Comments on "HbA1c for screening and diagnosis of diabetes?"
By Sten Westgard, MS
Dr. Westgard's recent essay on HbA1c for screening and diagnosis of diabetes and an accompanying post generated an interesting reply. Offered here in its entirety:
Jim,
Sten,
I would like to
comment on your article on HbA1c for screening and Diagnosis. First, the
comment by Sten that the tighter CAP criteria are not addressing the issue of
bias. It does indeed address both bias and imprecision because the CAP is using
an accuracy base - the NGSP assigned values. So if a method is either biased
from NGSP or shows variability within or between labs, this will decrease the
lab's chances of passing. The CAP no longer uses peer group grading for HbA1c.
I'm not sure what
you mean by "scientific marketing" and if this should be taken as a negative
comment. Sure manufacturers would like to see HbA1c used for screening since
this would increase use of the test. But use of HbA1c for screening has been
discussed for a long time and the discussion is based on very sound
arguments.
The main reason to
use HbA1c rather than fasting glucose is not for "convenience of any particular
group". It, in fact, will get people screened and diagnosed that may not
otherwise be diagnosed. There is still a large group of people who have
undiagnosed diabetes. This is why so many people (25% of those diagnosed) have
complications at the time of diagnosis! Using HbA1c will allow screening and/or
diagnosis more readily e.g. at regular doctors' appointments where patients
usually don't come fasting. Furthermore, many physicians already use HbA1c but
there are no specified cutoffs so it's hard to know exactly how it is being used
and it may not be used appropriately.
Another drawback of
fasting glucose is that it has a large (much larger than HbA1c) biological
variability. And measurement of glucose isn't perfect
either.
The screening
cuoff for HbA1c of 6% is equivalent to 125 mg/dL AVERAGE GLUCOSE, not fasting
glucose. The equation (eAG =28.7*HbA1c-46.7) gives you a mean blood glucose,
not fasting. You can't easily equate a patients mean glucose with his or her
fasting.
And I disagree that
discussions of using HbA1c for diagnosis/screening are guided by market forces
and not quality of care. The main goal is to get more people with undiagnosed
diabetes diagnosed. It is not for the pockets of manufacturers or scientists.
Randie
NGSP Network
Coordinator
Co-Director Diabetes
Diagnostic Laboratory
Depts. Of Pathology
& Anatomical Sciences and Child Health
University of Missouri School of Medicine
Columbia Missouri
Just to be clear, we meant no disrespect of the NGSP efforts in either the essay or the post. Indeed, they are doing far more to improve quality than most other organizations of a similar nature.
As they say, it's important to be able to disagree without being disagreeable. We welcome feedback, comments, and different points of view.
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