During acute kidney replacement therapy the amount (dose) of delivered therapy can be described by various terms: efficiency, intensity, efficacy, frequency, and clinical efficacy. Efficiency is represented by the concept of clearance (K), that is, the volume of blood cleared of a solute over a given time; the clearance does not reflect the overall solute removal rate but rather its value normalized by the serum concentration. Intensity can be defined by the product of clearance and time (Kt); this is more useful than clearance alone in comparing various therapies; however, it does not take into account the size of the pool from which the solute needs to be cleared. Efficacy is the effective solute removal resulting from the administration of a treatment dose to a patient. It can be described as a fractional clearance of a given solute (Kt/V) where V is the volume of distribution of the marker molecule in the body. Clinical efficiency can only be assessed by adequately powered prospective randomized clinical studies. The evidence available to date supports the use of at least 35 ml/h/kg for CVVH, CVVHD, or CVVHDF, or 1.2 Kt/V daily intermittent hemodialysis in treating patients with acute kidney failure.
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