Screening Infants at Risk for Fractures — Tubular Reabsorption of Phosphate (TRP) — Clinical Pathway: Inpatient

Screening Infants and Children at Risk for Fractures Clinical Pathway — Inpatient

Tubular Reabsorption of Phosphate (TRP)

  • TRP is the fraction of filtered phosphorus that is reabsorbed by the renal tubules
  • Its measurement is useful when evaluating infants with hypophosphatemia
  • Interpret TRP in conjunction with the serum phosphorus level
  • To calculate TRP, urine should be obtained at or around the same time as serum is obtained
  • In general, a reduced TRP in the presence of hypophosphatemia indicates increased urinary phosphate loss
TRPi =
1 −
PO4 (U) × Cr (S)
PO4 (S) × Cr (U)
× 100
  • TRP Tubular Reabsorption of Phosphate (%)
  • PO4 Phosphate, (S, mg/dl)
  • PO4 Phosphate, (U, mg/dl)
  • Cr Creatinine, (S, mg/dl)
  • Cr Creatinine, (U, mg/dL)
  • TRP = [1 − (FEPi/100)] × 100
  • U = urine S = Serum

The normal range for TRP is 85-100%, depending on the serum phosphorus concentration.

Urine Calcium/Creatinine (UCa/Cr)

  • As 24-hour collections are impractical in most children, spot samples of urine calcium/urine creatinine (UCa/Cr) are used to estimate daily urinary calcium excretion and screen for hypercalciuria, especially when children are on calcitriol or calcium supplementation.
  • The upper end of the typical range for Uca/Cr declines from over 1 at birth to < 0.21 after
    age 2 yrs.
  • Uca/Cr is falsely elevated in individuals with low creatinine levels due to low muscle mass. In those situations, Uca/osmolality can be obtained.
  • Levels of Uca/osm > 0.025 suggest excess urinary calcium loss.

Urine Ca/Cr Ratio Norms for age

Age Random (mg/mg)
0-6 mos < 0.8
7-24 mos < 0.6
> 24 mos < 0.21

References

Urolithiasis in Children  
Random Urine Calcium/Osmolality in the Assessment of Calciuria in Children with Decreased Muscle Mass