Current histological classification of NAFLD: strength and limitations

Nonalcoholic fatty liver disease (NAFLD) covers a spectrum of lesions ranging from pure steatosis without significant necroinflammatory injury (NAFL) to a complex pattern with lesions of hepatocyte injury and inflammation (nonalcoholic steatohepatitis, NASH) in the absence of significant alcohol intake. Because histopathological evaluation of liver biopsy samples remains central in NAFLD in the absence of a non-invasive test with enough accuracy, precise definition of each group is a key issue. When at least 5 % of hepatocytes display steatosis, patients can be qualified for NAFLD in an appropriate clinical context. When, in addition, lobular inflammation and liver cell clarification/ballooning are present, then the lesion can be qualified for NASH. Like for other chronic liver diseases, semiquantitative histological scoring has been proposed for NAFLD. These scoring systems are not useful in clinical practice, and each has some limitation. For comprehensive purposes, we suggest describing histopathological lesions in NAFLD using the SAF score, which assesses together and separately (1) the grade of steatosis (S, from S0 to S3) and (2) the grade of activity (A from A0 to A4 by adding the grades of ballooning and lobular inflammation, both from 0 to 2) and the stage of fibrosis (F from F0 to F4). Such a semiquantitative score could be used for assessing changes in paired biopsies spontaneously or during clinical trials. Finally, the SAF score could help to correctly describe borderline cases of fatty liver diseases that do not fulfill all of the diagnostic criteria of simple steatosis or steatohepatitis, but as for any scoring system, a comprehensive description of the different lesions in their appropriate context is still necessary.