TmP/GFR Calculator (Renal Phosphate Threshold)
TmP/GFR Calculator (Renal Phosphate Threshold)
Calculate the renal tubular maximum phosphate reabsorption threshold (TmP/GFR) using the Walton-Bijvoet formula, the most sensitive test for renal phosphate wasting.
TmP/GFR (Renal Phosphate Threshold)
Walton-BijvoetUrine phosphate 40 mg/dL, serum creatinine 0.9 mg/dL, serum phosphate 1.9 mg/dL, urine creatinine 90 mg/dL
Formula
If TRP ≤ 0.86: TmP/GFR = TRP × serum phosphate
If TRP > 0.86: TmP/GFR = [0.3 × TRP ÷ (1 − 0.8 × TRP)] × serum phosphate
- TRP
- tubular reabsorption of phosphate, a fraction between 0 and 1
- 0.86
- the threshold above which the TRP–TmP/GFR relationship becomes markedly non-linear, requiring the corrected form
- all concentrations
- mg/dL, from a fasting morning paired blood and spot urine sample
Worked example
Urine phosphate 40 mg/dL, serum creatinine 0.9 mg/dL, serum phosphate 1.9 mg/dL, urine creatinine 90 mg/dL
TRP = 1 − (40 × 0.9) ÷ (1.9 × 90) = 1 − 36 ÷ 171 = 0.789
TRP (0.789) is ≤ 0.86, so use the direct branch
TmP/GFR = 0.789 × 1.9 = 1.50 mg/dL
Normal TmP/GFR by age
| Group | Approximate TmP/GFR (mg/dL) |
|---|---|
| Adults | 2.5 – 4.2 |
| Children | higher than adult values — age-specific ranges apply |
What TmP/GFR measures, and why it is more sensitive than phosphate alone
TmP/GFR is the serum phosphate concentration above which the kidney begins to excrete phosphate into the urine — the renal phosphate threshold. Because it accounts for the filtered phosphate load as well as the amount reabsorbed, it is a substantially more sensitive test for renal phosphate wasting than serum phosphate alone, which can sit within the reference range even when tubular reabsorption is clearly abnormal.
The Walton-Bijvoet formula first derives the tubular reabsorption of phosphate (TRP) from a paired blood and spot urine sample. When TRP is 0.86 or below, TmP/GFR is simply TRP multiplied by serum phosphate. Above 0.86, the relationship between TRP and TmP/GFR becomes markedly non-linear, and a corrected formula is required — using the direct form in that range would meaningfully overestimate the threshold.
A low TmP/GFR accompanied by hypophosphataemia points to renal phosphate wasting, with three broad categories of cause: FGF-23-mediated disorders, including X-linked hypophosphataemia, tumour-induced osteomalacia, and the transient wasting seen after ferric carboxymaltose infusion; primary hyperparathyroidism, in which parathyroid hormone directly suppresses phosphate reabsorption; and Fanconi syndrome, where phosphate wasting accompanies glycosuria, aminoaciduria and a proximal renal tubular acidosis.
The sample should be taken fasting in the morning, since phosphate has a pronounced diurnal rhythm that can otherwise distort the result. Normal adult TmP/GFR is roughly 2.5 to 4.2 mg/dL, and reference values are higher in children — a fact that matters directly when investigating a child for suspected rickets.
Frequently asked questions
What does TmP/GFR measure?
The renal phosphate threshold — the serum phosphate concentration above which the kidney starts excreting phosphate into the urine. It is the most sensitive test for renal phosphate wasting.
Why does the formula change above a TRP of 0.86?
The relationship between tubular reabsorption of phosphate (TRP) and TmP/GFR becomes markedly non-linear above 0.86. The Walton-Bijvoet formula switches to a corrected expression there to avoid overestimating the threshold.
What causes a low TmP/GFR?
Renal phosphate wasting from FGF-23-mediated disorders (X-linked hypophosphataemia, tumour-induced osteomalacia, post-ferric carboxymaltose), primary hyperparathyroidism, or Fanconi syndrome.
Why should the sample be taken fasting in the morning?
Serum phosphate follows a marked diurnal rhythm. A non-fasting or afternoon sample can shift the calculated threshold enough to mislead the interpretation.
Related calculators
References
- Walton RJ, Bijvoet OL. Nomogram for derivation of renal threshold phosphate concentration. Lancet. 1975;2(7929):309–310.
- Payne RB. Renal tubular reabsorption of phosphate (TmP/GFR): indications and interpretation. Ann Clin Biochem. 1998;35(2):201–206.