Serum Inorganic Phosphorus

Excerpt

Phosphorus is an abundant element that is widespread in its distribution. It is a major intracellular anion in mammals. Total body phosphorus in a 70-kg man is about 700 to 800 mg, 85% of which is in the skeleton in hydroxyapatite phase; the remaining 15% is in soft tissues. Almost all of the phosphorus found in the extracellular fluid space is in the form of inorganic phosphate. Serum inorganic phosphate reflects only a very minor percentage of total body phosphorus; however, it is easily measurable and gives a clue to the status of body phosphorus stores. The majority of the phosphate in the body is in the organic form as a complex with carbohydrates, lipids, and proteins. Phosphorus is an essential element in the cellular structure, cytoplasm, and mitochondrium. It is necessary for several enzymatic processes in glycolysis, ammoniagenesis as well as in oxidative phosphorylation, resulting in energy from the formation of adenosine triphosphate from adenosine diphosphate. In addition, it influences oxygen-carrying capacity of hemoglobin by its role in the regulation of 2, 3-diphosphoglycerate (2,3-DPG) synthesis.

The normal serum phosphorus concentration is 3.4 to 4.5 mg/dl (1.12 to 1.45 mmol/L). This fluctuates with age (it is higher in children than adults), dietary intake, and acid–base status. There is a diurnal variation, which reaches its nadir between 8 and 11 a.m.

The concentration of serum phosphate is generally expressed in milligrams per deciliter because concentration in millimoles can vary with acid–base status. In serum, phosphate exists in two forms, dihydrogen phosphate (H₂PO₄) and its salt, mono-hydrogen phosphate (HPO₄). The relationship between these two can be determined by the Henderson-Hasselbalch equation. At the physiologic pH of 7.40, the pK of H₂PO₄ is 6.8 and the ratio of HPO₄ to H₂PO₄ is 4:1. The valence of phosphorus at a pH of 7.40 is 1.8. Serum levels expressed in milligrams can be converted to millimoles per liter by multiplying by 0.323. To convert to milliequivalents, multiply the concentration in millimoles by the valence (1.8 at pH of 7.40).

About 85 to 90% of serum phosphate is free and is ultrafiltrable; 10 to 15% is bound with protein.