How is hypophosphatemic rickets diagnosed?

Updated: May 18, 2020
  • Author: James CM Chan, MD; Chief Editor: Sasigarn A Bowden, MD  more...
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Begin clinical laboratory evaluation of rickets with assessment of serum calcium, phosphate, and alkaline phosphatase levels. In hypophosphatemic rickets, calcium levels may be within or slightly below the reference range; alkaline phosphatase levels will be significantly above the reference range.

Carefully evaluate serum phosphate levels in the first year of life, because the concentration reference range for infants (5.0-7.5 mg/dL) is high compared with that for adults (2.7-4.5 mg/dL).

Serum parathyroid hormone levels are within the reference range or slightly elevated, while calcitriol levels are low or within the lower reference range. Most importantly, urinary loss of phosphate is above the reference range.

The renal tubular reabsorption of phosphate (TRP) in XLH is 60%; normal TRP exceeds 90% at the same reduced plasma phosphate concentration. The TRP is calculated with the following formula:

1 - [Phosphate Clearance (CPi) / Creatinine Clearance (Ccr)] X 100

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