What is the role of a 24-hour urinary calcium test in the workup of hypercalciuria?

Updated: Apr 23, 2019
  • Author: Stephen W Leslie, MD, FACS; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

The obvious initial laboratory evaluation for hypercalciuria is the 24-hour urinary calcium determination, which is generally recommended when patients are feeling well and on their usual diet. A 24-hour urine test is of little value when patients are hospitalized with acute stone attacks or other medical problems, since their diet and activity levels are different from the home conditions under which they formed the stones. The 24-hour urine sample should be collected in a standardized fashion.

In addition to calcium, other 24-hour urine chemistries that are usually performed in stone formers include the following (if possible, these chemistries should be performed together):

  • Oxalate
  • pH
  • Volume
  • Creatinine
  • Specific gravity
  • Phosphorus or phosphate
  • Citrate
  • Sodium
  • Uric acid
  • Magnesium
  • Urea nitrogen or sulfate - These are increased in cases of high protein ingestion

Ensure that the laboratory performing the studies has a reliable methodology for urinary chemistry testing. In the United States, this most often requires sending most 24-hour urine tests to an outside reference laboratory. Because usually only a small portion of the total sample is actually sent, some potential errors are introduced if the urine sample is not handled properly or if the total volume is not measured and recorded accurately.

Instructions for proper 24-hour urine collection procedures must be reviewed carefully with every patient. (The most intelligent patients are often the ones who rush through the instructions and misunderstand, delivering grossly inaccurate specimens.)

One easy way to determine the accuracy of urine collection is to compare the total urinary creatinine collected with the expected levels. A properly performed 24-hour urine collection should show a mean urinary creatinine of 22.1 mg/kg in men and 17.2 mg/kg in women. Any values that are significantly different from the predicted ones probably represent improper or inaccurate collections.


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