FPCT Calculator — Fractional Phosphate Clearance Test
The Fractional Phosphate Clearance Test (FPCT / FE_PO4) measures what percentage of filtered phosphate the kidneys excrete in the urine, indicating whether tubular phosphate reabsorption is normal, insufficient (wasting), or excessive (retention). Enter urine and serum phosphate and creatinine values from the same collection.
mg/dL
mg/dL
mg/dL
mg/dL
Normal range — appropriate tubular phosphate reabsorption
- 1
Urine phosphate ÷ serum phosphate
45 ÷ 3.5 = 12.8571 - 2
Urine creatinine ÷ serum creatinine
120 ÷ 1 = 120Creatinine ratio corrects for urine concentration, allowing a spot sample. - 3
FE_PO4 (%)
(12.8571 ÷ 120) × 100 = 10.71
How does this calculator work?
FE_PO4 = (U_PO4 × S_Cr) / (S_PO4 × U_Cr) × 100. Normal range: 5–15%. Below 5% suggests phosphate retention; above 15–20% suggests renal phosphate wasting. The complementary measure TRP = 100 − FE_PO4. Results require physician interpretation.
Formula
How this is calculated
Phosphate is freely filtered at the glomerulus and then reabsorbed mainly in the proximal tubule via NaPi-IIa and NaPi-IIc co-transporters. Parathyroid hormone (PTH) and FGF23 are the principal regulators that inhibit reabsorption when their levels rise, causing more phosphate to appear in the urine.
FE_PO4 is calculated the same way as other fractional excretion indices: the urine-to-serum ratio of phosphate, divided by the urine-to-serum ratio of creatinine (the internal filtration marker), multiplied by 100. This creatinine correction adjusts for urine concentration so that a spot sample can be used instead of a 24-hour collection. The complementary measure TRP = 100 − FE_PO4 expresses what fraction of filtered phosphate was reabsorbed.
Reference ranges (which can vary between laboratories and guidelines): FE_PO4 < 5% typically reflects phosphate retention, seen in hypoparathyroidism, vitamin D deficiency, or dietary restriction. 5–15% is generally considered normal. Values > 15–20% suggest renal phosphate wasting, as occurs in primary hyperparathyroidism, X-linked hypophosphataemia (XLH), oncogenic osteomalacia (tumour-induced FGF23 excess), or Fanconi syndrome. This calculator does not replace clinical evaluation or laboratory reporting.
Frequently asked questions
In most guidelines a normal FE_PO4 is 5–15%, reflecting appropriate tubular reabsorption. Values below 5% suggest excessive phosphate retention; values above 15–20% suggest renal phosphate wasting, which may point to hyperparathyroidism, FGF23 excess, or proximal tubular dysfunction.
No. Using the urine creatinine to correct for urine concentration allows a spot (random) urine sample to be used, which is how the FPCT is most commonly performed. The sample must be collected at roughly the same time as the serum draw.
They are complementary: TRP (Tubular Reabsorption of Phosphate) = 100% − FE_PO4. TRP expresses what fraction of filtered phosphate was kept; FE_PO4 expresses what fraction was excreted. A normal TRP is 85–95%, corresponding to FE_PO4 5–15%.
Also known as
TG we-Calculate Editorial Team. (2026). FPCT Calculator — Fractional Phosphate Clearance Test [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/fpct-calculator
TG we-Calculate Editorial Team. "FPCT Calculator — Fractional Phosphate Clearance Test." TG we-Calculate. 2026. https://we-calculate.com/calculator/fpct-calculator.
TG we-Calculate Editorial Team, "FPCT Calculator — Fractional Phosphate Clearance Test," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/fpct-calculator
@misc{wecalculate_fpct_calculator, title = {FPCT Calculator — Fractional Phosphate Clearance Test}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/fpct-calculator}}, year = {2026}, note = {TG we-Calculate} }
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