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.

What is the Renal Stone Risk Index Calculator?

Computes simplified urinary saturation indices for calcium oxalate (the Tiselius AP-CaOx index) and uric acid risk from 24-hour urine chemistry, helping quantify kidney-stone formation risk in metabolic stone clinics. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Renal Stone Risk Index Calculator

Name: Renal Stone Risk Index Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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

Calcium oxalate: the Tiselius AP-CaOx index

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

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

where calcium, oxalate, citrate, and magnesium are in mmol/day and volume is in litres/day. Calcium and oxalate in the numerator raise the index; citrate, magnesium, and urine volume in the denominator lower it. The exponent on volume (1.03) means dilution is one of the most powerful modifiable factors — simply drinking more is the first-line intervention.

Uric acid: pH-dependent solubility

Uric acid risk is governed almost entirely by urine pH through the dissociation of uric acid into the more soluble urate form:

soluble fraction = 1 / (1 + 10^(pKa − pH)),  pKa ≈ 5.35

A low soluble fraction combined with a high uric acid excretion load signals high crystallisation risk. The practical implication is that raising urine pH from 5.5 to 6.5 shifts most excreted uric acid into the soluble urate form, dramatically reducing supersaturation without changing how much uric acid the kidneys excrete.

Interpreting the results

AP-CaOx value	Risk band
Below ~1.0	Low — crystallisation unlikely at typical conditions
~1.0 to ~1.7	Intermediate — monitor; dietary adjustment may help
Above ~1.7	High — metabolic intervention warranted

For uric acid, the key decision point is pH relative to 6.0. Below 6.0, a large fraction of uric acid remains undissociated; above 6.5, the majority is soluble urate. Alkalinising agents (sodium bicarbonate, potassium citrate) target a urine pH of 6.2–6.8.

What affects each index

To lower the AP-CaOx index:

Increase fluid intake — the volume term has the strongest leverage
Reduce urinary oxalate (dietary oxalate, vitamin C excess)
Increase urinary citrate (potassium citrate supplementation)
Reduce urinary calcium if hypercalciuria is present

To lower uric acid risk:

Raise urine pH with alkalinising agents
Reduce uric acid excretion via xanthine oxidase inhibition (specialist decision)
Increase fluid intake

Scope and limitations

These are screening estimates from published simplified formulae, not full thermodynamic outputs. Dedicated software such as EQUIL or Lithorisk solves hundreds of ion-pairing equilibria to give true relative supersaturation. This calculator is suitable for educational use, quick clinical screening, and tracking response to dietary interventions — not for replacing validated metabolic stone assessment. Always interpret alongside the complete 24-hour profile, stone composition analysis, and specialist review.