The polarity spectral coherence method (PSCM) of Seifritz has been proposed for measuring the subcritical reactivity of a nuclear reactor. However, due to the nature of the PSCM measurement technique, a systematic error in the subcritical reactivity measurement may occur due to unexpected changes in the neutron detection efficiency. This error is assessed through the development of the appropriate equations, and a hypothetical measurement in a typical liquid-metal fast breeder reactor is numerically evaluated. The results demonstrate that the PSCM is subject to systematic errors which can be quite serious and which must be considered when the PSCM technique is used.