Limits of preservation of samples for urine strip tests and particle counting

Preservation of urine samples is important for centralised laboratory services with automated instruments.

A multicentre evaluation was carried out to assess preservative tubes from BD Diagnostics-Preanalytical Systems and from Greiner Bio-One for test strip reading (documented at the level of remission values), for particle counting by flow cytometers (UF-100) and for visual microscopy. Failures were expressed as percentages of originally positive samples beyond a two-fold change (+100% or -50%) from the original values.

The preservative-containing BD Plus C&S plastic, BD Plus UAP and Greiner Stabilur tubes succeeded in preservation of test strip results for 6-24 h (exceptions were glucose and nitrite tests). Greiner boric acid tube showed false negative results in leukocyte, protein and ketone strip tests immediately after adding the preservative. Urine red blood cell counts (with Sysmex UF-100) were preserved for 5 h in BD Plus C&S plastic and Greiner Stabilur tubes (Greiner tubes having clearly larger preservative-related background). Bacteria or white blood cell counting succeeded in BD Plus C&S plastic tubes for 5 or 24 h, respectively, but up to 72 h in Greiner Stabilur tubes. In visual microscopy, the Greiner Stabilur tube was slightly better than the BD Plus C&S plastic tube.

Urine specimens can be transported at +20 degrees C on the day of collection if preserved properly. Longer delays need careful planning with current preservatives. Flow cytometry with UF-100 is sensitive to non-dissolved preservative remnants.