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The body usually is able to keep glucose concentrations stable. The normal fasting
blood sugar is usually between 3.5-6.7mmol/l. After a meal it would rarely exceed5
8mmol/l. Normally there is no glucose in urine since the normal threshold above
which glucose would appear in the urine would be 10mmol/l. Below a concentration
of 10mmol/l the kidneys reabsorbs glucose back into the blood stream and so glucose
does not appear in the urine unless the blood concentration of glucose is high. Dipsticking urine for the presence of glucose is therefore often used as a screening test for
diabetes mellitus.
The diagnosis of diabetes mellitus is made by finding a fasting blood glucose of over
6.7mmol/l or a random glucose of >10mmol/l. If a patient presents with symptoms of
diabetes and is found to have a single very high glucose measurement eg >15mmol/l
then this can be diagnostic. More commonly it would be appropriate to ask the
patient to fast overnight and attend for a fasting blood glucose to be taken the next
morning. Ideally this should be performed on two occasions before diagnosing
diabetes.
If there is any doubt about the diagnosis then a further test can be performed. This
test is called the oral glucose tolerance test and it measures how the body responds to
a glucose load. The patient is asked to fast overnight and then attends for the test.
The patient has a blood glucose level taken and is then given a drink, which contains
75gm of glucose. After two hours another blood sample is taken. From the results of
the glucose tolerance test the patient can be either diagnosed as having diabetes,
impaired glucose tolerance or no abnormality of glucose handling.

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