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Test ID: RTRPP    
Tubular Reabsorption of Phosphorus, Random

Useful For Suggests clinical disorders or settings where the test may be helpful

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's syndrome

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

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 (PTH), 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 Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

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

> or =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

> or =18 years: 2.5-4.5 mg/dL

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

 

CREATININE

Males

12-24 months: 0.1-0.4 mg/dL

3-4 years: 0.1-0.5 mg/dL

5-9 years: 0.2-0.6 mg/dL

10-11 years: 0.3-0.7 mg/dL

12-13 years: 0.4-0.8 mg/dL

14-15 years: 0.5-0.9 mg/dL

> or =16 years: 0.8-1.3 mg/dL

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

Females

12-36 months: 0.1-0.4 mg/dL

4-5 years: 0.2-0.5 mg/dL

6-8 years: 0.3-0.6 mg/dL

9-15 years: 0.4-0.7 mg/dL

> or =16 years: 0.6-1.1 mg/dL

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

Interpretation Provides information to assist in interpretation of the test results

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

 

TmP/glomerular filtration rate (GFR) is independent of dietary phosphorus intake, tissue release of phosphorus, and GFR.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

No significant cautionary statements

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

1. Suki WN, Lederer ED, Rouse D: Renal transport of calcium, magnesium, and phosphate. In The Kidney, Sixth Edition. Edited by B Brenner. WB Saunders Company, 2000, Chapter 12

2. Bijvoet OL: Relation of plasma phosphate concentration to renal tubular reabsorption of phosphate. Clin Sci 1969;37:23-36

3. Walton RJ, Bijvoet OL: Nomogram for derivation of renal threshold phosphate Concentration. Lancet 1975;2:309-310

4. Payne RB: Renal tubular reabsorption of phosphate (TmP/GFR): indications and interpretation. Ann Clin Biochem 1998;35:201-206