This paper describes a practical method for predicting signals of an adsorbed protein detected by field effect transistors (FETs). The FETs detect the change of the surface charge caused by protein adsorption. The sensing signals strongly depend on an ionic strength of the buffer solution. The prediction of signals is useful for optimizing the strength. A calculation method using three-dimensional conformation structure of proteins was proposed for the prediction tool in the latest paper citing biotin-avidin interaction as an example. To verify availability of the method for alternative adsorbed protein, an adsorption model of streptavidin, which also has a binding ability to biotin, was investigated. Streptavidin has little charge in a buffer solution at pH 7.4 calculated from the method. The results suggest that the signals caused by adsorption of streptavidin were barely detectable using a biotinylated FET.