At least in Portuguese, ANI stands for "atividade necro-inflamatória", a sort of expanded translation of the "activity" component of the METAVIR score (see http://hepatitis.about.com/od/diagnosis/a/Metavir.htm for instance). It's usually graded as mild/moderate/severe, though, not low/elevated or "marked" etc.

I translated a bunch of articles on liver transplantation last month and saw this quite often, but as I always say, your mileage may vary...

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Note added at 13 mins (2010-12-21 21:22:59 GMT)
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A "convenience" reference:

Considerou-se como resposta histológica a redução, entre a primeira e a segunda biopsias, de dois pontos na actividade necro-inflamatória (ANI) e de um grau na fibrose.
http://hjurbano.pt/caracter-informativo/doc_download/3-alter...

A liver biopsy sample is useful in this instance in allowing an assessment of the Hepatitis Activity Index (a necroinflammatory/fibrosis scoring system)...
http://www.hepatitis-central.com/hcv/biopsy/guidelines/clini...

Fibrosis Stages and Necroinflammatory Activity Grades
Activity (necroinflammation) and fibrosis are two major histologic features of chronic hepatitis C included in different proposed classifications. One of the few validated scoring systems is called the METAVIR scoring system. This system assesses histologic lesions in chronic hepatitis C using two separate scores, one for necroinflammatory grade (A for activity) and another for the stage of fibrosis (F). These scores are defined as follows... A0, no histologic necroinflammatory activity; A1, minimal activity, A2, moderate activity, A3, severe activity.
http://www.medscape.com/viewarticle/410846_2