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HelpMayo Test ID 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.
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, RandomSpecimen Type
SerumUrine
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.
Cautions
No significant cautionary statements
Clinical Reference
1. Kulasingam V, Jung BP, Blaustig IM, et al: Pediatric reference intervals for 28 chemistries and immunoassays on the Roche cobas 6000 analyzer--a CALIPER pilot study. Clin Biochem. 2010;43:1045-1050
2. Suki WN, Lederer ED, Rouse D: Renal transport of calcium, magnesium, and phosphate. In: Brenner B, ed: The kidney. 6th ed. WB Saunders Company; 2000:chap 12
3. Bijvoet OL: Relation of plasma phosphate concentration to renal tubular reabsorption of phosphate. Clin Sci. 1969;37:23-36
4. Walton RJ, Bijvoet OL: Nomogram for derivation of renal threshold phosphate Concentration. Lancet. 1975;2:309-310
5. Payne RB: Renal tubular reabsorption of phosphate (TmP/GFR): indications and interpretation. Ann Clin Biochem. 1998;35:201-206
6. Delaney MP, Lamb EJ: Kidney disease. In: Rifai N, Horvath AR, Wittwer CT, eds. Textbook of Clinical Chemistry. 6th ed. Elsevier; 2018:1256-1323
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)
Specimen Retention Time
See Individual Test IDsPerforming 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