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January 11, 2004

Blood Tests and Insulin Injections

Question from Dieppe, New Brunswick, Canada:

My son (diagnosed September 2001) is starting to complain that his fingers are getting sore. We are trying an alternate site tester for a few weeks. Today for the first time, I tested with his regular tester and the new tester on the finger for comparison. The results were 9.9 mmol/L and 10.0 mmol/L [178 and 180 mg/dl]. I then tested on his forearm with the new one and the result was 5.9 mmol/L [106 mg/dl]. Is it normal to have such a big difference between the arm and fingers? Can you explain why the difference? I'm assuming that if he is complaining that his sugars are low we are to check the fingers, and when he is feeling fine, we do the alternate sites before meals and adjust accordingly. I'm always on the Internet looking for information on diabetes, I know a lot about diabetes. But this is new to me as this situation is just starting to occur. In this case when testing on the arm, should we be adding 4 mmol/L (72 mg/dl) to the results to get a better idea of what his sugars are? Is there an average? If a person is not aware of these differences, the result could be too much or too little insulin. I want to make sure I understand how this works so I can play with the insulin properly and keep my son's sugars where they should be. Thanks for your time, a long question I know, but the more informed I am the better I can control the diabetes.


You are very astute. I’m not sure that I would base any conclusions of glucose values site-to-site based on one single comparison. You could have somehow shorted the sample as you point out.

The advantages of alternate site testing really hinges on the smaller amount of blood required, which then has lead to changes in the technology and special strips. But in theory, if you are using less blood, any error will be ‘magnified’ so to speak.

The US FDA has required comparisons of alternate site and fingerstick testing. The resulting recommendation is that which you intuitively concluded: If you find a low on alternate site testing, confirm with a fingerstick. Personally, I think a little common sense is required here. If you feel LOW and the alternate site confirms that, then just treat. If you feel well and then are ‘surprised’ with an alternate site low, then check the fingerstick before you just treat.

There are concerns that the degree of blood flow to the fingers relative to, say the forearm, can have a differing rate of glucose utilization and thus lead to discrepant values site-to-site.

Note that some meters are approved both for fingerstick and alternate site testing. For sore fingers, try resetting your lancet device for a lower setting. There are also a variety of moisturizing lotions to help soften the pads of the fingers.