Primary Biliary Cirrhosis (cont.)

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What is the role of testing for antimitochondrial antibodies?

AMA are detectable in the serum in 95 to 98% of patients with PBC, as noted earlier. 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 the 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 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 is highly specific for a diagnosis of PBC. 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, as discussed earlier. So, newly developed tests for antibodies that bind to PDC-E2 are more specific and are now available to confirm the diagnosis of PBC.

It is noteworthy that approximately 20% of patients with AMA also have antinuclear (ANA) and/or anti-smooth muscle (SMA) autoantibodies in their blood. The ANA and SMA are more characteristically found in a disease called chronic autoimmune hepatitis. It turns out that patients who have persistently undetectable AMA but otherwise have clinical, laboratory, and liver biopsy evidence of PBC, all have either ANA or SMA. These patients have been referred to as having AMA-negative PBC, autoimmune cholangiopathy, or autoimmune cholangitis. The natural history, associated diseases, laboratory test abnormalities, and liver pathology are indistinguishable between the AMA-positive and AMA-negative patients. Thus, it seems inappropriate, for now at least, to classify this AMA-negative disease as different from PBC. Accordingly, this situation should be referred to as AMA-negative PBC. Rarely, some other patients appear to concurrently have features of both PBC and chronic autoimmune hepatitis. Such patients are said to have an overlap syndrome.

Medically Reviewed by a Doctor on 1/9/2014

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