Hypophosphatemic Rickets Workup
- Author: James CM Chan, MD; Chief Editor: Stephen Kemp, MD, PhD more...
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 are 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). Hypophosphatemia can easily be missed in a baby.
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.
In all cases of rickets, the study of choice is radiography of the wrists, knees, ankles, and long bones. However, no pathognomonic sign on radiographs distinguishes hypophosphatemic rickets from any other etiology.
Renal Tubular Phosphate Reabsorption
The renal tubular reabsorption of phosphate (TRP) is calculated with the following formula:
1 - [Phosphate Clearance (CPi) / Creatinine Clearance (Ccr)] X 100
The following formula calculates CPi:
[Urine Phosphate (mg/dL) X Volume (mL/min)] / Plasma Phosphate (mg/dL)
By substituting creatinine values for phosphate in the same formula, Ccr can also be calculated. A single early morning urine sample can be used, because CPi divided by Ccr causes units of urine volume to cancel each other.
The TRP in X-linked hypophosphatemia is 60%; normal TRP exceeds 90% at the same reduced plasma phosphate concentration.
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