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PTD - 302

Test Directory - TmP/GFR

Synonyms

Renal tubular maximum reabsorption rate of phosphate to glomerular filtration rate

Clinical Indications

Measurement of TmP/GFR (together with plasma 1,25-DHCC) is required for the diagnosis of X-linked hypophosphataemic rickets and in the diagnosis of hereditary hypophosphataemic rickets with hypercalciuria. Measurement of TmP/GFR and 1,25-DHCC are central to the diagnosis of oncogenic osteomalacia

Test Includes

Serum and urine phosphate and creatinine. TmP/GFR is calculated from these parameters

Request Form

Combined Pathology Blood form (Yellow/Black)

Availability

On request.

Patient Preparation

Patient should be fasted (overnight if possible). The overnight urine is discarded. The next untimed urine is collected and a blood specimen is taken.

Turnaround Time

Same day

Specimen

Fasting serum/plasma and spot urine

Volume

2 ml

Container

Yellow top (SST) tube or

Paediatric orange top (lithium-heparin)

White Capped Universal

Causes for Rejection

Unlabelled samples. Not collected fasting.

Reference Range

Sex

Age

TmP/GFR

Male and Female

Birth1

1.43 - 3.43

3 months1

1.48 - 3.30

6 months1

1.15 - 2.60

2 - 15 years2

1.15 - 2.44

Male

15 - 35 years3

1.00 - 1.35

35 - 55 years3

0.90 - 1.35

Female

15 - 35 years3

0.96 - 1.44

35 - 55 years (premenopausal)3

0.88 - 1.42

Male and Female

> 55 years3

0.80 - 1.35

References

1. Bistarakis L, Voskaki I, Lambadaridis J et al. Renal handling of phosphate in the first six months of life. Arch Dis Child 1986; 61: 677-681

2. Shaw NJ, Wheeldon J, Brocklebank JT. Indices of intact serum PTH and renal excretion of calcium, phosphate and magnesium. Arch Dis Child 1990; 65: 1208-11

3. Minospla S, Pacitti MT, Scarda A et al. Serum ionised calcium, PTH and related variables: effect of age and sex. Bone Mineral 1993; 23: 183-93

Interpretation

Conditions that cause phosphate redistribution (e.g. glucose infusion, respiratory alkalosis) also increase renal phosphate loss, so if hypophosphataemia persists, a low (fasting) TmP/GFR indicates the need for phosphate replacement. In the treatment of severe phosphate deficiency, TmP/GFR can be used as an indicator of intracellular repletion.

Lab. Handling

Processing: Aliquot the serum and urine. Stored at 4C in the cold room (CB23)


Version 1.0 / August 2014                                                                                                       Approved by: Consultant Biochemist