i started out in nursing in the days when we did urine glucose testing four times a day on our diabetic patients. how i remember the pain in the arse it was to have the patient empty their bladder and then go back a half hour later to get those few precious drops of urine (if the patient could give them) and carry them in a plastic medicine cup to the utility room where we put 5 drops of urine and 10 drops of water into a test tube and dropped a clinitest tablet into it. we watched the bubbling and then compared the color of the resulting liquid in the test tube to a color chart to pronounce the patient none, 1+, 2+, 3+ or 4+ glucose. you may have done something similar to this in your chemistry class at school.
glucose does not spill into the urine until the circulating blood sugar excedes the renal threshhold for it which is somewhere around 180. so, when a person's clinitest, or urine glucose is showing positive for glucose their blood sugar is going to be elevated above normal levels. however, the clinitest tablet, or it's more easier version, the dipstick, can only give you a possible range of what the blood sugar might be
. so, that is part of the answer to your question.
a big problem with urine glucose testing is that if the urine specimen is not collected correctly the test results may be wrong. i knew of nurses who were in a hurry and took whatever urine the patient was able to give them and test it. the problem was that if the sample came from a bladder full of urine that had been collecting in the person over several hours, they were getting a mixture that was not reflective of what was going on "now". this was a big, big problem. especially when the patient was receiving sliding scale coverage. there were also patients who for some reason or another just couldn't squeeze out those few drops of urine (we only needed five drops, remember) a half hour after voiding. now, you need to realize that one of the symptoms of elevated blood glucose is excessive urination. this is because the kidneys are trying to dump the excessively high amounts of glucose in the blood, a homeostatic compensatory mechanism.
another big problem with urine glucose testing is that there are medications the patient might be taking that will give you false positives on this testing method. some of the antibiotics and steroids were a big problem. we used to do urine glucose testing on people who were on prednisone because prednisone increases blood glucose.
so, there is a little history on urine glucose testing for you. to get back to your original question which is "why blood glucose testing is recommended to monitor glucose rather than urine dipstick testing". . .the other half of the answer is because a testing of blood glucose is far more efficient and accurate
. compared to using urine, blood by fingerstick is far more easier to obtain. you will also not have to deal with substances such as ascorbic acid or antibiotics that may caused false positive results like they can with clinitest tablets or dipsticking.
i am listing a web page for you to check out:
the piddling pup (a tale of a pedigreed piddlin pup ten piddles and a puddle) :chuckle http://www.user.shentel.net/nbaker/laffs/limerick.htm
it is a bit of fun for you, but will help to reinforce one of the symptoms of hyperglycemia which is excessive urination (due to the kidneys trying to clear the high glucose content from the blood). my uncle used to recite this poem for us as an entertainment when we were children back in the '60's. i don't know that he understood the meaning of the final lines of the poem although we, as nurses, most certainly do. have your speakers on because this particular version has the music that schroder plays from the peanuts cartoons accompanying it. enjoy!