Pathology of non‐alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) covers a spectrum of lesions ranging from steatosis (Non-alcoholic Fatty Liver or NAFL) to a complex pattern with hepatocellular injury and inflammation (non-alcoholic steatohepatitis; NASH) in the absence of alcohol intake. However, it is increasingly clear that intermediate patterns may exist. The histopathological evaluation of liver biopsy samples is central in the diagnosis of NAFLD and NASH in the absence of sufficiently accurate non-invasive tests because a precise definition of each group is a key issue. When at least 5% of hepatocytes display steatosis, patients can be defined as having NAFLD in an appropriate clinical context. When, in addition, lobular inflammation and liver cell clarification/ballooning are present, then the lesion is usually defined as NASH. Evaluation of the stage of fibrosis is even more fundamental than necroinflammation since it is the main prognostic factor of this disease. Semi-quantitative histological scoring systems have been proposed for NAFLD, but they are not useful in clinical practice and each has certain limitations. For comprehensive purposes, we suggest describing histopathological lesions in NAFLD using the SAF (Steatosis, Activity, Fibrosis) score which assesses separately the grade of steatosis (S, from S0 to S3), the grade of activity (A from A0 to A4 by adding grades of ballooning and lobular inflammation, both from 0 to 2) and the stage of fibrosis (F from F0 to F4).