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RE: [IP] Re: A1C

<< Professional mode = ON >>

*When discussing laboratory results, we are very close to what I do for a
living.  I'm a statistician in the Division of Laboratory Systems, Public
Health Practice Program Office, CDC.  One of the branches within this
division is responsible for establishing the standards that laboratories are
required to meet in order to be certified to do the specific tests.
Certification is required in order to be compensated by HCFA (Health Care
Financing Agency) for any tests done for Medicare patients.

When the subject of HbA1c testing came up once on another list, I went to
those who work with these tests and I asked.

Ryan Suchy [mailto:email @ redacted] wrote:

> Now I am confused.  Which is it, An average or NOT?????
> Kristin

It depends on what you mean by "average".  It is not a simple mean or median
of one's blood sugars.  During the life of red blood cells, a portion of the
A1 components of the hemoglobin (of which there are three fractions: A1a,
A1b, and A1c) binds to a carbohydrate.  This attachment is glycation (or
glycosylation), and the total component is called glycohemoglobin.  The
*rate* of attachment is proportional to the serum glucose levels.  IOW, the
more glucose in the serum (or blood), the more glycation occurs.  Because
the life of any one red blood cell is known (six to eight weeks) and we can
measure the rate of glycation (in vitro), we can determine (calculate) a
weighted measurement of the mean serum (or blood) glucose levels over the
past six to eight weeks.  It is incorrect that the HbA1c is a measurement
over the past three months.

HbA1c results, like our daily BG measurements, should probably be evaluated
against each other rather than against other's results.  More importantly,
they should not be evaluated between laboratories unless the exact test
methodology and instrument manufacturer is known.  This is particularly true
of most HbA1c tests that are done in the physician's office because these
test are, for the most part, not subject to proficiency testing requirements
(that is, they are waived from this requirement).  The most accurate testing
must be done by a laboratory that uses more rigorous methodology.

HbA1c tests done in physician's offices are generally waived tests, that is
they meet the same standards as home test meters.  In order to be waived,
the device must be usable by someone with no specific training other than
the written instructions, which must be readable and followed by someone
with a seventh grade education, and the results have to be within 5% of a
standard (i.e. non-waived laboratory test) 50% of the time.  Informally,
this works out to +/- 20%.  Such waived tests are generally used for
screening only, or watching for trends, and are not useful (nor are they
intended) for diagnostic or prognostic purposes.

<< Professional mode = OFF >>

Jim Handsfield
Centers for Disease Control and Prevention
mailto:email @ redacted OR
mailto:email @ redacted

The opinions expressed are my own and do not necessarily represent those of
the Centers for Disease Control and Prevention, the United States Public
Health Service or any other agency of the United States government.
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