Background. The concentration of magnesium in urine reflects its intake from food resources. The main regulatory mechanism of magnesium homeostasis is the kidney filtration-absorption process. So far the reference values are not known of the magnesium concentration in urine in dependence on age and sex for the normal population. Therefore, we focused on investigating these problems in a randomly selected sample of persons of both sexes, aged 1-93 years. Beside magnesiuria we also assessed creatininuria. Methods and Results. The selection of 2715 persons of both sexes aged from one to ninety three years was made by the random selection method. As we have found no statistically significant difference between men and women as regards concentrations of magnesium and creatinine in urine, the evaluated groups include persons of both sexes. The whole is divided into age groups as follows: children 1 year (n = 24), 2 years (n = 15), 4 years (n = 18), 6 years (n = 378), 10 years (n = 424), 13 years (n = 416), and adults 18-35 years (n = 396), 36-49 years (n = 482), 50-65 years (n = 453), 66-75 years (n = 53), 76-85 years (n = 42), 86-93 years (n = 14). In the morning urine after twelve-hour fasting we determined the concentration of magnesium and creatinine be using Merck Vitalab Eclipse apparatus. The concentration of magnesium in urine rises from 3.37 ± 0.28 mmol/l at 1 year to the top value 6.64 ± 0.58 mmol/l (p < 0.001) at 4 years, and since this age it is continuously dwindling to the value 2.35 ± 0.17 mmol/l (p < 0.001) at the age of 93 years. The creatinine concentration in urine rises from 2.47 ± 0.30 mmol/l at the age of one year to 15.33 ± 0.36 mmol/l (p < 0.001) in the age group 18-35 years, and further it goes down with age to the value 6.23 ± 0.99 mmol/l (p < 0.001) at 93 years. The highest ratio of the urine concentrations of magnesium and creatinine 1.98 ± 0.32 is at the age of 2 years, and then it is continuously subsiding to 0.41 ± 0.15 (p < 0.001) at the age range 66-75 years. Conclusions. When evaluating the magnesium concentration in urine as the index of a sufficient supply of organism with this ion, and also in various studies of metabolism, it is essential to consider the patient´s age. The ratio of concentrations in urine of magnesium and creatinine does not improve the meaningfulness of magnesium analysis.

        Key words: magnesium, creatinine, urine, age.