Salmonella to Salmon, What Is Impedance Measuring?

In 2002 at AFS, Cox and Hartman presented a paper titled “Is compensatory growth in fish all wet?”.  This initial research led them to pursue non-lethal methods to measure the energy content of fish.  The resulting method that they came up with was bioelectrical impedance analysis (BIA).  Fast forward to 2013.  What has impedance been used to measure and what is it actually measuring?  BIA has been used to measure vegetable oil in cucumbers, Salmonella in a petri dish, condition of fish, body composition of fish, time of death in fish, presence of frozen tissue, degradation of tissue, cell growth, and energy content.  BIA is measuring resistance (R) and reactance (Xc) at the cellular level, but how does this relate to the aforementioned uses?  R and Xc are highly specific measures that are related to extra-cellular volume (via R), and the amount or volume of cellular lipid bilayers (via Xc).  Unfortunately, it is not as simple as measuring the R and Xc of anything biological.  In a paper to determine sources of error in BIA measures, we found that BIA was sensitive (so therefore measuring) electrode needle depth and distance, ionic properties of surface, deviations from a standard, user experience, and temperature. Sensitivity analyses showed errors in predictive estimates of composition can be large (>50%) when these errors (extra measures) are experienced. Adherence to a strict protocol can help avoid these sources of error and provide BIA estimates that are both accurate and precise in a field or laboratory setting.  In 2011, the North Pacific Research Board (NPRB) provided funding to develop an automated fish specific BIA board capable of electronic recording automated measures of GPS location, length, weight (motion compensated), resistance, reactance, distance between electrodes and core temperature of the fish.