Untimed or random urine specimens are suitable only for few chemical tests (mainly qualitative test). Usually urine sample is collected over a predetermined period of time, such as 1, 4, or 24 hr.
A clean, early morning, fasting specimen is usually the most concentrated specimen and thus preferred for microscopic examination and for detection of abnormal amounts of constituents such as proteins, or of unusual compounds such as chorionic gonadotropin.
Urine may be collected by voiding, from an inserted catheter, or from suprapubic tap specimen.
Timed urine specimens: the collection period should be long enough to minimize the influence of shot term biological variation. The patient should adhere to the instructions, the bladder should be emptied at the time the collection is to begin, and this urine is discarded. Thereafter, all urine should be collected until the end of the scheduled time. collection should be done in an adequate clean container (3 or 4 liter bottle is usually enough for 24 hr collections). Collection from infants is done using special adhesive plastic bags.