The AMA (M2) test is a chemiluminescent immunoassay (CLIA), for use on IDS automated analyzers. It is used for the quantitative determination of the specific IgG antibodies directed against mitochondrial M2 subtype antigen in human samples of serum or plasma (EDTA and Na-Citrate).

Primary biliary cirrhosis (PBC) is an autoimmune disease that causes a chronic inflammation of the intrahepatic bile ducts. Progressive destruction of the liver cells leads to hepatic fibrosis and cirrhosis and over time may lead to hepatic insufficiency and to the need for a liver transplant1,2. The exact cause of PBC is unknown; genetic factors, linked to a dysfunction of the immune system are implicated, as well as environmental factors, such as interaction of the organism with a number of infectious agents3,4,5.

PBC is more frequent in women with a maximum incidence between ages 40 and 60 years6, with no racial difference, but with a wide geographical variability ranging from 40 to 400 individuals in the general populations, and higher frequency in Northern Europe and in the USA7.

In half of the cases, PBC is casually diagnosed when, during other tests or screening, anomalous levels of liver pathology markers are detected: transaminases (AST and ALT) and above all cholestasis indexes (γ-GT and Alkaline Phosphatasis).

Diagnosis is given by the presence of at least two of the following criteria:

  • positive result of a search for anti-mitochondrial antibodies (AMA) at a sufficient concentration (> 1:40)
  • persistence for over 6 months of alkaline phosphatasis (higher than 1.5 times the upper limit of the reference range)
  • liver biopsy with a compatible histological context

Definite PBC can be diagnosed when all three criteria are met, while the term probable PBC is used when only two criteria are met8.

High AMA titres are the most sensitive and specific PBC immune-serologic markers, since they can be found in 90-95%9,10 of patients with a specificity close to 100%. However, it is worth underlining that the frequency of the antibodies detected depends on the group studied and the sensitivity may be lower11.

Based on the immunochemical structure, 9 sub-types of mitochondrial antigens have been classified, named from M1 to M912. Only the anti-M2, anti-M4, anti-M8 and anti-M9 autoantibodies are specific to PBC and, of these, only the anti-M2 antibodies (specificity for pyruvate–dehydrogenase) are of diagnostic importance since they are present in high titres in almost all patients, while the other three may be found in low titres and in any case are never found in isolation, but are always associated with anti-M2 antibodies13. In reality, antigen M2 comprises 4 different autoantigens belonging to the structural complex of the pyruvate–dehydrogenase enzyme14, to which 4 different types of non cross-reagent antibodies correspond. The antibodies most frequently detected are those acting against the E2 (2-oxo acid dehydrogenase) component14; those others are found less frequently and are normally associated with anti-E2 antibodies13. The specific antigens have been identified as sub-units of the pyruvate–dehydrogenase complex (PDC-E2), of the 2-oxo acid dehydrogenase complex of the branched chain (BCOADC-E2) and of the 2-oxoglutarate dehydrogenase complex (OGDC-E2).

The test to determine AMAs based on the use of the three combined antigens give higher performances than the IFA test, making it possible to detect AMA antibodies in over two thirds of the serums of PBC patients who tested AMA negative with the IFA test15,16. Since the presence of AMA antibodies may precede the onset of the symptomatic disease, the ability to identify more accurately the presence of the PBC marker could contribute to early diagnosis and treatment and could slow down the progression of the disease17,18.

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