Phosphorus (PHOS), Serum or Plasma

CPT Code: 



Serum and Plasma 


5 mL 

Minimum Volume: 

2.2 mL 


Red-top tube, gel-barrier tube, or green-top (lithium heparin) tube



Patient Preparation:

Patient should be fasting.


Complete clot formation should take place before centrifugation.  Serum or plasma should be separated from red cell within 1 hour because erythrocytes contain phosphate concentrations several times greater than those found in the serum.

Reference Range:



Temperatures  Period 
Room Temperature  8 Hours 
Refrigerated (2-8 ⁰C)  2 Days 
Frozen (≤ -20 ⁰C)  Longer Storage 



Phosphorus is a mineral widely distributed throughout the body, the majority complexed with calcium in bones. In addition to bone formation, phosphorus contributes to carbohydrate metabolism and acid-base balance. This level can be affected by diet and tends to fluctuate rhythmically throughout the day. Since there are many causes for low or high values, the significance of an abnormal phosphorus must be considered in relationship to calcium levels.

Magnesium excretion controls magnesium balance. Magnesium urinary excretion is enhanced by increasing blood alcohol levels, diuretics, Bartter syndrome, corticosteroids, cis-platinum therapy and aldosterone. Renal magnesium wasting occurs in renal transplant recipients who are on cyclosporine and prednisone. Renal conservation of magnesium is diminished by hypercalciuria, salt-losing conditions, and the syndrome of inappropriate secretion of antidiuretic hormone. Magnesium deficiency is often inadequately documented by serum magnesium levels. Urinary magnesium analyses have been advocated before and after therapeutic magnesium administration to further investigate the significance of an apparent low serum magnesium.

Causes for Rejection: 

Hemolysis; gross lipemia; improper labeling 

Interfering Substances:

Hemolyzed samples may give spuriously elevated results.  Bias from hemolysis may result from inorganic phosphates produced by the action of phosphatases on organic phosphates, both of which are released from red blood cells upon hemolysis.

  • Hemoglobin (hemolysate) at 200 mg/dL (monomer) increases a PHOS result of 1.1 mg/dL by 27%.
  • Bilirubin at 20 mg/dL decreases a PHOS result of 1.0 mg/dL by 21%.
  • Lipemia (Intralipid) at 600 mg/dL decreases a PHOS result of 1.0 mg/dL by 16%.
  • Mannitol at concentrations of 500 mg/dL or greater causes a decrease of 10% or more.
  • Lipemia with triglyceride of 600 mg/dL and bilirubin of 20 mg/dL increases the PHOS result by 0.2 mg/dL and 0.8 mg/dL respectively, at phosphorus concentration of 6.9 mg/dL.
  • Creatine phosphate and phosphoenol pyruvate liberate phosphorus under the reaction conditions of this method and can thereby increase results.