A fluid separation system for separating a plasma-containing fluid into separated plasma and a concentrated fluid, the fluid separation system being configured to cooperate with a fluid flow circuit including a fluid separation chamber and a plasma outlet line associated with the fluid separation chamber for removing separated plasma from the fluid separation chamber, the fluid separation system comprising: an optical sensor assembly configured to monitor the contents of the plasma outlet line and produce an output indicative of the concentration of free plasma hemoglobin in the plasma outlet line; a controller programmed to calculate the amount of free plasma hemoglobin in at least a portion of the concentrated fluid based at least in part on the output of the optical sensor assembly by calculating the amount of free plasma hemoglobin in the separated plasma and multiplying said amount of free plasma hemoglobin in the separated plasma by a ratio factor, wherein the ratio factor is determined experimentally and derived by measuring the concentration of free plasma hemoglobin in the plasma of the concentrated fluid at one or more different times during a procedure and comparing the measured concentration to the concentration of free plasma hemoglobin in the separated plasma at the same times during the procedure to determine the ratio between the concentration of free hemoglobin in the separated plasma and concentrated fluid at the one or more different times, and a pump configured to deliver at least a portion of the concentrated fluid to a fluid recipient, wherein the controller is programmed to generate an alarm or alert signal when the amount of free plasma hemoglobin in the concentrated fluid delivered to and/or to be delivered to the fluid recipient meets or exceeds a preselected or predetermined value. an optical sensor assembly configured to monitor the contents of the plasma outlet line and produce an output indicative of the concentration of free plasma hemoglobin in the plasma outlet line; a controller programmed to calculate the amount of free plasma hemoglobin in at least a portion of the concentrated fluid based at least in part on the output of the optical sensor assembly by calculating the amount of free plasma hemoglobin in the separated plasma and multiplying said amount of free plasma hemoglobin in the separated plasma by a ratio factor, wherein the ratio factor is determined experimentally and derived by measuring the concentration of free plasma hemoglobin in the plasma of the concentrated fluid at one or more different times during a procedure and comparing the measured concentration to the concentration of free plasma hemoglobin in the separated plasma at the same times during the procedure to determine the ratio between the concentration of free hemoglobin in the separated plasma and concentrated fluid at the one or more different times, and a pump configured to deliver at least a portion of the concentrated fluid to a fluid recipient, wherein the controller is programmed to generate an alarm or alert signal when the amount of free plasma hemoglobin in the concentrated fluid delivered to and/or to be delivered to the fluid recipient meets or exceeds a preselected or predetermined value.