An interferometric measurement system for measuring an optical fiber comprising multiple primary cores in the optical fiber and an auxiliary core configured in the optical fiber, the system comprising: interferometric detection circuitry configured to detect measurement interferometric pattern data associated with each of the multiple primary cores and the auxiliary core; and data processing circuitry (20, 22) configured to: determine a shape of the optical fiber based on the detected measurement interferometric pattern data for the multiple primary cores, determine a difference between a predicted phase or strain value of the auxiliary core and a measurement-based phase or strain value of the auxiliary core, the predicted phase or strain value of the auxiliary core being predicted based on the detected measurement interferometric pattern data for the multiple primary cores, and the measurement-based phase or strain value being determined based on the detected measurement interferometric pattern data for the auxiliary core, determine, based on the difference, whether the determined shape of the optical fiber is considered unreliable, and causing one or more actions in response to a determination that the determined shape of the optical fiber is considered unreliable, the one or more actions comprising at least one action selected from the group consisting of: generating a fault signal, stopping operation of a system associated with the shape sensing fiber, stopping operation of a machine associated with the shape sensing fiber, and generating an alarm interferometric detection circuitry configured to detect measurement interferometric pattern data associated with each of the multiple primary cores and the auxiliary core; and data processing circuitry (20, 22) configured to: determine a shape of the optical fiber based on the detected measurement interferometric pattern data for the multiple primary cores, determine a difference between a predicted phase or strain value of the auxiliary core and a measurement-based phase or strain value of the auxiliary core, the predicted phase or strain value of the auxiliary core being predicted based on the detected measurement interferometric pattern data for the multiple primary cores, and the measurement-based phase or strain value being determined based on the detected measurement interferometric pattern data for the auxiliary core, determine, based on the difference, whether the determined shape of the optical fiber is considered unreliable, and causing one or more actions in response to a determination that the determined shape of the optical fiber is considered unreliable, the one or more actions comprising at least one action selected from the group consisting of: determine a shape of the optical fiber based on the detected measurement interferometric pattern data for the multiple primary cores, determine a difference between a predicted phase or strain value of the auxiliary core and a measurement-based phase or strain value of the auxiliary core, the predicted phase or strain value of the auxiliary core being predicted based on the detected measurement interferometric pattern data for the multiple primary cores, and the measurement-based phase or strain value being determined based on the detected measurement interferometric pattern data for the auxiliary core, determine, based on the difference, whether the determined shape of the optical fiber is considered unreliable, and causing one or more actions in response to a determination that the determined shape of the optical fiber is considered unreliable, the one or more actions comprising at least one action selected from the group consisting of: generating a fault signal, stopping operation of a system associated with the shape sensing fiber, stopping operation of a machine associated with the shape sensing fiber, and generating an alarm