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  1. This test is done in the serum of the patient.
  2. The heparinized plasma can be used.
  3. Separate the serum from blood as soon as possible, maximum within one hour.
  4. Fasting serum is preferred.
  5. The separated serum is stable at 4 °C for several days.
  6. Frozen sample is stable for several months.

Precaution

  1. Avoid venous stasis.
  2. Hemolysis, icteric serum, and fluoride interfere with the chemical reaction.
  3. Be Careful about the phosphorus contamination of glassware.
  4. There is diurnal variation with an increased level in the PM sample. So fasting (AM) sample is preferred.
  5. Exercise leads to increase the level.
  6. Avoid anticoagulants like oxalate, citrate, and EDTA.
  7. 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)

  1. This will give an idea of renal and bone diseases.
  2. This test is done to investigate calcium abnormality.
  3. This test is done to evaluate parathyroid abnormality.

Pathophysiology

  1. Most of the phosphorus in the body is in phosphate from, so these are used interchangeably. So it exists in the body:
    1. Inorganic phosphate.
    2. Organic phosphate esters.
    3. Most of the phosphorus is in organic form and the very small amount is in inorganic form (2.5 to 4.5 mg/dL).
  2. So we measure inorganic phosphate when there is a request for phosphorus or phosphate or inorganic phosphate.
  3. The organic phosphate esters which are not measured are part of  or present in:
    1. Synthesis of phospholipids in the cell membranes (present within cells).
    2. Associated with nucleoproteins.
    3. Hexoses (glucose-6-phosphate).
    4. Deoxygenated hemoglobin in the RBCs.
    5. ATP (adenosine triphosphate) as an energy source in metabolism.
    6. The energy source for enzymes like 2,3 diphosphoglycerate.
  4. In our body, 85 % of the phosphorus is combined with Calcium in the bone.
    1. Rest 15 % is in the cells.
  5. 10% phosphate in serum is protein bound.
    1. 35% in serum is complexed with sodium, magnesium, calcium.
    2. Inorganic phosphate ions (H2PO4¯, HPO4¯ ¯  ) are mostly confined to the extracellular fluid. Their main role is a buffer system.
    3. 80% of inorganic phosphate at pH 7.4 is in the form of  HPO4¯ ¯.
    4. Rest is free is in the serum.
  6. Distribution of the phosphate and Calcium in the body shown in the following table. This is relative distribution.
  Phosphate Calcium
Bone  85% 99%
Extracellular fluid            <0.1 % <0.2%
Soft tissue  15% 1%
Total weight in grams 600 1000

 

  1. Most of the phosphorus in the blood exists as phosphate.
  2. Phosphate in blood exists in two forms:
    1. Monovalent Phosphate (H2 PO4)¯.
    2. Divalent Phosphate (HPO4)2¯.

  1. Phosphate is required for:
    1. Formation of the bone:
    2. In the metabolism of glucose and lipids.
    3. In the maintenance of acid-base balance.
    4. It is needed for the storage and transfer of the energy from one site to another site.

  1. Phosphorus enters the RBC with glucose so its level is lowered after the ingestion of carbohydrate.
  2. The dietary absorption of phosphate is very efficient, rarely there is a deficiency of phosphate.
  3. Malabsorption and antacids can decrease the absorption in the GI tract.
  4. The renal excretion maintains the balance of dietary intake of phosphorus.
  5. Phosphate level varies during the day :
    1. Low values around 10 AM.
    2. High values after 12 hours later.
  6. Phosphorus level is dependant upon :
    1. Calcium metabolism
    2. Parathyroid hormone PTH.
    3. Renal excretion.
    4. Intestinal absorption.
    5. PTH tends to decrease the phosphate reabsorption in the kidney.
    6. PTH and Vit.D stimulate the absorption of phosphate from the intestinal.

  1. When calcium levels are decreased, then phosphorus level increases.
  2. When calcium level is increased then the phosphorus level is decreased.
  3. This inverse ratio is maintained by the kidney by increasing the excretion. The principal route of excretion is urine.

NORMAL

Source 1

Age mg/dL
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
>60 year   
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

Source 2

Increased Phosphorus or hyperphosphatemia:

The level is more than 4.7 mg/dL

  1. Renal diseases with increased blood urea ( BUN) and creatinine.
  2. Hypoparathyroidism with raised phosphate and decreased calcium. But renal function will be normal.
  3. Hypocalcemia.
  4. Excessive intake of Vit.D.
  5. Milk-alkali syndrome.
  6. Bone tumors and metastases.
  7. Liver diseases and cirrhosis.
  8. Addison’s disease.
  9. Acromegaly.
  10. Increased dietary intake.
  11. Sarcoidosis.
  12. Acidosis.
  13. Hemolytic anemia.

Decreased level of phosphorus or hypophosphatemia:

The level is less than 2.4 mg/dL

  1. Decreased intestinal absorption.
  2. Increased renal excretion
    1. Hyperparathyroidism.
  3. Hyperinsulinemia.
  4. Rickets ( Vit.D deficiency ).
  5. Diabetic coma.
  6. Vomiting and severe diarrhea.
  7. Liver diseases.
  8. Acute alcoholism.
  9. Severe malnutrition and malabsorption.
  10. Hypercalcemia due to any cause.
  11. Gram-negative septicemia.
  12. Chronic intake of antacid.
  13. Alkalosis.
  14. Causes according to the mechanism of Hyperphosphatemia:
  15. Increased renal reabsorption:
  16. Excess vit.D
  17. Hypogonadism
  18. Hypoparathyroidism
  19. Pseudohypoparathyroidism
  20. Hyperthyroidism
  21. Growth hormone excess 
  22. Increased body fluid overload:
  23. Hyperalimentation
  24. High phosphorus laxative
  25. High phosphorus enema
  26. Blood transfusion
  27. Massive cell necrosis or destruction:
  28. Hypoxia
  29. Hyperthermia
  30. Crushing injuries
  31. Cytotoxic therapy

Possible References Used

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