Not all dialysis units use "the crit-line", and its utility remains somewhat controversial. As an example, in our fellowship program, one of our hospitals (Mass General Hospital) believes in the use of the crit-line, whereas the other (Brigham and Women's Hospital) does not.
What is the "crit-line" and how is it useful? The crit-line is a device built into dialysis machines which monitors absolute hematocrit and oxygen saturation. It works by attachment of a small cuvette to the arterial side of the dialyzer which, via optical measurements, determines the hematocrit concentration. Assuming that the red blood cell mass stays constant during dialysis (e.g., in the absence of blood transfusion), the hematocrit will change only as a result of changing blood volume, a characteristic that may be very useful in determining fluid status. Thus, as fluid is being removed, the hematocrit will increase and vice versa. The readout on the screen is that of percent blood volume change. If the crit-line goes down rapidly (as in the cartoon) you might be concerned that you are being too aggressive with the ultrafiltration rate, and you may need to increase that patient's dry-weight. In contrast, a crit-line which stays relatively constant throughout the dialysis treatment suggests that there is still fluid available for removal, and that patient's dry-weight may need to be lowered.
The benefit in using crit-line monitoring is therefore to avoid intradialytic hypotension and to help determine dry weight. It can also be used to determine access recirculation, and there is recently some suggestion that it may help in more accurate EPO dosing.