Fasting glucose tests

As most people with Type 1 diabetes will know, when visiting your endocrinologist they’ll usually scan over your latest lab blood test results in the appointment. There are many important things tested there, but I’m talking here only about the numbers that relate to blood glucose.

Blood glucose (BG) is our primary indicator of how our diabetes management is going, especially in the moment. We live by these numbers. The checks we do during the day (either spot checks with a fingerprick or a glance at our CGM) are what guide our intake of insulin and carbohydrates, let us decide whether we’re safe to drive a vehicle, etc.
Low BG can kill us, and high BG will increase the risk of secondary health complications (as well as putting us at risk of DKA, which again can kill us).

Our average BG (which can be measured via the HbA1c lab test) has been used for years by clinicians as a rough measure of our management, but we know that because it doesn’t measure the highs or the lows (just the average) it’s an incomplete measurement by itself. “Time in range” and other measures give more indication of how much time we’re spending “in range” (i.e. without highs/lows) and gives a better picture than just HbA1c by itself. But to measure this needs continuous measurements throughout the day¬†(or at least frequent: CGMs usually record every 5 minutes), and not everyone has this. Apparently only 1-3% of insulin users worldwide have access to CGM. Without CGM all you can do is a series of spot checks throughout the day, and try to capture the post-meal highs and other details of the continuous BG variations.

HbA1c is measured in either mmol/mol (which in Australia is the “official” unit) and also in the older “percentage” format (which is also used in the US). It can be converted to an indication of an average BG value (for example in mmol/l) for the prior 3 months. The blood test to measure HbA1c does NOT require you to have fasted.

The other test result some doctors look for is a “fasting glucose” value (reported in mmol/l just like your BG meter). It’s being interpreted as the level your body is able to maintain after a solid sleep (with no food intake to mess things up). Unfortunately for those of us on insulin (as in everyone with Type 1) our food intake the previous night and insulin activity overnight (along with any overnight hypos) can mean this value changes a lot from day to day. So I’ve always wondered how useful this test is for anyone with Type 1 diabetes.

Obviously this test does require you to have fasted, and getting to the blood collection location safely before having had any breakfast is often a complicated process for us. Not only do we run the risk of a hypo, but many people have significant “dawn syndrome” or “feet on the floor” syndrome which means that their liver seems to release glucose into the bloodstream automatically at the start of the day, and a BG test even 30 minutes after getting up doesn’t always have a good relationship to the overnight figure.

I used to often deliberately run my sugars a little higher than normal for the night before a fasting test so that I would be able to get myself to the hospital without worrying about a hypo on the way. And then just laugh at any comments from my doctor about a “high” fasting glucose (e.g. 7.0 mmol/l). For many of us it seems like a test that gives no useful result!

Is it needed?

I think the only other common test we have that requires fasting is that for cholesterol, which is often not done every regular test. So why do doctors keep getting patients with Type 1 to have fasting glucose tests every 3 months or so? Even without CGM we can test our blood glucose when we wake up.

Is it that they don’t trust us or our home BG meters and CGMs to record accurate results? Do they think we’re going to fudge the system somehow because we’re ashamed of our actual numbers? Things have come a long way from the days of having to keep written journals of BG results (today meters remember things).

These days I take CGM reports to my endo and he can see how my BG has behaved, in much more detail than just HbA1c and fasting-glucose could tell him. In fact the CGM gives a prediction of the HbA1c result, and for me it’s always been very close. The actual HbA1c test now only really acts as a verification that my CGM data isn’t “off” in some way.

And my closed-loop system (which links my CGM to my insulin pump) means that usually my fasting glucose tests are somewhere around 5.5 mmol/l.

I’m hoping that my endo will soon get out of the habit of ordering fasting glucose tests for me. It might be useful for some patients who don’t have access to other data, but it seems like a waste of time otherwise.

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