Blood Glucose Self-Monitoring: Part 2

Monitoring Technique

This article is part two of a four-part series on blood glucose self-monitoring. The first installment, “Part 1: The Gear,” can be found here.

To measure the concentration of glucose in your blood, you put a strip in the meter, poke a hole in your finger to get a sample of blood, touch the tip of the test strip to the drop of blood, and wait for the result. It sounds pretty straightforward. But, of course, the devil is in the details.


Setting up

Let’s start at the start. Once you have your meter, strips, lancing device, and lancet assembled, what’s the first thing you should do? While there’s no right or wrong answer, the most efficient way to get started is to put a strip in the meter. Once the strip is in the meter and, if it’s a coded meter, the code has flashed, the meter will do a self-test and then signal that it’s ready for a blood sample. The amount of time a meter takes to do its self-test varies by brand and model. This self-test is the main reason it’s more efficient to turn on the meter by inserting a strip first, before you lance your finger for a blood sample. It’s better to have the meter waiting for your blood than to have blood drying on your finger while you get the meter ready.

Speaking of getting a blood sample…

Lancing issues

The next step in measuring your blood glucose is to get some blood for the meter, and thus we arrive at one of the debates surrounding self-monitoring of blood glucose: Do you first need to clean your finger with an alcohol pad? The answer is no.

At one time, cleaning the lancing site with alcohol was the standard recommendation for self-monitoring. This advice stemmed from unsubstantiated worries about infection from the environment. But times have changed, and the evidence simply does not support the need to clean the skin with alcohol prior to lancing it. In fact, using alcohol may be counterproductive, as it can dilute the blood sample and lead to an erroneous result.

The only situation in which cleaning a lancing site with alcohol might conceivably be helpful is in a hospital, where the potential for contact with infectious pathogens is higher than in the rest of the world. But even so, bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) and other hospital-dwelling menaces are tough bugs that are unlikely to be killed by lowly alcohol pads, and there’s no evidence that such pathogens can be spread through a fingerprick. A quick wash of your hands with soap and water will ensure that any surface residue that might interfere with the test is removed. Most notorious in this department is the residue of fresh fruit, but some brands of hand lotion, especially the scented kind, can also be problematic.

So your meter has a strip in it and is ready to rock and roll. The lancing device is loaded, cocked, and ready. Where should you poke? Anywhere between the fingernail and the first knuckle of any finger is fine. Some people like to use the side of the finger, some people like the pad, and some people prefer the top. It doesn’t make any difference. It really comes down to the size and shape of your hands and the type of lancing device you’re using. Choose a spot that’s easy to get a sample from, but don’t play favorites. Use different fingers, and vary the exact lancing spot each time so that you don’t develop calluses.

I also advise people to avoid their little finger, simply because it often seems to be more sensitive than the others. Also, many people have difficulty lancing their thumb, while others find it easy and even preferable. Again, there’s no right or wrong choice when it comes to selecting a finger, nor any right or wrong choice regarding the lancing location on the tip of that finger.

Where there can be a right or wrong technique, however, is in the pressure you bring to bear on the lancing device. Timid folks barely kiss the skin with the tip of the device, while others mash it down to the bone. It’s important to be consistent. Apply the same amount of pressure to the device each time you use it so that you can adjust the lancing depth, if needed, and get repeatable results. The goal is sufficient blood with minimal cursing.

If lancing your finger makes you release a string of expletives more appropriate for the locker room or the deck of a merchant sailing vessel, then you’ve got the depth set too deep, or the lancet is overused and dull. You should feel something, but unless you have severe neuropathy, which can make lancing painful, lancing should not hurt.

On the other hand, if you have to milk your finger for five minutes to get a decent-size blood drop out, then you aren’t lancing deeply enough. A very gentle squeeze around the lancing site should cause the blood to well up. If you “milk” the site too much, with many meters you’ll get an inaccurate result because the sample is watered down with interstitial fluid (fluid surrounding cells in the body).

How much blood do you need? Newer meters require laughably small drops, about the size of a pinhead. Older meters – which, as we discussed in Part 1, are making a comeback – need a significantly larger drop of blood. Failing to feed either type of meter enough blood will result in a frustrating error message at best, and a false reading at worst.

Many test strips have a viewing port at the top of the strip to indicate visually if they’ve received enough blood for a proper test. But as a means to double-check, there should always be a small amount of extra blood left behind on your finger, showing you that the strip drank its fill.

Now, what should you do with that extra blood? Most nurses and doctors are horrified to learn that most people with diabetes, especially the ones who test a lot, just lick it off their finger. Social norms aside, there’s no harm in this. Other people carry a tissue or cotton pad in their meter case to dab off their finger after testing, and that’s fine, too.

To dispel another piece of legacy mythology, it is no longer necessary to do a dual wipe: wiping away the first drop of blood and testing the second drop. In the early days of blood glucose testing, the amount of blood needed was much larger and the speed of the tests was much slower. The first drop of blood from a lancing site contains a greater volume of platelets, which could make the lancing site seal up before enough blood was obtained for the test, and the dual wipe ensured a longer, larger flow of blood. Additionally, the first drop of blood can contain a greater volume of interstitial fluid and/or higher potassium levels, either of which could throw off the results of the first generation of test strips. With the enzymes used in today’s test strips, and with the lower blood volume requirements and greater speed of testing, a dual wipe is obsolete and unnecessary.

Fingertips versus forearms

What about those TV ads that say you don’t have to poke your finger anymore? This particular type of misleading advertising has caused me no end of clinical grief and has burst an untold number of patient bubbles. Let me be clear: Not poking a hole in your finger means poking a hole in your arm instead.

Sorry, but there’s only one “noninvasive” meter in the world right now, it’s not available here in the United States, and it’s only accurate enough to be used in screenings for diabetes, not for regular monitoring. For years, dozens of companies have been working night and day to develop some way of accurately measuring blood glucose without breaking the skin, but so far it’s a road littered with bankruptcies.

Those misleading TV ads are making an oblique reference to alternate site testing, which means taking a blood sample from a less sensitive, relatively low-circulation area where capillary glucose lies just below the surface of the skin. This is most commonly the forearm, but other areas approved by the US Food and Drug Administration (FDA) include the upper arm, palm, thigh, and calf. Compared to fingertips, there are significantly fewer nerve endings in alternate sites, so testing is relatively painless. But keep in mind that if it’s done correctly, fingertip testing shouldn’t be that painful in the first place.

But while the FDA has approved the vast majority of today’s meters for alternate site testing, the author of this article has not. Here’s the deal: There are a number of problems with using blood samples from alternate sites, and chief among them is that the glucose level of blood drawn from them is old news. Your fingertips are a high-circulation area, which makes fingertip blood glucose an excellent proxy for venous blood glucose (what lab tests measure). Alternate sites, however, are a bit like a stagnant area just off the main channel of a river. Sure, fresh water gets in there, but it’s not getting the kind of changeover you’d see in the middle of the stream.

The glucose level in the capillary bed of your forearm could easily be as much as 20 minutes out of date. If you were testing just once in the morning (and you’d better not be, but more on that in a later installment), then a 20-minute delay wouldn’t be such a big deal. But when you’re experiencing hypoglycemia and your blood glucose level is falling as fast as a rock from a cliff, a lot can happen in 20 minutes – including seizure, coma, and death.

If you read the fine print of any manual that comes with a meter approved for alternate site testing, you’ll see that – in less blunt terms than I use – this risk factor is acknowledged, and users are advised to switch to fingertip testing if low blood glucose is suspected. Here’s the problem: People are creatures of habit, even in life-and-death situations. In fact, studies of police officers under fire suggest that we humans are even more likely to resort to habit in times of stress. In 1970, four young police officers in California were slain in an epic shootout with a pair of ex-cons. After the smoke cleared (literally), it was discovered that one of the slain officers had taken the time to put the empty shells in his pocket while reloading his pistol. Why would someone waste precious time, exposing himself to hostile fire, to do this? The answer is: habit. As cadets, the officers were taught to pocket their used shells to keep the firing range neat and orderly.

Police firearm training has since changed.

I submit that we have an additional risk factor in “shootouts” with our diabetes. When our blood glucose is low, we are not at our mental best. I like to joke that IQ drops with blood glucose levels, but this is actually true to some degree. Cognitive function takes an enormous amount of fuel, and glucose is brain fuel. When blood glucose drops to low levels, the brain literally doesn’t have enough fuel to run correctly. People with diabetes experiencing extreme lows are often mistakenly thought to be intoxicated or drugged, and more than one has ended up being arrested and thrown in jail as a consequence.

This is why it’s important to test blood from a location on your body that is the best proxy for the blood in your veins that flows to your brain. If you do this all the time, you won’t have a habit you’ll need to break at a time when your brain is running out of fuel. You’ll already be using the technique that befits the emergency.

Beyond the safety issue cited above, there are other problems with alternate site testing. It’s actually more work to get blood from your forearm than from your fingertips. Blood at the fingertip is right at the surface – a tiny puncture brings it up into the air – while blood in the arm is deeper inside and needs to be pumped out.

Alternate site testing requires heating the skin using friction (rubbing your hand vigorously back and forth will do) and putting a special clear plastic cap on the lancing device. The clear cap has two functions: It positions the lancing needle for a much deeper thrust at all depth settings, plus it lets the user view the lancing site. This last part is critical, as a simple poke isn’t going to produce blood at an alternate site. After lancing, the device will need to be pumped up and down a number of times to force blood up though the skin. The tip of the clear cap is concave, pushing the tissue around the puncture into a domelike shape and forcing blood out of the skin. All of that is a lot more work than a quick prick and gentle squeeze of a finger.

If all of these reasons still aren’t enough to dissuade you from alternate site testing, consider that it leaves bruises on many people. A bunch of bruises on your forearm isn’t going to help if someone mistakes your behavior during an episode of hypoglycemia for drunkenness or being under the influence of drugs.

Alternate site testing has been a marketing boon for meter manufacturers, allowing them to target people who are having a hard time with fingertip lancing. It’s high on my list of things that should be banned: It isn’t necessary; it’s less accurate, especially in the case of rapidly changing glucose levels; it takes longer; and it has negative cosmetic side effects.

Putting blood on the strip

Now that you have the meter ready to go, and an adequate drop of blood waits on your fingertip, is there any protocol for how to actually do the test? Yes. The best strategy is to bring the meter in at a 45° angle and touch the end of the strip to the very edge of the blood drop. The strip will suck in the blood like paper towels cleaning up spilled wine in those old TV commercials.

Don’t try to apply blood to the strip; the strip is designed to draw the blood into itself. Let it do its job. Also, shoving the strip deep into the blood sample seems to overwhelm the strip in many cases, leading to an error message and requiring you to start over with a new strip.

Generally speaking, the higher your blood glucose level, the longer a meter takes to display its reading. If your meter is silent for a while after the strip has sucked in the blood, know what to expect. Most of the meters on the market today take 5–10 seconds to test blood samples and display results, but some meters can still take as long as the full minute the original 1978 Ames Eyetone took.

OK, enough about how to monitor. Part 3 of this series will cover when you should monitor and how often to do it. In the meantime, click here for some blood glucose self-monitoring field tips.