We discuss inaccuracies associated with neglecting radiation modes in the calculation of waveguide mode propagation and sensitivity in "Surface plasmon interferometer in silicon-on-insulator: novel concept for an integrated biosensor" [Opt. Express 14, 7063-7072 (2006)]. Numeric calculation shows that due to the short length of the proposed waveguide structure, radiation modes cannot be ignored. When radiation modes are added to the calculation, sensitivity values decrease from above 10,000 dB/RIU to below 1000 dB/RIU. The influence of the radiation modes in such a short structure is due to the recapture of optical power from radiation modes by the output guide, thus reducing the contrast in the interference and the sensitivity.