Antimitochondrial Antibodies (cont.)
How is the blood test for AMA done?
The most economical test for AMA applies diluted samples of a
patient's serum onto tissue sections from rat stomach or kidney in the
laboratory. (Remember that mitochondria are present in all cells, not just the cells of
the liver and bile ducts.) Serum antibodies that attach (bind) to mitochondrial
membranes within the tissue cells can then be observed with a microscope. The
most dilute sample of serum showing this binding reaction is reported, using the
term titer. The titer indicates the most dilute serum sample that reacts with
the tissue mitochondria. A higher titer means there is a greater amount of AMA
in the serum.
The antigen recognized by AMA in patients with PBC is now known to be PDC-E2
and is also often referred to as the M2 antigen. So, newly developed tests for
antibodies that bind to PDC-E2 are more specific and are now available to
confirm the diagnosis of PBC.
What is the value of the AMA blood test?
AMA are detectable in the serum in 95 to 98% of patients with PBC. So, AMA
are tremendously important as a diagnostic marker in patients with PBC. The AMA
titers in PBC are almost universally greater than or equal to 1 to 40. This
means that a serum sample diluted with 40 times its original volume still
contains enough antimitochondrial antibodies to be detected in the binding
reaction. A positive AMA with a titer of at least 1:40 in an adult with an
elevated alkaline phosphatase in the blood is highly specific for a diagnosis of
PBC.
Last Editorial Review: 2/23/2004
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