NAFLD fibrosis score can predict but NOT REPLACE LIVER BIOPSY for diagnosing degree of statosis and fibrosis. Patients can have different degrees of steatosis and fibrosis with normal ALT.

Not my area of expertise.

In USA general population 30% of individuals may have liver steatosis. Most of them have clinically-non-relevant simple steatosis. Nevertheless  5% have inflammation and fibrosis, a condition known as NASH. The presence of fibrosis is associated with high rate of liver related complications and mortality. So the necessity for detecting fibrosis in patients with liver steatosis is clear. Liver biopsy is the gold standard test for diagnosis of fibrosis.  The NAFD fibrosis score was proposed for clinical use in 2007 by Angulo et al. It is composed of six parameters: age, BMI, fasting blood glucose, serum platelets count, plasma albumin and plasma AST/ALT ratio. It is a non invasive and easy available tool for estimating fibrosis in patients with NAFLD. It can be calculated on line on:http://nafldscore.com/ .

When NAFLD score results is -1.5 there is a very low probability of fibrosis (Negative predictive value is 93%). Medium values -1.6 to 0.67 have moderate probabilities while 0.67 or higher values have a PPV of 90%.

This score has been validated in diverse publications in NAFLD patients in order to estimate prognosis of liver complications and death. It does not replace liver biopsy but it may be useful for optimizing the use of this procedure. Particularly for avoiding unnecesary biopsies.

For answering your question if a NAFLD patient with normal enzymes has NAFLD score inferior to -1.5, liver biopsy should not be performed. If it is 0.67 or higher,  liver biopsy is guaranteed.  Values from -1.5 to 0.67 make us monitoring the patient and repeat NAFD score every 6 months.   

 Treeprasertsuk S et al World J Gastroenterol 2013; 19: 1219-1229

Angulo P  et al HEPATOLOGY 2007;45: 846-854