Phosphate: Introduction, Clinical significance and measurement

An adult contains 600 g of phosphorus in inorganic and organic phosphates, of which about 85% is in skeleton and rest in soft tissue and extracellular fluid. Though plasma has both organic and inorganic phosphate but inorganic phosphate (H₂PO₄^- and HPO₄^2-) is measured. Approximately 10% is protein bound and 35% complexed with sodium, calcium, magnesium; and remainder 55% is free. The organic phosphates are located within the cell of blood.

Inorganic phosphate is a major component of hydroxyapatite in bone and is the source of intracellular and extracellular pool. Organic phosphate in cells is found to be incorporated into nucleic acid, phospholipids, phosphoproteins and ATP, GTP, Creatine phosphate, etc. Phosphate is important for activity of adenylate cyclase, 25-hydroxy vitamin D-1α-hydroxylase and those involved in 2, 3-diphosphoglycerate.

CLINICAL SIGNIFICANCE

Hypophosphatemia

This may be caused by (1) shift of phosphate from extracellular to intracellular space due to increase in glucose, insulin, respiratory alkalosis, in this condition there is carbohydrate induced stimulation of insulin secretion, which increases the transport of glucose and phosphate into insulin-sensitive cells, where it is incorporated into sugar phosphates and ATP. Respiratory alkalosis cause increase in pH that activates phosphofructokinase and accelerates glycolysis causing shift of phosphate into the cell. (2) renal phosphate wasting due to hyperparathyroidism, renal tubular defects, fanconi syndrome, (3) decreased intestinal absorption as in vomiting, diarrhea, malabsorption syndrome, vitamin D deficiency. (4) loss from intracellular phosphate as in ketoacidosis, lactic acidosis.

Hypophosphatemia impair cellular function due to inadequate glycolysis, ATP formation, there is muscle weakness, decreased cardiac output, in RBC there will be decreased 2, 3-DPG which causes tissue hypoxia and severe hypophosphataemia cause hemolysis.

Fig. Concept Map : Hypophsphatemia

Hyperphosphataemia

Hyperphosphataemia is usually secondary to the inability of the kidney to excrete phosphate as occurs in renal failure. Acidosis can also lead to hyperphosphatemia due to hydrolysis of intracellular organic phosphate containing compounds.

MEASUREMENT

Measurements of inorganic phosphate are based on the reaction of phosphate ions with ammonium molybdate to form a phosphomolybdate complex that is measured spectrophotometrically. The colourless phosphomolybdate complex is measured directly by UV absorption (340 nm), or reduced to molybdenum blue and measured at 600 to 700 nm. An acidic pH is maintained by addition of acid. Use of sulphuric acid reduces and produces blue phosphomolybdate complex. Many enzymatic methods are also developed.

Serum or heparinized plasma is preferred specimens for phosphate measurement. Hemolyzed specimens are unacceptable as RBC contains high organic phosphate esters which are hydrolyzed to inorganic phosphate during prolonged storage. Long storage can increase phosphate concentration. Hemolysis, icterus and lipemia can interfere.

REFERENCE INTERVAL

Serum phosphate in adults = 2.5-4.5 mg/dL (0.81-1.45 mmol/L)