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Objectives: To measure the mass transfer and clearance of procalcitonin (PCT) in patients with septic shock during continuous venovenous hemofiltration (CVVH), and to assess the mechanisms of elimination of PCT., Setting: The medical department of intensive care., Design: A prospective, observational study., Patients: Thirteen critically ill patients with septic shock and oliguric acute renal failure requiring continuous venovenous postdilution hemofiltration with a high-flux membrane (AN69 or polyamide) and a 'conventional' substitution volume (< 2.5 l/hour)., Measurements and Main Results: PCT was measured with the Lumitest PCT Brahms(R) in the prefilter and postfilter plasma, in the ultrafiltrate at the beginning of CVVH (T0) and 15 min (T15'), 60 min (T60') and 6 hours (T6h) after setup of CVVH, and in the prefilter every 24 hours during 4 days. Mass transfer was determined and the clearance and the sieving coefficient were calculated according to the mass conservation principle. Plasma and ultrafiltrate clearances, respectively, at T15', T60' and T6h were 37 +/- 8.6 ml/min (not significant) and 1.8 +/- 1.7 ml/min (P < 0.01), 34.7 +/- 4.1 ml/min (not significant) and 2.3 +/- 1.8 ml/min (P < 0.01), and 31.5 +/- 7 ml/min (not significant) and 5 +/- 2.3 ml/min (P < 0.01). The sieving coefficient significantly increased from 0.07 at T15' to 0.19 at T6h, with no difference according to the nature of the membrane. PCT plasma levels were not significantly modified during the course of CCVH., Conclusions: We conclude that PCT is removed from the plasma of patients with septic shock during CCVH. Most of the mass is eliminated by convective flow, but adsorption also contributes to elimination during the first hours of CVVH. The effect of PCT removal with a conventional CVVH substitution fluid rate (<2.5 l/hour) on PCT plasma concentration seems to be limited, and PCT remains a useful diagnostic marker in these septic patients. The impact of high-volume hemofiltration on the PCT clearance, the mass transfer and the plasma concentration should be evaluated in further studies.