14. Inorganic phosphate - common data for the parameter, indications for investigation, principles of the analytical methods, reference ranges, interpretation of results.

Inorganic phosphate - common data for the parameter, indications for investigation, principles of the analytical methods, reference ranges, result interpretation.

The total body phosphorus content in normal adults is around 700-800 g. Approximately 80-85% is present in the skeleton; the remaining 15% is in the ECF in the form of inorganic phosphate and intracellularly in the soft tissues as organic phosphates such as phospholipids, nucleic acids, and ATP. The skeleton contains primarily inorganic phosphate, predominantly as hydroxyapatite and calcium phosphate. Inorganic phosphate in serum exists as both divalent (HPO4 2-) and monovalent (H2PO4 - ) phosphate anions, both of which represent important buffers.

Function: In addition to its role in the skeleton, an important constituent of nucleic acids in that both RNA and DNA represent complex phosphodiesterases; in phospholipids and phosphoproteins; - high-energy compounds (ATP) and cofactors (NADP) and is involved in intermediary metabolism and various enzyme systems (adenylate cyclase); - essential for normal muscle contractility, neurologic function, electrolyte transport, and oxygen carrying by hemoglobin (2,3-diphosphoglycerate – 2,3-DPG)

Reference ranges: Serum phosphate is best measured in fasting morning specimens and should be 0.89-1.44 mmol/L

Principles of the analytical methods:

1. Most commonly used methods for determination of inorganic phosphate are based on the reaction of phosphate with ammonium molybdate to form phosphomolybdate complex. The formation of phosphomolybdate complex is pH dependent, and the rate of its formation is influenced by protein concentration.
2. Direct UV measurement of the colourless unreduced complex by absorption at 340 nm, has been adapted for use on most of the automated analysers.
3. An enzymatic method - phosphorus undergoes successive enzymatic reactions. The NADPH produced can be quantitated fluorometrically or spectrophotometrically

Indications for investigation and Result interpretation:

Hyperphosphatemia is caused by increased absorption, decreased loss or increased production. Increased intake: occur as a result of excessive intravenous administration or oral supplementation or Vitamin-D intoxication. There is reduced excretion in renal failure, hypoparathyroidism and magnesium deficiency. Reduced Loss: Renal failure; Hypoparathyroidism; Acromegaly; Tumoral calcinosis; Vitamin D intoxication;

Hypophosphatemia is caused by inadequate intake, excessive loss or redistribution within the body. 1) Decreased Absorption: Malnutrition; Phosphate binding antacids (Agents that bind with phosphate may reduce its absorption. Malabsorption syndromes – short bowel syndrome, tropical sprue, Crohn’s disease, celiac disease, radiation enteritis etc.; Vitamin E deficiency. 2) Increased Loss: Diuretics; Steroids; Alcoholism; 3) Redistribution: Shifts from serum into cells – carbohydrate infusions, hyperglycaemia;