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August 17, 2002

Blood Tests and Insulin Injections

Question from Cicero, Indiana, USA:

At a recent support group meeting for families of children with diabetes, a sales rep from a meter company stated that those with diabetes should not use anything other than their fingertips for testing if they've eaten within two hours. She said the reading could be inaccurate due to the recent consumption of food. This was not a misunderstanding. We questioned her about this statement. Is this true? If so, should those who eat every two hours or so use alternate site testing? Most children with diabetes are eating approximately every two hours during their waking hours, this would mean the results could be inaccurate, thus potentially dangerous.


The story is perhaps a little different. In a study of the FreeStyle meter at this center where the subjects were persuaded to take measurements every 15 minutes from both forearm and finger tip over a six hour period (there was a significant reward!) that included an insulin injection and a meal the following conclusions were reached:

The flexor aspect of the forearm needs always to be rubbed vigorously before taking a sample.
There were no clinically significant differences between the two sites when blood sugars were stable or were rising after a meal.
When blood sugars were falling rapidly there was about a 20 minute delay between the forearm reading and the fingerstick.

Because the meters that read from the forearm have the important advantage that they are essentially painless, a considerable asset in these days of pressing for ever better control the compromise was to tell parents that they should rely on the forearm reading except when there were symptoms of hypoglycemia. There were no subjects who had hypoglycemia unawareness which would also be a temporary contraindication.

Additional comments from Dr. Jim Lane:

Good question. As you know, there is great pressure from patients with diabetes and their families for less invasive means for blood sugar monitoring. One alternative is to try testing blood from other sites besides the fingertips. The fingers are very sensitive and have the disadvantage of a limited surface area, especially in smaller children. There are even meters on the market that are advertised as being acceptable for monitoring on other skin surfaces besides the fingertips. However, there are some problems with interpreting the data generated by monitoring at sites other than the fingertips.

First, the blood supply to the skin over the arms is less generous than the fingers. This means that with rapidly changing blood sugars, the results over the limbs will be slower to change than on the fingertips. The consequences of this would be to underestimate a rising blood sugar or get a more normal blood sugar when the value is truly rapidly falling. This is the reason for the conditional instructions for using monitoring at sites other than the fingertips.

My feeling that is that the only thing worse than no information is bad information. I am hesitant to fully endorse monitoring at sites other than the fingertips at this time. There is always the feeling in the back of mind that I may not be getting the most accurate data.

Additional comments from Jane Seley, diabetes nurse specialist:

It is true that there may be a difference in the blood sugar test result from fingertip to forearm under certain conditions. Since the blood flow in the fingertip is greater than alternative sites, the lag time may be from five to twenty minutes for the alternative site to “catch up” to the fingertip. The FDA agrees that the good outweighs the bad and has decided to continue to approve alternative site testing equipment.

There are several things you can do to get the most accurate results. One important way to minimize any difference is to rub the alternative site first to increase the blood flow. Another is to avoid using an alternative site when the blood sugar is rapidly rising or falling (such as post meals and during exercise) and whenever you suspect hypoglycemia (low blood sugar). I advise my patients to use the fingertip for the two hour post meal test and use the alternative sites for pre-meal testing. This gives them the welcome opportunity to give the fingers a rest!