An apparatus for performing haemodialysis on a patient comprises: first blood transfer means (3) for selectively withdrawing blood from a patient and storing it in a first storage portion (5); second blood transfer means (11) for removing filtered blood from a filtration device (7) and storing it in a second storage portion (13); and a fluid measurement system (51a, 51b) for periodically measuring the respective volumes of blood in the first and second storage portions; adding the volume of blood in the first storage portion to the volume of blood in the second storage portion at that time in order to calculate the total volume of blood within the first and second storage portions at that time; and comparing the total measured volumes of blood within the first and second storage portions measured over a predetermined time interval to calculate the volume of fluid removed from the blood during that predetermined time interval.