Estimation of 24-h aldosterone secretion in the dog using the urine aldosterone:creatinine ratio

One potential method of evaluating renin-angiotensin-aldosterone system (RAAS) activation involves the quantification of urinary aldosterone excretion. While blood concentrations of aldosterone are easily obtained, results may be misleading because of minute-to-minute variation in aldosterone secretion and subsequent blood concentrations. Urinary aldosterone concentration measurement represents a more consistent "pooled" index of aldosterone secretion, but obtaining 24-h urine samples is time-consuming, difficult, and fraught with potential error. We postulated that the urinary aldosterone:creatinine ratio, measured from spot urine samples, would correlate well with 24-h urinary aldosterone excretion, and would provide a simple index of aldosterone excretion that would eliminate the need for 24-h urine collection.

After validating an assay for aldosterone in canine urine, 24-h urinary aldosterone excretion was determined by radioimmunoassay from 8 normal, male beagle dogs under control conditions, after RAAS stimulation with amlodipine administration, and after RAAS attenuation with the addition of enalapril to amlodipine administration. Spot urine samples, each obtained at the same time of day, were used to determine the aldosterone:creatinine ratio during control conditions, RAAS stimulation, and RAAS attenuation.

The aldosterone:creatinine ratio from spot-checked urine samples correlated well with 24-h urinary aldosterone excretion (r=0.77, P<0.0001).

A spot urinary aldosterone:creatinine ratio might be substituted for 24-h urinary aldosterone determination.