The test measures the amount of phosphorus or the phosphate in the blood or serum (from the clotted blood). Phosphorus determination is very significant to those who are malnourished or to those who are being treated for ketoacidosis, caused by high concentrations of ketone bodies. It is performed to check for the blood level of phosphorus, particularly if a person has a disorder known to cause abnormal phosphorus levels. Phosphorus testing may be ordered when symptoms suggest kidney and gastrointestinal disorders. The severity of conditions and diseases that influences the GIT, interfering with the absorption of phosphorus, calcium, and magnesium can be assess and diagnose through phosphorus testing. It can also help to diagnose disorders that affect the kidneys, interfering with mineral excretion and conservation, and phosphorus levels are cautiously monitored in people with kidney failure. If a person has a kidney disorder, kidney stones, or uncontrolled diabetes, the doctor may monitor his phosphorus levels to make sure that he’s not excreting or retaining excessive amounts.
Phosphorus is a mineral that is essential to bone metabolism and is found in all cells in the body. Most of the body's phosphorus is combined with calcium within the skeleton, but about 15% exists in the blood and other soft tissues and body fluids as phosphate (PO4) ions. Although abnormal phosphorus levels usually cause no symptoms, phosphorus testing often is performed as a follow-up to an abnormal calcium level and/or related symptoms, such as fatigue, muscle weakness, cramping, or bone problems. It is also present in proteins and enzymes necessary to energy transfer and life called ATP, Adenosine Triphosphate. Phosphorus is regulated with calcium by hormones including one from the parathyroid gland and vitamin D and is obtained through the digestion of food in the small intestine. Secretion of most of the Phosphorus is through the kidneys from the blood. A phosphorus test is often ordered to help detect diseases and conditions that cause problems with the body’s utilization of calcium. The test may serve in the diagnosis of problems with hormones, such as parathyroid hormone (PTH), and Vitamin D, which functions as a hormone, that regulate the body’s calcium level and, to a lesser degree, phosphorus levels.
Principle behind Phosphorus test: (Fiske Subbarrow Method)
Phosphate reacts with molybdate in strong acidic medium to form a complex. The absorbance of this complex in the near UV is directly proportional to the phosphate concentration.
Fiske and Subbarow Method
The method uses 1: 2: 4- aminonaphtholsulphonic acid, which has the great advantages of giving the full complement of blue colour of solution in 5 minutes or less, and of giving tremendous proportionality of colour over a wide range of concentration of phosphate ion. It has the added advantage of being far less affected by various substances, like ammonium and iron salts, nitrites and nitrates, chlorides and silicates, all of which interfere in varying ways with the colour production when quinol is used as the reducing agent.
Normal Range for Phosphorus
The normal range in the blood depends on the age and gender of the person. The normal range is between 2.3 and 4.5 mg/dL.
Decreased phosphorus in the blood could be a result of:
- Diabetic ketoacidosis
- Hypercalcemia
- Hyperinsulinism
- Hyperparathyroidism
- Inadequate dietary intake of PO4 or Vitamin D resulting in rickets (childhood) or osteomalacia (adult)
High levels of phosphorus in the blood could be a result of:
- Bone metastasis
- Hypocalcemia
- Hypoparathyroidism
- Increased dietary or IV intake of PO4
- Liver disease
- Renal failure
- Sarcoidosis
References:
http://www.labtestsonline.org/understanding/analytes/phosphorus/test.html
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