The urine sample should be collected in a clean, dry container and should
be examined fresh. For cultures, sterile containers should be used. With time, RBC, and leucocytes tend to
be destroyed due to hypotonicity of the urine. Casts too tend to get decomposed. Bacterial contamination of
stale urine is frequent and causes alkalinization of the urine due to conversion of urea to ammonia and loss
of glucose. This rise in pH accelerates loss of leucocytes and epithelial cells. For ordinary qualitative, tests
a random sample is enough. For diabetes mellitus, a 2-hour postprandial sample is desirable; for nephritis, a
morning specimen is best as it has higher specific gravity and lower pH desirable for preservation of formed elements. Repeated samples are necessary
sometimes, as for orthostatic proteinuria. Whenever needed, a 24-hour urine should be
collected in a large container. Have patient void and discard urine at any
particular time, save all urine for the next 24 hours, and then void at the
same hour to finish the collection.
Preservation
of Specimen
Urinary
decomposition occurs quickly in warm temperatures. Hence, fresh specimens should
be examined, if not, then it should be refrigerated. As far as possible, the need
for preservation should not arise. However, the following preservatives can be
used:
1. Toluol Best for preservation of chemical
constituents. Add 2 mL Toluol/100 mL urine.
2. Thymol A small floating lump of thymol
can preserve the urine for several days in a
bottle. Thymol may, however, cause
a false-positive reaction for protein.
3. Formalin 1
drop/30 mL urine. Is good for preserving formed elements. It can precipitate
proteins
and can reduce Benedict’s solution.
4. Boric
acid 0.3 g/120 mL of urine. However, yeasts can still grow and uric acid
crystals
get precipitated.
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