NAS is a semi-quantitative scoring system rather than a diagnostic procedure. NAS <3 points can exclude NASH, NAS >4 points can diagnose non-alcoholic steatohepatitis (NASH), and scores in between indicate possible NASH.

Non-alcoholic fatty liver (NAFL) is defined as steatosis >33% without lobular inflammation, ballooning, and fibrosis. Steatosis less than this level is simply called hepatocyte steatosis.