Test ID: RTRPP    
Tubular Reabsorption of Phosphorus, Random

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

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.

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 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.

No significant cautionary statements

1. Suki WN, Lederer ED, Rouse D: Renal transport of calcium, magnesium, and phosphate. In The Kidney, 6th 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