The reliability of rapid immunoassay is a concern due to an incomplete incubation to a non-equilibrium state and is susceptible to different error factors causing variance. The most critical point in the process should be found in order to improve the accuracy, and reproducibility of immunoassays, and enhance the system robustness. In this paper, the behavior of rapid assays is predicted by simulations using mechanistic assay model, based on antibody-analyte binding reaction kinetics. This antibody-analyte binding reaction kinetics model was constructed for a generic three-component (immunometric) assay and the parameters were chosen to be those of a known surface binding assay. The effects of the exact incubation timing and the initial reagent concentrations were studied focusing on the early phase of incubation, the non-equilibrium state. The magnitudes of errors in the input parameters were estimated using knowledge from practical immunoassays. According to simulations, inaccurate incubation timing adds error in the results at very short incubation times, especially in low analyte concentrations. The inaccurate reagent concentrations increase variance at short incubation times, as well. The error decreases rapidly after the first few minutes of incubation.