Posted on 15 Jan 2015
Canadian health and care mall > Diseases

Health and Care Mall: Diabetes and Urine

URINE

I do not understand why it is that the glucose from the blood only spills into the urine above a certain level. I gather this level is known as the renal threshold – could you explain it for me in a little more detail? URINE

Urine is formed by filtration of blood in the kidneys. When the glucose concentration in the blood is below about 10 mmol/L, any glucose filtered into the urine is subsequently reabsorbed back into the bloodstream. When the level of glucose exceeds about 10 mmol/L (the renal threshold) more glucose is filtered than the body can reabsorb, and as a result it is passed in the urine. Once the level has exceeded 10 mmol/L, the amount of glucose in the urine will be proportional to the level of glucose in the blood. Below 10 mmol/L, however, there will be no glucose in the urine and, since the blood glucose level never exceeds 10 mmol/L in people without diabetes, they will not find glucose in the urine, unless they have a particular inherited condition called renal glycosuria.

Why does one not always get a true blood glucose reading through a urine test (as in my case)?

In most people, urine contains glucose only when the glucose concentration in the blood is higher than a certain figure (usually 10 mmol/L), so below this level urine tests give no indication at all of the concentration of glucose in the blood. The level at which glucose spills out into the urine (the renal threshold, discussed earlier in this section) varies from one person to another and you can assess it in yourself only by making many simultaneous blood and urine glucose measurements.

If you have a low renal threshold for glucose, you should abandon urine testing and rely entirely on blood tests to provide information about your diabetes. However, at early stages of Type 2 diabetes, when people are well controlled on diet alone, or perhaps with a single tablet, there may be a place for urine testing. Some people prefer this to pricking their finger for a blood glucose measurement and the strips have the advantage of being very much cheaper than blood glucose testing strips. If you use urine tests to monitor control of Type 2 diabetes, you should aim at keeping your urine free from glucose at all times and confirm with HbA1c measurements that you are maintaining good control.

I have a strong family history of diabetes. My daughter recently tested her urine and found 2% glucose. However, her blood glucose was only 8 mmol/L She underwent a glucose tolerance test and this was normal. Could she have diabetes or could there be another reason why she is passing glucose in her water?

It is very unlikely that she has diabetes if a glucose tolerance test was normal. If she had glucose in her urine during the glucose tolerance test when all the blood glucose readings were strictly normal, then this would indicate that she has a low renal threshold for glucose (as discussed at the beginning of this section).

If this is the correct diagnosis, it is important to find out whether she passes glucose in her urine first thing in the morning while fasting or only after she has eaten. People who pass glucose in their urine during the fasting state do not have an increased incidence of development of diabetes, and the condition (called renal glycosuria) is inherited. If, on the other hand, your daughter passes glucose in the urine only after meals containing starch and sugar, this condition sometimes progresses to diabetes.

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