Another Pitfall Implicit in the Relation Described By

Ficheux et al. may result from the method of TMP calculation. If only three pressure points are known, the fourth one (inlet dialysate pressure) must be inferred or assumed, so introducing great variability related to dialyser characteristics and operational setting. The impact of the fourth point on TMP calculation is highly significant and the error is increased by disregarding the contribution of oncotic pressure (Ponc). In high-efficiency post-HDF, haemoconcentration inside the capillaries may double the protein concentration, and Ponc opposing filtration pressure may achieve values of 80–90 mmHg. Based on the same data as above, we could show that substantial differences arise from the different methods in TMP computation during post-dilution HDF (Figure 2). In conclusion, KUF max seems not to be a reliable index to characterize dialysers or modulate QUF during HDF. In addition, its identification would be cumbersome and its value highly variable between and within patients, as also admitted by the authors. On the other hand, the authors disregard that efficient feedback systems to modulate QUF have already been implemented and validated experimentally on different HDF systems. Some of them provide continuous measurement and control of four-point TMP and allow maximal QUF to be safely achieved, accounting for effective blood flow, haematocrit changes (blood volume monitoring) and dialyser characteristics. This TMP/QUF feedback adapts QUF to the individual needs automatically and without the intervention of nurses whatever the patient and treatment operational conditions in different HDF modalities [3,5].