Preface: When I saw my endo last week, she praised me on my latest A1c level (6.2, Baby!), but said that my standard deviation score was "unacceptable." Yikes! What does this mean for my health going forward? ...
You've all heard of standard deviation, correct? It's that mathematical measurement that became popular a few years ago as a "back up" for showing how well a diabetes patient was doing with their glucose control.
Sure, you can have a good A1c, but we all know that a low A1c often comes with a lot of lows, and a lot of lows can come with rebound highs. So, having a low A1c may not the perfect measure of good D-health. The theory with using standard deviation in diabetes went so far as to assert that it could actually help predict how likely you were to develop future complications.
But now it turns out that our trust in standard deviation might be unfounded — while our trust in A1c remains inflated (ugh).
For those who struggled in statistics class, standard deviation, mathematically speaking, shows how much variation there is from the average. In diabetes, a low standard deviation indicates that blood sugar tends to be very close to the ideal mid-range most of the time. A high standard deviation, on the other hand, shows that a patient has a wide swing of blood sugar values, regardless of their A1c result.
The theory went that low standard deviation, i.e. less blood sugar swings, would be a good indicatior that a person's risk of complications is lower than someone with a high standard deviation, even if their A1c was higher than recommended (above 7%). This is supposedly so because the wild swings in blood sugar levels supposedly take a toll on a body — possibly even more so than having slightly elevated BG levels that remain steady.
"The fundamental question is does variability give you an independent risk for getting diabetes complications? It's the 500 pound gorilla and we just don't know," says Dr. Irl Hirsch, endocrinologist at University of Washington and a lifetime type 1 himself, who's been one of the most famous proponents of applying standard deviation to diabetes care. What's that? We don't know? Not enough evidence here?!
According to an April 2008 Diabetes article, analysis of the participants in DCCT found that A1c was only responsible for 11% of the overall risk for developing microvascular complications. That means 89% of our risk for complications is coming from somewhere else. But whether that's standard deviation, environment, genetics, or something else entirely, no one knows.
In a recent Journal of the American Medical Association article entitled, "Beyond HbA1c: Need for Additional Markers of Risk for Diabetic Microvascular Complications," co-authored by Dr. Hirsch and Dr. Michael Brownlee of the Albert Einstein College of Medicine, the authors state that it is "crucial" to find out what causes the remaining 89% of microvascular risk. They write, "Physicians will have to realize that much remains to be done in identifying important factors contributing to microvascular complications risk, which are not captured by the HbA1c."
And it's still unclear whether or not standard deviation actually plays a role in complications risk. Dr. Eric Kilpatrick, a doctor in the UK who contributed to a July 2006 Diabetes article, says, "We looked at the DCCT database and found that within-day glucose variability did not seem to contribute to the risk and therefore did not seem to help make up the remaining 89%." He adds that their findings were based at "face-value" of the DCCT study, and that "this is still speculation." Oy vey.
So the A1c may only contribute 11% of our overall risk of complications? Does that mean we should stop paying so much attention to it? Not exactly, no. It seems that patients whose A1c levels are all over the map may be at highest risk for complications. This notion is supported by a July 2008 Diabetes article that states, "The DCCT data has shown that in patients with type 1 diabetes, increasing variability in A1C adds to the risk of microvascular complications over and above that predicted by the mean A1C value alone." So variation in A1c levels over the years could be a real risk factor.
Confused yet? Yeah me too. The researchers from that last study cited admit that their findings seem to conflict with other results indicating that day-to-day blood sugar variations do not play a huge role in complications.
At the end of the day, what we learned from our conversations with Dr. Hirsch and Dr. Kilpatrick is that researchers really just don't know what role standard deviation plays in the risk for complications because this hasn't been studied as intensely as needed. Luckily, Dr. Hirsch was recently approved for a grant to study just this: the effects of standard deviation in 12 clinics around the United States starting in 2011. So we have some clarity to look forward to in the New Year, and will certainly keep you all (and my endo!) posted as results unfold.