Are preservatives necessary in 24-hour urine measurements?

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.

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 microL of HCl (6 N), another containing 200 microL 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.

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).

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.