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I asked how tTG worked and why it would be useful for a diagnosed
celiac. I got many responses (about half of them asking me to
summarize). The main thought on why they'd be useful to a diagnosed
celiac was "to test for diet compliance" which is the same use for which
the traditional battery of tests is used. I had been hoping for a test
capable of diagnosing the disease while on a gluten-free diet. I know
there is one of these in testing (I believe it's called a "rectal
challenge" which probably tells you more than you want to know about
it).

I got a very concise reply from Tom Ryan, Technical Service Specialist
at INOVA Diagnostics, Inc. who make the test. He answered with just
about everything you could want to know about the test, so I will quote
his message here:

"Celiac disease and dermatitis herpetiformis, more specifically termed
gluten sensitive enteropathy are essentially the same disease but with
different clinical presentations.  It is an inability of the body to
process the protein gliadin properly.  Gliadin is a fraction of the
gluten protein found in wheat and some other cereal grains.  The body
produces IgA and IgG antibodies to this protein.

"This results in an inability to digest and absorb it properly and gives
rise to the production of antibodies to the sheaf of fibers that
surround the smooth muscle layer of the small intestine.  This is called
the endomysium and it is the presence of these IgA type antibodies that
is being tested for with the endomysial antibody test.  The IgA
antibodies are very specific for celiac disease.  IgA type endomysial
antibodies are not found in any other condition.  This test is very
subjective, however.  It involves applying some of the patient's serum
to a section of tissue fixed onto a glass slide, treating it with
various solutions, and viewing it under a fluorescent micrscope.  If the
endomysium surrounding the muscle fibers shows a green, glowing
fluorescence, the test is positive.  This involves a great deal of
expertise and judgement viewing the slide and is subject to
interpretation.  Tissue transglutaminase has been identified as the
protein that is actually being targeted in the endomysium and the tTG
test has been developed to measure antibodies to this protein directly
rather than looking for its visual effect on the endomysium so it's
easier, faster and less subjective than the endomysial test.

"A very small percentage of celiac patients are IgA deficient, not all
of them.  Only about 2-4% of them show this deficiency.  Still, - - this
is significantly higher than the general population where only about one
half of a percent of the people are IgA deficient.  It is significant
enough to look for an additional tool to diagnose this disease.  It has
been found that those who are IgA deficient are usually IgG positive and
that is the reason for testing for IgG.  To catch the rare patient who
is IgA deficient.

"None of these tests, or any others, can report accurate results if the
patient has been following a GF diet because the challenge by gluten in
the diet is necessary to trigger the body to produce these antibodies or
to cause a change in the jejunum that would be observable by endoscopy
or detectable by biopsy."

My thanks to Tom Ryan for the detailed answer, and to the many of you
who shared what they knew about the test, too.


--
Linda Blanchard
Midland, Texas, USA
http://www.nowheat.com
celiac safe foods database, primer and more