Hypophosphatemic Rickets Workup

Updated: May 02, 2018
  • Author: James CM Chan, MD; Chief Editor: Sasigarn A Bowden, MD  more...
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Workup

Approach Considerations

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). 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.

Imaging studies

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.

On the other hand, a study by Lempicki et al indicated that in hypophosphatemic rickets, magnetic resonance imaging (MRI) reveals characteristics of the knee—including maximum physial widening and transverse degree of widening—that correlate with alkaline phosphatase levels. [26]

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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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