In the MAY Q&A Column in CAP TODAY, the following question was posed:
I work in an 800-bed university hospital core lab. My question is about the processing of 24-hour urines. We get so many per day that the jugs have become difficult to manage. When a test needs acidification, our lab assistants aliquot all tests that do not need any additive. To the remaining urine, they add 25 mL of 1 N HCl (remaining volume must be > 500 mL), mix it up, and aliquot into a sample tube. This tube is set aside for one hour for equilibration before testing is performed. The jug is then discarded. Is this an acceptable practice? In the past, we left the jug for one hour after acidification before aliquoting into a sample tube. But the process is messy and hard to manage.
The answer, provided by Gifford Lum, M.D., Associate Chief, Clinical Pathology Pathology and Laboratory Medicine Service VA Boston Healthcare System, discussed what to do with urine samples. According to Dr. Lum, urine samples may be aliquoted and frozen for analysis at a later time but to prevent specimen degradation as a consequence of repeated freeze-thawing, the urine sample should be frozen in aliquots.
Dr Lum referenced a urine collection study conducted by the Mayo Clinic Department of Laboratory Medicine and Pathology. The study was initiated to determine the stability of analytes under different preservatives and temperature conditions. According to the study, Mayo Clinic suggests that 30 mL of 6 N HCl be added per 24-hour collection.