Laube et al developed the Bonn Risk Index (BRI) to predict the risk of calcium oxalate crystal formation in a patient's urine. This can help identify a patient who may benefit from an intervention to reduce the risk. The authors are from Bonn University.

Molecular weight of oxalic acid: 90.04 grams



(1) 24 hour urine ionized calcium in mmol/L

(2) 24 hour urine oxalic acid in mmol/L


Bonn risk index =

= (concentration of ionized calcium) / (concentration of oxalic acid)



• An index > 1 is considered at risk for calcium oxalate stone formation.

• An index <= 1 is low risk for calcium oxalate stone formation.



• The index for healthy patients in the study had a mean value of 1.05 with a range of 0.06 to 4.89. This means that some low risk patients will have an index > 1.

• The patients with calcium oxalate stones had a mean value of 2.87 with a range of 0.278 to 23.

• Because there is some overlap in the populations, it might be useful to have low, moderate and high risk groups.

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