Urine is an excretory product of the body and presence of certain substances in urine reflects the metabolic state of the body.
Composition
- 95% of volume of normal urine is due to water.
- Others:- Organic components – urea, urobilinogen, uric acid, amino acids, creatinine, metabolities of hormones.
Inorganic components – Cations:- Na+, K+, Ca2+, NH4+ Anions:- Cl-, SO42-, HCO3-, HPO4-
Collection
- Containers for collection should be wide mouthed, clean and dry.
- Analyzed within 2 hours of collection else requires refrigeration.
Types of sample collection
- Random specimen:- No specific time, taken anytime, most common. For routine screening.
- Morning sample:- First urine in the morning, most concentrated. For pregnancy test, microscopic tests.
- Clean catch midstream:- First discard few ml of urine and then collect the rest. For culturing.
- 24 hours:- All the urine passed during the day and night and next day first sample is collected. Used for qualitative and quantitative analysis of substances.
- Postprandial(PP):- 2 hours after meal. Determine glucose in diabetic monitoring.
- Supra-pubic aspired:- Needle aspiration. For obtaining sterile urine.
Urine sample must be tested in laboratory within 2 hours of collection to get the correct results.
Changes which occurs in standing urine @room temperature
•Increase in pH due to production of ammonia from urea by urease – producing bacteria.
•Formation of crystals due to precipitation of phosphates and calcium (making urine turbid).
•Loss of ketone bodies, since they are volatile.
•Decrease in glucose due to glycolysis and utilization of glucose by cells and bacteria.
•Oxidation of bilirubin to biliverdin causing false negative results for bilirubin.
•Bacterial proliferation.
•Oxidation of urobilinogen to urobilin causing false negative results for urobilinogen.
Preservation
•Refrigeration:- At 4-6 degree celcius.
Best method of preservation of sample upto 8 hrs.
•Chemical preservation:-
1.Hydrochloric acid – used for 24 hr sample.
2.Toluene – forms a thin layer over the surface and acts as a physical barrier for bacteria and air.
3.Boric acid – a general preservation.
4.Thymol – inhibits bacteria and fungi.
5.Formalin – excellent chemical for preservation of formed elements.