Are preservatives necessary in 24-hour urine measurements?

Background 24-h urine measurements are used in the routine diagnosis and follow-up of many diseases in the clinical laboratory. Calcium (Ca 2+ ), magnesium (Mg 2+ ), phosphate (PO 4 3− ) and uric acid are frequently requested markers in 24-h urine samples. Because of the different solubilities of these parameters, different urine collection conditions – urine in base for uric acid and urine in acid for Ca 2+ , PO 4 3− and Mg 2+ measurements – are recommended. Methods We aimed to test the effect of addition of preservatives and heating of the urine specimen on the results obtained for Ca 2+ , Mg 2+ , PO 4 3− and uric acid by comparison with untreated samples results. Spot ( n = 20) and 24-h urine ( n = 50) samples were obtained from patients for routine urine analysis. A single spot urine sample was divided into five aliquots of 10 mL each: one containing 200 µL of HCl (6 N), another containing 200 µL of sodium bicarbonate, NaHCO 3 (5 g/L), two others in which the same preservative agents were added 24 h after the collection, and one without any preservative (untreated). Ca 2+ , PO 4 3− , uric acid and Mg 2+ were measured in triplicate and at three different time points during the study: at the time of sampling (0 h), 24 h after sampling, and after heating the samples. The 24-h urine samples were collected without preservatives and analytes were measured promptly before and after acidification/alkalinization. Results There was no statistically significant difference between untreated and treated samples ( p > 0.05). Heating also failed to show any difference in the results ( p > 0.05). Conclusion According to our results, addition of preservatives is not necessary for measurement of Ca 2+ , Mg 2+ , PO 4 3− and uric acid in promptly assayed 24-h urine samples.