The Science of Urine
The color of a urine specimen is determined by the concentration of its constituents. For example, if a urine sample is dark or unusually colored, it could be due to the presence of substances derived from food pigments, medications, or a particular disease that causes the body to produce excessive amounts of chemical substances known as analytes.
On the other hand, a urine sample could be relatively colorless. This may be the result of excess hydration due to the patient's diet or a specific medical condition, or it could be caused by the patient's deliberate consumption of excess water. In some cases, excessive water intake is innocent, but some patients may drink large quantities of water before urine drug screening in an effort to produce a dilute urine sample that cannot be properly tested. In the absence of underlying renal pathology, a patient who repeatedly provides dilute urine samples may be drinking too much water prior to testing. In this case, the patient should be advised to drink less water prior to testing and to give a sample in the early morning when the urine is most concentrated. Not surprisingly, a concentrated urine sample is generally more reliable than a diluted one.
The temperature of a urine sample within four minutes of voiding should fall within the range of 90 degrees Fahrenheit to 100 degrees Fahrenheit if the sample is of sufficient volume- 30 milliliters or more. The pH of urine, a measure of its alkalinity or acidity, generally fluctuates throughout the day, but it should be within the range of 4.5 to 8.0. A sample with a pH greater than 9.0 is likely a "degraded" urine sample, possibly because of improper storage or prolonged transportation – or adulteration.
Urinary creatinine varies with a person's water intake and state of hydration. Creatinine is a breakdown product of creatinine phosphate in muscle tissue, and it is normally produced at a fairly consistent rate by the human body. In the urine, creatinine is measured in milligrams per deciliter (mg/dL). A normal urine specimen has a creatinine concentration greater than 20 mg/dL.
Specimens with a creatinine concentration of less than 20 mg/dL and a specific gravity (the density of the urine relative to the density of water) of less than 1.003 are considered to be dilute samples. A creatinine concentration of less than 2 mg/dL is not consistent with human urine. Patients with renal insufficiencies or kidney failure can have elevated creatinine levels. Physicians should discuss aberrant test results with the patient as well as the Medical Review Officer as necessary.
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