Sunday, June 25, 2006 | 12 comment(s)
Ladies and Gentlemen!Step Right Up!
Admission is FREE!
Don't Be Afraid!
Kids, don't try this at home!
Ahhhh, the H(a)emoglobin
ApparentlyThe Brits spell it "Haemoglobin" but in the States the 'a' is dropped. From here on out, I'll just use "Hemoglobin" but I wanted to give a nod to the folks over there on the other side of the pond. Oh and this is kinda interesting...(man, I love wikipedia!).
At Least…We diabetics are certainly familiar with it, whether friends and family members (and thus some readers of this blog) are familiar with it, is another matter completely.
For those who don't know, the Hemoglobin A1c test is the 'gold standard' test of how well controlled one's diabetes has been over the previous 3 months. Below 5% is the normal, non-diabetic range, and while the American Diabetes Association suggests diabetics shoot for a reading of <7%, the American Association of Clinical Endocrinologist recommends a more stringent reading of <6.5% as the target for diabetic patients.
More Detail (just in case)Hemoglobin are protein structures that are contained in your red blood cells. Lab tests measure how much glucose binds to these proteins. Because red blood cells are constantly dying and being created, the HbA1c test can give a measure of how much glucose one has had floating in their system over the lifespan of the red blood cell population (which is approximately 3 months). Since this population of cells is constantly in flux, HbA1c results are essentially a weighted average of blood sugar control. That is: about 50% of the reading represents the previous month's control and then 25% represents the previous two month's control, and the final 25% measures the previous 3-4 month's worth of control.
So I went to my endo appointment on Thursday morning and I got my test result back. As I wrote about previously, I was pretty anxious / excited about getting the test done this time 'round. In that post, I said I'd be "dancing a jig if I [hit my target A1c], and crying to fugues if I don't." Well, I have my result, and wouldn't say I'm dancing or crying, really. Instead I'd say I've got quite a salad of emotions about the dumb number. A partial list of ingredients would include: pissed off, confused, determined, depressed, and sprinkled with a little bit of happy, too (just for the crunch). Soooo... (without any further ado), my A1c reading was 7.1,
I know, I know...This is a decent reading... a good reading, even. And I may be lambasted for complaining about such a score, and if so, I completely understand. I realize that HbA1c scores can be a sensitive topic (not unlike asking how one scored on a final exam or asking how much someone's salary is). And I've felt this way before too. When I've seen or heard people complain about blood sugars that bounce above 200 occasionally or A1c's that are in the low 6's or even the upper 5's I think they're crazy to be complaining and I get a little angry, too. But I think that largely stems from envy. But overall I'm happy for them, and I respect the amount of discipline they probably must exert to attain such great scores on a consistent basis. And so, I apologize if anyone reading this thinks that I have nothing to complain about, but I'm a wee-bit anal retentive and I've been particularly goal-oriented on this lately. Please excuse me.
Like I mentioned before, I've been busting my ass to maintain the best control I can (and I think that I have probably gotten to the best control that I ever have had). I've kept detailed records, tested my blood sugar many, many times per day, I've been eating the right things, I've been exercising, I've even lost some weight, yadda, yadda, yadda, etc., etc., etc., ad infinitum, ad nauseum. So beforehand, I was looking for a conversion chart between what my average bloodsugars have been and what I might expect for my A1c reading. I came across the following table in various formats and in various locations (though this one is lifted from the Children with Diabetes web page on the HbA1c test. And this table seems to originally come from a 2002 "Diabetes Care" article that analyzed data from the Diabetes Control and Complications Trial (DCCT) and I've seen the same table in numerous other places including ADA (which publishes "Diabetes Care") and NIH (i.e., reputable sources).
|A1c (%)||Mean Plasma Glucose|
|From Diabetes Care - 26 (Supplement 1): Table 1|
Based on this table, my 7.1 HbA1c reading indicates that my average blood sugar has been around 170 mg/dl. But based on the data I've collected with my glucometer, I again have to protest: "This simply cannot be!" Below is a table of data from my glucometer for the 7, 14, 30, 60, and 90 days prior to getting the blood drawn for the HbA1c test:
|Kevin's Average Readings|
So, a few things: my average blood sugars tend to be around 150 mg/dl (not great, but not awful, either). I test like a maniac -- on average about 20 times per day. And my standard deviation is in the mid-to-upper 60s (I believe I've read that one should be aiming for below 60 in terms of S.D. as there's a bunch of new research suggesting that the amount of glycemic variation one has may be more important than the actual average level in terms of complication risks).
I then took the HbA1c-to-Mean Plasma
This is importantSo apparently when a lab takes a sample of blood to perform the HbA1c test, it filters out some of the blood first (though I don't know which parts or why) and only tests the blood plasma. When we prick our fingers, however, we are getting whole blood samples. The concentration of glucose in the whole blood sample is therefore slightly lower than in the plasma basically because the denominator is larger. Most meters these days do a quick internal calculation to adjust for this fact so that the readings displayed on your glucometers are in "plasma equivalents." (I think that almost all glucometers simply multiply your whole blood reading by 1.11 to get the "plasma equivalent").
Until about 6 months ago, I was still using the OneTouch Profile II glucometer. This might be the only meter on the market that still produced results in whole blood values (and in addition took 45 seconds to give up it's results (I really have no idea why it took me so long to upgrade to a more modern glucometer)). But I have actually had arguments with my endocrinologist about the importance of this matter when I originally started to notice a disconnect between what my average readings were telling me and what my HbA1c readings came back as. She basically mocked my concern and told me that there is no difference and that there are no meters that still use the whole blood measure -- two facts that are patently wrong and only a small portion of the list of why I need to find another endocrinologist (but that's a whole 'nother post).
|A1c (%)||Mean Plasma Glucose|
Based on this table and my glucometer readings, I was going into the test expecting a HbA1c result in the 6.5 range. 7.1? 6.5? What's the difference, right? (0.6). I know I shouldn't be upset about this, but I am. I even argued about it with my endocrinologist. Since then, I've been doing research on my own and reading medical articles with such riveting titles as: "Effects of Hemoglobin Variants and Chemically Modified Derivatives on Assays for Glycohemoglobin" and "Defining the Relationship Between Plasma Glucose and HbA1c." The later of which seems to be the original source where they analyzed average plasma glucose readings and statistically
Be ThankfulI originally intended to go over the linear regression coefficients and discuss their methods of calculating "average plasma glucose" but that would have required yammering on about arithmetic averages, weighted averages, the "area under the glycemic curve" and then perhaps some integral calculus and THAT would have been overboard.
************* KEY QUESTIONS *************
So before I wrap this up, here are my questions for you:
Do your average glucose readings from your meter fit well with the expected HbA1c readings you get back from the lab (or from an HbA1c meter if you're lucky enough to have access to one at your endocrinologist's office)? Or do you regularly experience a disconnect between what you would expect your A1c reading to be based on what your average blood sugar readings have been?
To make a short story (even) long(er), I actually had a new blood sample drawn to be sent to a different lab on Thursday. In addition to an HbA1c test, I am also having a hemoglobin electrophoresis test done to see if I have any abnormalities in my hemoglobins that may bias my HbA1c test scores. I'll be sure to report back what I find out.
And so I'll wrap up this long-ass post with a selection from
Defining the Relationship between Plasma Glucose and HbA1c:
Several studies have suggested that, although intraindividual variation in HbA1c is minimal, there is evidence of wide fluctuations in HbA1c between individuals that are unrelated to glycemic status, suggesting that there are "low glycators" and "high glycators" (14–16). However, a recent study showed that when multiple observations per patient are used to minimize the effects of assay variation, the interindividual range of HbA1c results in nondiabetic individuals is actually quite narrow, <1% HbA1c (17). Therefore, for any individual patient, a consistent discrepancy between patient-monitored PG determinations and estimated HbA1c should be investigated; there may be other factors causing this discrepancy, such as improper meter use, laboratory error, a physical condition that alters red cell life span, or a variant hemoglobin interfering with the HbA1c assay method.