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Test Code RTRP2 Tubular Reabsorption of Phosphorus, Random Urine and Serum


Specimen Required


Both serum and urine are required.

 

Specimen Type: Serum

Patient Preparation: Fasting

Collection Container/Tube: 

Preferred: Serum gel

Acceptable: Red top

Submission Container/Tube: Plastic vial

Specimen Volume: 0.5 mL

Collection Instructions:

1. Centrifuge and aliquot serum into a plastic vial.

2. Label specimen as serum.

 

Specimen Type: Urine

Container/Tube: Plastic, 5-mL tube

Specimen Volume: 4 mL

Collection Instructions:

1. Collect a random urine specimen.

2. No preservative.

3. Label specimen as urine.


Secondary ID

614047

Useful For

Assessing renal reabsorption of phosphorus in a variety of pathological conditions associated with hypophosphatemia including hypophosphatemic rickets, tumor-induced osteomalacia, and tumoral calcinosis

 

Adjusting phosphate replacement therapy in severe deficiency states monitoring the renal tubular recovery from acquired Fanconi syndrome

Profile Information

Test ID Reporting Name Available Separately Always Performed
PHOS Phosphorus (Inorganic), S Yes Yes

Method Name

RTRP: Calculation

CRETR, ACREA: Enzymatic Colorimetric Assay

PHOS: Photometric, Ammonium Molybdate

Reporting Name

Tubular Phosp Reabsorption, Random

Specimen Type

Serum
Urine

Specimen Minimum Volume

Urine: 1 mL; Serum: See Specimen Required

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Frozen (preferred) 7 days
  Refrigerated  7 days
Urine Refrigerated (preferred) 30 days
  Frozen  14 days
  Ambient  7 days

Reject Due To

Gross hemolysis Reject

Clinical Information

The tubular reabsorption of phosphate (TRP) is the fraction (or percent) of filtered phosphorus that is reabsorbed by renal tubules. Its measurement is useful when evaluating patients with hypophosphatemia. In general, a reduced TRP in the presence of hypophosphatemia is indicative of a renal defect in phosphate reabsorption.

 

The ratio of the maximum rate of tubular phosphate reabsorption to the glomerular filtration rate (TmP/GFR) is considered the most convenient way to evaluate renal phosphate transport and is referred to as the theoretical renal phosphate threshold. This corresponds to the theoretic lower limit of plasma phosphate below which all filtered phosphate would be reabsorbed. Although direct measurements of parathyroid hormone, which increases renal phosphate excretion, have replaced much of the utility of TmP/GFR measurements, it may still be useful in assessing renal reabsorption of phosphorus in a variety of pathological conditions associated with hypophosphatemia.

Reference Values

TUBULAR REABSORPTION OF PHOSPHORUS

>80%

(Although, tubular reabsorption of phosphorus levels must be interpreted in light of the prevailing plasma phosphorus and glomerular filtration rate.)

 

TUBULAR MAXIMUM PHOSPHORUS REABSORPTION/GLOMERULAR FILTRATION RATE (TmP/GFR)

2.6-4.4 mg/dL (0.80-1.35 mmol/L)

 

PHOSPHORUS (INORGANIC)

Males

1-4 years: 4.3-5.4 mg/dL

5-13 years: 3.7-5.4 mg/dL

14-15 years: 3.5-5.3 mg/dL

16-17 years: 3.1-4.7 mg/dL

≥18 years: 2.5-4.5 mg/dL

Reference values have not been established for patients that are <12 months of age.

Females

1-7 years: 4.3-5.4 mg/dL

8-13 years: 4.0-5.2 mg/dL

14-15 years: 3.5-4.9 mg/dL

16-17 years: 3.1-4.7 mg/dL

≥18 years: 2.5-4.5 mg/dL

Reference values have not been established for patients that are <12 months of age.

 

PHOSPHORUS, Random Urine

No established reference values

 

Random urine phosphorus may be interpreted in conjunction with serum phosphorus, using both values to calculate fractional excretion of phosphorus.

 

The calculation for fractional excretion (FE) of phosphorus (P) is

FE(P)= ([P(urine)XCreat(serum)]/[P(serum)XCreat(urine)]) X 100

 

CREATININE Serum

Males(1)

0-11 months: 0.17-0.42 mg/dL

1-5 years: 0.19-0.49 mg/dL

6-10 years: 0.26-0.61 mg/dL

11-14 years: 0.35-0.86 mg/dL

≥15 years: 0.74-1.35 mg/dL

 

Females(1)

0-11 months: 0.17-0.42 mg/dL

1-5 years: 0.19-0.49 mg/dL

6-10 years: 0.26-0.61 mg/dL

11-15 years: 0.35-0.86 mg/dL

> or=16 years: 0.59-1.04 mg/dL 

 

CREATININE, Random Urine

16-326 mg/dL

Reference values have not been established for patients who are less than 18 years of age.

Interpretation

Interpretation of tubular reabsorption of phosphate (TRP) and the maximum rate of TRP to the glomerular filtration rate (TmP/GMR) is dependent upon the clinical situation and should be interpreted in conjunction with the serum phosphorous concentration.

 

TmP/GFR is independent of dietary phosphorus intake, tissue release of phosphorus, and GFR.

Method Description

Creatinine is performed by the enzymatic method, which is based on the determination of sarcosine from creatinine with the aid of creatininase, creatinase, and sarcosine oxidase. The liberated hydrogen peroxide is measured via a modified Trinder reaction using a colorimetric indicator. Optimization of the buffer system and the colorimetric indicator enables the creatinine concentration to be quantified both precisely and specifically.(Package insert: Creatinine plus ver 2. Roche Diagnostics; V15.0 03/2019)

 

Inorganic phosphate forms an ammonium phosphomolybdate complex with ammonium molybdate in the presence of sulfuric acid. The concentration of phosphomolybdate formed is directly proportional to the inorganic phosphate concentration and is measures photometrically.(Package insert: Phosphate (Inorganic) ver 2. Roche Diagnostics; V11.0 07/2019)

Performing Laboratory

Mayo Clinic Laboratories in Rochester

Test Classification

This test has been cleared, approved, or is exempt by the US Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.

CPT Code Information

82565

82570

84100

84105

LOINC Code Information

Test ID Test Order Name Order LOINC Value
RTRP2 Tubular Phosp Reabsorption, Random In Process

 

Result ID Test Result Name Result LOINC Value
PHOS Phosphorus (Inorganic), S 2777-1

Day(s) Performed

Monday through Sunday