Using Alternate Test Sites

Fingers get tired of being poked all the time. Some people use alternate-site testing (AST) to give fingers a rest. But are those alternate sites accurate? Choosing the right site can make a big difference.

A recent article on Diabetes In Control told a scary story: A woman in her 30s, diagnosed Type 2, was “slender, active, [and] did carb counting.” She had a log of her glucose tests, and they were “awesome.” Yet her A1C was extremely high, possibly as high as 15. How could this be?

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Maybe her glucose meter was wrong? The diabetes educator had her bring her meter in and checked her glucose from her thigh, which was her usual site. Both her meter and the hospital’s meter read the same, about 67. The meter was OK. The woman said she would treat this as a low by eating one carb choice.

Then came the shock. They did a fingerstick glucose check with both meters. Her reading was over 305! The problem was the site she was using. This woman was heading for severe complications while having excellent glucose readings, because her alternate site didn’t work for her.

Why should different sites give different glucose readings? It’s because some areas of our bodies need less blood than others, so they get slower circulation. They don’t have the latest blood, so they don’t have up-to-the-minute news about the blood’s glucose level. It’s like reading a day old paper to find out today’s weather.

Fingertips have very good blood flow, so they are usually the most accurate sites to use. Other sites can be OK, but you need to test them on yourself to see how they compare with your fingertips.

Some meters are approved for alternate sites. That means they have shown they give results within 20% of correct values when used on particular alternative sites, such as the palm, forearm, thigh, or abdomen. However, you should still check alternate site readings on yourself before trusting them too much.

Fingertip monitoring is especially important when glucose levels may be changing rapidly, like after eating or exercise or when you have low blood glucose or are ill. That’s when you need the latest news.

Continuous glucose monitoring (CGM) is probably even more accurate and less painful than fingersticks, so you might consider that (although it is important to note that CGM readings shouldn’t be used to make treatment decisions). We wrote about CGM here.

The only way to know if an alternate site is accurate for you is to check it a couple of times against a fingertip test to be sure you get similar results. Because of the slow-news effect, you should not do this comparison shortly after eating or exercising, nor upon waking up, when the dawn phenomenon (early morning blood sugar spikes) could throw you off.

The articles I read advise that AST works for some, but not for all. If your glucose tests don’t seem to match up with your A1C results, check the meter and the way the tests are being done. When numbers are not as expected, use fingertip monitoring.

There is a lot more to say about AST, and we have a good article about it here.

What has been your experience with alternative-site testing? Does it seem to work for you?