• Clearance describes the volume of blood that is cleared of a substance per unit of time
  • The determinant of clearance in CRRT depends upon the mechanisms that are being utilised
    • In continuous haemofiltration:
      • A volume of ultrafiltrate is produced following the passage of the pressurised plasma over the membrane
      • Clearance is of a solute determined by the:
        • Rate at which ultrafiltrate is produced (Quf)
        • Sieving coefficient of the filter membranes (determined by the ratio of solute concentrations in the ultrafiltrate)
      • Most small molecules have a sieving coefficient of 1, meaning they pass freely through the membrane (though this is less true of middle molecules)
      • Therefore, the clearance is analogous to the ultrafiltrate production rate (Quf)
    • In continuous haemodialysis:
      • The concentration gradient across the membrane determines clearance
      •  The gradient is affected by the dialysate flow rate (Qd) and the blood flow rate (Qb)
      • Qd is much slower than Qb, and the dialysate becomes fully saturated.
      • Therefore, the dialysate rate (Qd) becomes the rate-limiting factor for solute removal and is analogous to clearance
    • Overall consequently, clearance is equivalent to the combination of the ultrafiltration rate (Quf) and dialysate (Qd), which is termed the effluent rate (Qef)
    • This is often discussed in terms of the ‘dose’ of a continuous replacement therapy which is given