Hyperphosphatemia Workup

Updated: Dec 15, 2017
  • Author: Eleanor Lederer, MD, FASN; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Workup

Approach Considerations

The following measurements are indicated in patients with hyperphosphatemia:

  • Serum phosphate level [40] : Reference range in adults, 2.5-4.5 mg/dL; reference range in children, 3-6 mg/dL; hemolysis or hyperlipidemia of the serum sample may lead to falsely elevated phosphorus levels
  • Serum calcium level
  • Electrolytes
  • Blood urea nitrogen (BUN) and creatinine levels
  • Serum magnesium level (may be low)

No specific procedures are indicated to evaluate hyperphosphatemia. Bone biopsy findings, however, may help in differentiating parathyroid bone disease and osteomalacia in patients with chronic or end-stage renal disease.

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Full Chemistry Profile

Measures of serum calcium, magnesium, BUN, and creatinine are of critical importance. The levels of calcium and magnesium, for example, yield information on the status of all divalent ion metabolism.

Low serum calcium levels along with high phosphate levels are observed with renal failure, hypoparathyroidism, and pseudohypoparathyroidism. BUN and creatinine values help to determine whether renal failure is the cause of hyperphosphatemia. Patients with renal failure are also more likely to have elevated intact PTH levels. On the other hand, patients with hypoparathyroidism, either primary or acquired, will have relatively low levels of intact PTH and normal renal function.

High serum calcium and high phosphate levels are observed with vitamin D intoxication and milk-alkali syndrome. Patients with vitamin D intoxication should show relatively low levels of intact PTH and high 25 and 1,25 vitamin D. Patients with milk-alkali syndrome should show low levels of both PTH and vitamin D.

If renal function is normal, then more a unusual disorder, such as one of the following, may be the cause:

  • Vitamin D intoxication
  • Laxative (Phospho-soda) abuse
  • Tumor lysis
  • Rhabdomyolysis
  • Isolated hypoparathyroidism

Urinalysis

Urine studies are rarely indicated, but if renal function is normal and PTH levels are high or normal, then a 24-hour urine measurement of cyclic adenosine monophosphate (cAMP) levels can be obtained. Patients with pseudohypoparathyroidism have abnormally low cAMP levels.

Note, however, that most cases of pseudohypoparathyroidism are diagnosed based on clinical grounds, ie, characteristic physical features of Albright hereditary osteodystrophy (eg, short phalanges, short stature, obesity, round face, mental retardation) accompanied by low calcium levels, high phosphate levels, and positive findings from the family history.

In a patient with hyperphosphatemia, the fractional renal excretion of phosphate should be well in excess of 15%. If not, this suggests that renal excretion is impaired either because of renal failure or hypoparathyroidism. If the fractional renal excretion exceeds 15%, this suggests either massive ingestion (eg, laxative [Phospho-soda] abuse) or lysis of tissue with release of intracellular phosphate.

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Imaging Studies

Imaging studies are not generally indicated in the evaluation of hyperphosphatemia. If, however, renal failure is discovered, then renal imaging studies (eg, ultrasonography) are indicated.

If significant secondary hyperparathyroidism due to renal failure is found, then long-bone studies may help to assess for the presence of hyperparathyroid bone disease. Likewise, bone densitometry may be desirable for individuals in whom significant bone loss is suggested. Bone biopsy findings may be helpful to differentiate parathyroid bone disease and osteomalacia.

Evaluation of vascular calcification in coronary arteries and peripheral vasculature is being used increasingly, although it is still not in widespread use. Electron beam computed tomography (CT) scanning is the most commonly used modality for imaging and quantitation of coronary artery calcification. The presence of coronary artery and valvular calcification in patients with renal failure and in those on dialysis indicates a poor outcome in some studies. Some investigators suggest that these patients should take sevelamer and not calcium-containing phosphate binders for control of serum phosphorus.

Renal ultrasonography, bone studies, and coronary calcification studies yield data on the chronicity of the process and the patient's prognosis. Shrinkage of the kidneys due to renal failure; changes associated with hyperparathyroidism, based on bone survey results; and coronary calcification are highly suggestive of chronic processes.

Radiography is not necessary for the workup of hyperphosphatemia, but it may reveal evidence of metastatic calcifications (eg, bilateral, symmetrical calcifications of the basal ganglia; periarticular calcifications around large joints; soft tissue calcifications at pressure point areas).

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