Patent ID: 8216478

Claim:
A method for monitoring the supply of substitution fluid for an apparatus for extracorporeal blood treatment having a dialyzer or filter divided by a membrane into a first chamber and a second chamber, an extracorporeal blood circuit that includes the first chamber of the dialyzer or filter, and a fluid system that includes the second chamber of the dialyzer or filter, wherein blood is fed at a blood flow rate Q B to the first chamber of the dialyzer or filter and substitution fluid is fed to the blood at a substitution rate Q S upstream or downstream of the first chamber of the dialyzer or filter and fluid is withdrawn from the blood at a flow rate Q M via the membrane of the dialyzer or filter, the method comprising: changing at least one of a) the substitution rate Q S , b) the blood flow rate Q B , and c) the flow rate Q M ; measuring the density of the blood, or the density of a constituent of the blood, in the extracorporeal blood circuit; and detecting whether a supply of substitution fluid is upstream or downstream of the dialyzer or filter on the basis of a change in the density of the blood, or the density of the constituent of the blood, caused by the changing of the at least one of a) the substitution rate Q S , b) the blood flow rate Q B , and c) the flow rate Q M .