferred.
- Exercise leads to an increase in level.
- Avoid anticoagulants like oxalate, citrate, and EDTA.
- The phosphate level in serum increases if the sample is left at 37 °C at room temperature for a long time.
Purpose of the test (Indication)
- This will give an idea of renal and bone diseases.
- This test is done to investigate calcium abnormality.
- This test is done to evaluate parathyroid abnormality.
- Most of the phosphorus in the body is in phosphate, so these are used interchangeably. So it exists in the body:
- Inorganic phosphate.
- Organic phosphate esters.
- Most of the phosphorus is in organic form and the very small amount is in inorganic form (2.5 to 4.5 mg/dL).
- So we measure inorganic phosphate when there is a request for phosphorus or phosphate or inorganic phosphate.
- The organic phosphate esters which are not measured are part of or present in:
- Synthesis of phospholipids in the cell membranes (present within cells).
- Associated with nucleoproteins.
- Hexoses (glucose-6-phosphate).
- Deoxygenated hemoglobin in the RBCs.
- ATP (adenosine triphosphate) as an energy source in metabolism.
- The energy source for enzymes like 2,3 diphosphoglycerate.
- In our body, 85 % of the phosphorus is combined with Calcium in the bone.
- Rest 15 % is in the cells.
- 10% of phosphate in serum is protein bound.
- 35% in serum is complexed with sodium, magnesium, calcium.
- Inorganic phosphate ions (H2PO4¯, HPO4¯ ¯ ) are mostly confined to the extracellular fluid. Their main role is a buffer system.
- 80% of inorganic phosphate at pH 7.4 is in the form of HPO4¯ ¯.
- The rest is free is in the serum.
- Distribution of the phosphate and Calcium in the body shown in the following table. This is relative distribution.
|Extracellular fluid||<0.1 %||<0.2%|
|Total weight in grams||600||1000|
- Most of the phosphorus in the blood exists as phosphate.
- Phosphate in blood exists in two forms:
- Monovalent Phosphate (H2 PO4)¯.
- Divalent Phosphate (HPO4)2¯.
- Phosphate is required for:
- Formation of the bone:
- In the metabolism of glucose and lipids.
- In the maintenance of acid-base balance.
- It is needed for the storage and transfer of the energy from one site to another site.
- Phosphorus enters the RBC with glucose so its level is lowered after the ingestion of carbohydrates.
- The dietary absorption of phosphate is very efficient, rarely there is a deficiency of phosphate.
- Malabsorption and antacids can decrease the absorption in the GI tract.
- The renal excretion maintains the balance of dietary intake of phosphorus.
- Phosphate level varies during the day :
- Low values around 10 AM.
- High values after 12 hours later.
- Phosphorus level is dependant upon :
- Calcium metabolism
- Parathyroid hormone PTH.
- Renal excretion.
- Intestinal absorption.
- PTH tends to decrease the phosphate reabsorption in the kidney.
- PTH and Vit.D stimulate the absorption of phosphate from the intestinal.
- When calcium levels are decreased, then the phosphorus level increases.
- When the calcium level is increased then the phosphorus level is decreased.
- This inverse ratio is maintained by the kidney by increasing the excretion. The principal route of excretion is urine.
|Cord blood||3.7 to 8.1|
|Premature one week||5.4 to 10.9|
|0 to 10 days||4.5 to 9.0|
|2 to 12 year||4.5 to 5.5|
|12 to 60 year||2.7 to 4.5|
|Male||2.3 to 3.7|
|Female||2.8 to 4.1|
|Urine 24 hours|
|Constant daily diet||<1.0 g/day|
|Nonrestricted diet||0.4 to 1.31|
- The constant daily diet contains 0.9 to 1.5 g of Phosphorus and 10 mg calcium/ kg.
- To convert into SI unit x 0.323 = mmol/L
- Adult = 3 to 4.5 mg/dL (0.81 to 1.45 mmol/L).
- Child = 4.4 to 6.5 mg/dL (1.29 to 2.26 mmol/L).
- Newborn = 4.3 to 9.3 mg/dL (1.43 to 3 mmol/L)
- Urine (on non-restricted diet) = 0.4 to 1.3 g/day (12.9 to 42.0 mmol/day).
- These values may be varying from different sources.
Increased Phosphorus or hyperphosphatemia:
The level is more than 4.7 mg/dL
- Renal diseases with increased blood urea ( BUN) and creatinine.
- Hypoparathyroidism with raised phosphate and decreased calcium. But the renal function will be normal.
- Excessive intake of Vit.D.
- Milk-alkali syndrome.
- Bone tumors and metastases.
- Liver diseases and cirrhosis.
- Addison’s disease.
- Increased dietary intake.
- Hemolytic anemia.
Decreased level of phosphorus or hypophosphatemia:
The level is less than 2.4 mg/dL
- Decreased intestinal absorption.
- Increased renal excretion
- Rickets ( Vit.D deficiency ).
- Diabetic coma.
- Vomiting and severe diarrhea.
- Liver diseases.
- Acute alcoholism.
- Severe malnutrition and malabsorption.
- Hypercalcemia due to any cause.
- Gram-negative septicemia.
- Chronic intake of antacid.
- Causes according to the mechanism of Hyperphosphatemia:
- Increased renal reabsorption:
- Excess vit.D
- Growth hormone excess
- Increased body fluid overload:
- High phosphorus laxative
- High phosphorus enema
- Blood transfusion
- Massive cell necrosis or destruction:
- Crushing injuries
- Cytotoxic therapy
- Dangerous value is < 1.0 mg/dL