Background: Breast cancer-related lymphedema in the arm is commonly detected by bioelectrical impedance spectroscopy as an increased inter-arm impedance ratio due to the presence of excess lymph in the at-risk arm relative to that of the unaffected arm. The presence of lymphedema is determined by a value of this ratio greater than the mean ratio, plus three standard deviations observed in a comparable healthy population. This threshold value has not been established using the measurement protocols in current practice. The aim of the present study was to determine the reference range of the inter-arm impedance ratio to allow a cut-off value to be established as a criterion for the detection of breast cancer-related lymphedema. Methods: The mean and variation (3 SD) of the inter-arm impedance ratio for the arms of 172 healthy female control participants were determined from an accumulated database of impedance data obtained using present generation impedance instrumentation and methodology. This reference range and threshold value was compared to the original threshold ratio determined a decade ago but still in current use. Results: The presence of lymphedema is indicated when the impedance ratio exceeded 1.106 when the nondominant limb is at risk, and 1.134 when the dominant limb is at risk compared with the currently used values of 1.066 and 1.139, respectively. Although the difference in these values was statistically significant, this difference was determined to be of minor importance to clinical practice. Conclusions: The impedance ratio thresholds for early detection of lymphedema remain suitable for clinical use with present day bioimpedance spectroscopy analyzers and measurement protocols.