In diabetes mellitus, glucose appears in the urine because a hyperglycemia condition exists in the blood. Therefore, the glucose concentration should also be measured in the blood.
Glucosuria can occur temporarily from emotional stress or pain, hyperthyroidism, alimentary hyperglycemia, and meningitis.
Apparently, the kidney acts as a safety valve against the excessive accumulation of glucose in the blood. If the glucose level becomes too high, then the renal threshold in the kidneys may be exceeded. The renal threshold is a concentration level above which all glucose is not reabsorbed in the blood, but the excess above the threshold concentration remains in the urine. A threshold is analogous to a dam and only when the water level becomes too high, does the water spill over the dam. Many other substances in the blood have their own threshold levels and when the threshold is exceeded, the substance appears in the urine.
In renal diabetes, the threshold is abnormally low and glucose appears in the urine at a much lower concentration than normal. The relationships between glucosuria and the renal threshold are illustrated in the diagram on the left.
Another relatively easy laboratory test can be made for ketone bodies in the urine. The condition is known as acetonuria from the acetone present. Ketones bodies result in diabetes mellitus for the very same reasons as given for starvation. Ketone bodies are not normally found in urine nor are they present with the other types of diabetes listed. Ketone bodies are present in various amounts depending upon the severity of the diabetes mellitus.
Glucose Tolerance Test: In order to measure the metabolic response of a patient to glucose, the glucose tolerance test was devised. The test consists of giving 100 g of glucose in place of breakfast and then testing the concentrations of glucose in the blood and urine at specific timed intervals. This test establishes when the blood glucose reaches its highest concentration, when glucosuria occurs, and how rapidly the blood glucose concentration returns to normal.
In normal persons, the venous blood sugar value usually does not exceed 200 mg/100 ml blood and returns below 120 mg in two hours. In diabetes mellitus, the glucose level peaks above 200 mg and does not return below 120 mg after two hours. In renal diabetes, the blood glucose curve is normal. Glucose tolerance curves may produce different results at different times on the same person.
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