What is the Tiselius AP-CaOx index?

It is a simplified relative-probability index for calcium oxalate crystallisation derived from 24-hour urine. It multiplies urinary calcium and oxalate, divides by the protective factors citrate and magnesium and by urine volume, with empirical exponents. Higher values indicate greater supersaturation and stone risk.

What thresholds separate low and high risk?

For the AP-CaOx index a value below about 1.0 is generally considered low risk and a value above about 1.7 high risk, with the band between regarded as intermediate. These cutoffs come from Tiselius' original cohorts and should be read alongside the full metabolic picture.

Why does urine pH dominate uric acid risk?

Uric acid solubility is exquisitely pH dependent. At a urine pH of 5.5 the soluble urate fraction is low, so most uric acid stays as poorly soluble undissociated acid and readily crystallises. By a pH of about 6.5 the great majority is dissociated and soluble, which is why alkalinisation is the cornerstone of uric acid stone prevention.

What units should I use?

Enter calcium, magnesium, citrate, and uric acid in millimoles per day, oxalate in millimoles per day, and volume in litres per day, matching the Tiselius formulation. The tool labels each field with its expected unit; mixing units will distort the index.

Can this replace dedicated software like EQUIL or Lithorisk?

No. Full thermodynamic programs solve hundreds of ion-pairing equilibria to give true relative supersaturation. This calculator reproduces published simplified indices for quick screening and education only and should not replace validated metabolic stone software or specialist interpretation.

Renal Stone Risk Index Calculator

Recurrent kidney stones are driven by urinary supersaturation: when the concentration of stone-forming ions exceeds their solubility, crystals nucleate and grow. Simplified indices let a clinic quantify that risk from a 24-hour urine collection without a full thermodynamic model.

How it works

The Tiselius AP-CaOx index estimates calcium oxalate crystallisation probability:

AP-CaOx = 1.9 x Ca^0.84 x Ox / ( Cit^0.22 x Mg^0.12 x Vol^1.03 )

where calcium, oxalate, citrate, and magnesium are in mmol/day and volume is in litres/day. Calcium and oxalate raise the index, while citrate, magnesium, and dilution lower it. Uric acid risk is governed almost entirely by urine pH through the dissociation of uric acid:

soluble fraction = 1 / (1 + 10^(pKa - pH)),  pKa ~= 5.35

A low soluble fraction with a high uric acid load signals high uric acid stone risk.

Interpretation and notes

For the AP-CaOx index, values below about 1.0 are low risk, above about 1.7 high risk, and in between intermediate. For uric acid, raising urine pH toward 6.5 dramatically increases the soluble fraction and is the basis of alkalinising therapy. These are screening estimates from published simplified formulae and do not replace full thermodynamic software such as EQUIL or Lithorisk, nor specialist metabolic assessment. Always interpret indices alongside the complete 24-hour profile and the patient’s stone composition.