“The Fidelity Index (FI) is defined as the ratio of the maximum enzyme amount showing no star activity to the minimum amount needed for complete digestion at the cognate recognition site for any particular restriction endonuclease.” The FI value translates into the minimum number of units that was required to produce star activity. The FI value can be influenced by substrate, buffer and additives like glycerol.

The following reference explains how the Fidelity Index is determined. Hua Wei et. al. “The Fidelity Index provides a systematic quantitation of star activity of DNA restriction endonucleases”, Nucleic Acids Research, 2008, Vol.36, No. 9: e50.