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Sialic acids comprise all N- and O-acyl derivatives of neuraminic acid and are components of glycoproteins and glycolipids. Their concentrations vary physiologically with age but also in diseases such as inflammation, neoplastic tumours or in inborn genetic disorders causing abnormal sialic acid metabolism. Determination of free and bound sialic acids in urine using the thiobarbituric acid method according to Warren (J Biol Chem 1959; 234:1971-5) was shown to be useful for the diagnosis of diseases that involve sialic acid metabolic disorders. This test-also used for the diagnosis of inborn errors of metabolic diseases, such as sialidosis, infantile sialic acid storage disease, Salla's disease, neuraminidase deficiency and others-should be included in the selective screening for storage diseases. With the reported number of mild, juvenile and adult forms of genetic disorders increasing, this diagnosis will also be useful for teenagers and adults. We therefore considered it important not to confine our investigation to children and compared the diagnostic value of 24-hour and spot urines. As shown in 24-hour urines (n = 242, 128 males, 114 females) the average excretion of sialic acids increases constantly during life, from 67.6 mumol to 444.0 mumol per day, as does the free (27.5 mumol to 217.1 mumol) and bound fraction (40.1 mumol to 226.9 mumol). The relative proportion of free and bound sialic acid shows only slight lifetime variations, the free fraction increases from about 40 percent the first few years to about 53 percent of total in the fifth decade. In the spot urines, the mean ratio of total free sialic acids and urinary creatinine (mmol/mol) decreases constantly during the first few decades, with a sharp drop during the first years of life (from 3 months-2 years: from 203.9 to 94.2 and 82.1 to 42.3 respectively; with 10 years: 52.3 and 22.4 respectively; in the sixth decade: 44.8 and 21.9). Similar findings could also be observed in the investigated 24-hour urines (correlation coefficient of ratios, R = +0.981). The comparison of 24-hour urines and spot urines confirms the reliability of results for spot urines, however, the urine collection over an extended period yields additional information.

Comparison of Sialic Acids Excretion in Spot Urines and 24-Hour-Urines of Children and Adults