Can spot urine samples replace the 24-h urine specimen in the metabolic evaluation of stone-forming patients?

The gold standard for metabolic evaluation of stone-forming patients is the 24-h urine specimen, Recently, some authors have suggested that for routine metabolic evaluation spot urine samples are as valuable as the 24-h urine specimen. The purpose of our study, was to determine the value of the spot urine sample in comparison with the 24-h urine specimens. Eighty-eight healthy volunteers on different diets were investigated (32 vegetarians, 12 body-builders without protein concentrates, 28 body-builders on protein concentrates, and 16 subjects on a regular European diet), Using 24-h specimens, excretion rates of oxalate, calcium, sodium and potassium were determined, The concentration ratio of these electrolytes to creatinine was calculated for spot urine samples. A highly significant correlation between the excretion rates and the results of the spot urine samples was found for all parameters. However, the correlations showed considerable variations, On the other hand, we were able to show that creatinine excretion is highly dependent on daily protein intake, body weight and glomerular filtration rate. This leads to a considerable inter- and intraindividual variation in creatinine excretion, This variation of the creatinine excretion is the major cause for the variation in the results of spot urine samples. It is concluded that spot urine samples are an inadequate substitute for the 24-h urine specimen and that the 24-h urine specimen is still the basis for metabolic evaluation in stone patients.