An “Invalid Result” in the analysis report means that the OD for the mean of an unknown replicate falls outside of the range of the standard curve and as a result it is not possible to provide a predicted value.
Often the reason for this problem is that the unknown values have much lower or higher OD than then standard curve ODs. One possible solution to the problem may be to rerunning the experiment with additional standard curve points with lower or higher concentrations as is appropriate. Though it should be noted that if the lower standard curve ODs are close to the background noise then this may not improve the results and a more sensitive assay may be required.
The 4PL curve has upper and lower asymptotes that define the range of the curve on the OD axis. The lower asymptote is given by d and the upper asymptote is given by a in the 4PL equation.