Urinary Citrate Ratios (Citrate-to-Creatinine, Citrate-to-Calcium) and the CMC Index (Citrate*Magnesium-to-Calcium) in Patients with Urolithiasis

 

Pathophysiology: Urinary citrate binds with calcium to form a soluble salt. Reduced urinary citrate means that calcium may be free to bind with oxalate.

 

Parameters:

(1) urinary citric acid (citrate) in mmol per 24 hours

(2) urinary calcium in mmol per 24 hours

(3) urinary magnesium in mmol per 24 hours

(4) urinary creatinine in mmol per 24 hours

 

24 hour excretion =

= (measured concentration in mmol/L urine) * (urine volume in L per 24 hours)

 

where:

• The formula weight for citric acid is 192.12 grams per mole.

• Conversion of urinary citrate in mg/dL to mmol/L: multiply by 0.052

 

citrate-to-creatinine ratio =

= (urinary citrate in mmol per 24 hours) / (urinary creatinine in mmol per 24 hours)

 

citrate-to-calcium ratio =

= (urinary citrate in mmol per 24 hours) / (urinary calcium in mmol per 24 hours)

 

citrate-magnesium-magnesium (CMC) index =

= (urinary citrate in mmol per 24 hours)* (urinary magnesium in mmol per 24 hours) / (urinary calcium in mmol per 24 hours)

Test Findings	Mean Value in Normal Controls	Mean Value in Stone Formers
urinary citrate in mmol per 24 hours	2.05 +/- 0.22	1.41 +/- 0.15
urinary calcium in mmol per 24 hours	2.6	5.7 +/- 0.35
urinary citrate-to-creatinine ratio	0.18 +/- 0.02	0.10 +/- 0.01
urinary citrate-to-calcium ratio	1.25 +/- 0.31	0.25 +/- 0.03
CMC index in mmol per 24 hours	5.12 +/- 1.38	1.06 +/- 0.13

from Table 3, page 383, Nikkila et al, 1989

 

Test Findings	Normal Controls	Stone Forming Patients	p value
urinary citrate < 1.5 mmol per 24 hours	29%	68%	< 0.001
urinary citrate-to-creatinine ratio < 0.10	26%	66%	< 0.001
urinary citrate-to-calcium ratio < 0.50	26%	88%	< 0.001
CMC index < 1.0 mmol per 24 hours	13%	69%	< 0.001

from Table 4, page 384, Nikkila et al, 1989

 

where:

• Comparing the 2 tables, the distributions must be skewed.

 

In patients with nephrolithiasis:

(1) low urinary citrate excretion potassium citrate may be given (the sodium salt may increase calcium excretion).

(2) urinary calcium excretion may be reduced with thiazide diuretics.