Adjustment to concentration-dilution of spot urine samples: correlation between specific gravity and creatinine

Spot urine samples were investigated to determine correlations between urinary creatinine and specific gravity, and intra- and inter-day variations other than gender- and age-dependence of urinary concentrations.

Urinary creatinine concentrations and specific gravity were determined in 534 spot samples (385 from men and 149 from women). Subjects' ages ranged between 18 and 68 years. Spot urine samples were also collected from 14 male subjects before and after a 1-week work-shift for the evaluation of intra- and inter-day variations of creatinine and specific gravity.

In spot samples, creatinine concentrations ranged between 0.16 and 4.36 g/l and specific gravity between 1.002 and 1.037. A high correlation (r = 0.82, P < 0.001) was observed between creatinine and specific gravity; male subjects showed significantly higher values of creatinine (P < 0.001) than did female subjects (1.90 +/- 0.74 and 1.41 +/- 0.72 g/l, respectively) and specific gravity (1.023 +/- 0.006 and 1.020 +/- 0.007, respectively). In addition, creatinine but not specific gravity significantly decreased (P < 0.02) in subjects older than 50 years, compared with those under 40.

Results confirm the gender-dependence of creatinine concentrations in spot specimens and also show age-dependence, indicating the need for these aspects to be considered when the range of acceptable samples is to be set. No significant intra- or inter-day variations were observed for the two parameters. Lastly, the possibility of a comparison of differently adjusted values was indicated by a conversion formula derived from adjustments to creatinine and the corresponding specific gravity of a hypothetical urinary value, as follows: specific gravity adjusted values = 1.48 x creatinine adjusted values.