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GD, Hb A1c, GTTFrom: Bert Gold (bgold@ktb.net)Tue Mar 23 23:09:51 1999
I really appreciated Dr. Braun's reminder that screening doesn't mean confirming, but weeding out and using Bayesian (statistical and probabilistic) principles to confirm. That said, I still feel a little confused by resistance among the GD cognoscenti to research Hemoglobin A1c as a worthwhile indicator of GD. I thought of this about three years ago, while reading relevant chapters in 'When to Screen in Obstetrics and Gynecology', during my genetics fellowship at UCSF. It came up especially during discussions I had at Stanford about what factors were most importantly screened for during pregnancy. The Ob-Gyn author of the chapter on GD of the aforementioned review book never returned my phone calls to UCSF, when I inquired about this issue. I recently posted a query about this on a MedLab list [Knowing that there was a raging controversy going on about GTT screening here]. One Harvard physician (pathologist), wrote privately back: "If haemoglobin concentration is constant, then Hb A1c reflects a time averaged glucose concentration. Older RBC, having had more exposure to blood glucose, have proportionately more glycated haemoglobin, but this averages out. In pregnancy, however, haemoglobin concentration is not constant: blood volume is expanding, and there is a relative increase in young and relatively unglycated RBC. So even though a decrease in diabetic control (and an increase in average glucose level) is typical in pregnancy, equally typically, Hb A1c levels are _lower_ than they would otherwise be." So, this physician's opinion, if I understand it correctly, is that A1c would underscreen GD (have too many false negatives), based on the higher total blood volume and younger erythrocytes. One MT from Hopkins wrote back (also privately): "I guess my point is, it's common for pregnant women to present with iron-deficiency anemia. Most types of anemia will falsely lower hemoglobin A1c results (because of the same type of "lab logic" that I used in my example, substituting hemoglobin A1c for hemoglobin A2)--including iron-deficiency anemia. Therefore, you may get a falsely normal hemoglobin A1c result on a patient who has gestational diabetes. In fact, I've been diabetic for 33 years now, and my endocrinologist asked me about a year ago if I was anemic because he thought my hemoglobin A1c result was too low." I guess one of my points is that nobody wants to be quoted on this contentious issue (hence the private email responses to my posts). Another point is that I am sad that I haven't gone to medical school. But, I guess the real point of my writing this lengthy discussion is that I am trained as a scientist, and that IMHO alot of this reasoning is supposition and not science. I would still be interested in seeing a study of Hb A1c in pregnant ladies and whether or not there exists a relationship to GD. I feel pretty sure, based on attending clinics during genetics fellowship at UCSF while on faculty at UMDNJ, Temple and Jefferson, that GTT is not an infallible screen for GD. Please send data if you know where it is. [Disclaimer: These opinions are my own, not the property of my employer.]
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