[en] The 4πβ–γ anticoincidence method is used for the primary standardization of β−, β+, electron capture (EC), α, and mixed-mode radionuclides. Efficiency extrapolation using one or more γ ray coincidence gates is typically carried out by a low-order polynomial fit. The approach presented here is to use a Geant4-based Monte Carlo simulation of the detector system to analyze the efficiency extrapolation. New code was developed to account for detector resolution, direct γ ray interaction with the PMT, and implementation of experimental β-decay shape factors. The simulation was tuned to "5"7Co and "6"0Co data, then tested with "9"9"mTc data, and used in measurements of "1"8F, "1"2"9I, and "1"2"4I. The analysis method described here offers a more realistic activity value and uncertainty than those indicated from a least-squares fit alone. - Highlights: • Monte Carlo approach to 4πβ–γ anticoincidence counting extrapolation. • Geant4 simulations of the detector systems employed. • Demonstrates more realistic estimates of activity values and uncertainties.