February 21, 2002
Blood Tests and Insulin Injections
Question from Michigan, USA:
At a clinic visit last Fall, my almost three year old son had a fingerstick A1c performed and the reading was 8.2%. Several days later, I took a venous sample to the hospital lab where I work where we performed a "glycosylated hemoglobin" and the result was 6.8% (normal range = 0-6.9%). Two months later, his fingerstick A1c was 8.1%, and, once again, I did a comparison with the hospital lab, wondering if it was a "fluke" that there was such a discrepancy in the results. However, the venous glycosylated hemoglobin was 6.7%. What, exactly, is the difference between the "A1c", and the "glycosylated hemoglobin"? Why would there be such a large discrepancy in these results? Which result should I believe?
Hemoglobin A1c and glycosylated hemoglobin are essentially the same thing, but the tests are not standardized. Labs can have the test standardized to the DCCT. lab, but this is voluntary and not required. So, you really cannot compare results from two different labs most of the time. What is normal for the two labs? Are they standardized to DCCT?
My best advice is to use one test and not shop around. Finally, both results are correct most likely — just different.
[Editor’s comment: agree completely with Dr Deeb. Since there is so much variation in methodology, you cannot compare results, and you need to evaluate results with others done the same way and compare them to the normal for that method in that lab (or clinic). Standards require validation of testing procedures with blind, known samples, so you can be reassured of accurate results. For these reasons, rather than state an absolute value, the American Diabetes Association recommends having an A1c (or glycosylated hemoglobin) less than 1% above the upper limit of normal for the lab performing the test.