hemodiafiltration or hdf is a renal replacement modality that combines diffusion and convection to improve removal of molecules in the middle molecular weight range versus hemodialysis like hemodialysis blood and dialysate flow in opposite directions in the dialyzer cleaning the blood of toxins and excess fluid while diffusion allows solutes to passively filter through a high flux dialyzer down a concentration gradient from blood to dialysate convection uses pressure to actively drag larger potentially uremic molecules such as beta-2 microglobulin across the dialyzer into the ultrafiltrate during an hdf treatment approximately 20 to 30 percent of the plasma water is removed by ultrafiltration the convection volume or total ultrafiltration volume consists of the substitution volume plus the net ultrafiltration volume for fluid removal the substitution volume is the amount of replacement fluid infused into the patient's bloodline to maintain fluid balance this fluid must be sterile and non-pyrogenic in high volume hdf a large volume of plasma water is ultra filtered and must be replaced in online hdf potable water undergoes extensive treatment with carbon filters reverse osmosis and sterilizing ultra filters with one ultra filter being installed into the circuit before the site of infusion this allows delivery of sterile non-pyrogenic fluid to the patient and use of online substitution fluid instead of saline for priming infusion or as a bolus administration commonly used hdf treatment modalities include pre-dilution and post-dilution hdf during pre-dilution substitution fluid is administered before the dialyzer meanwhile in post-dilution the fluid is administered after the dialyzer pre-dilution hdf can use greater than 35 liters of substitution fluid leading to hemodilution such large volumes could also reduce solute clearances hematocrit viscosity and oncotic pressure and increase membrane fouling post-dilution hdf uses 21 liters or more of substitution fluid it can increase solute clearance with lower substitution volumes but can cause hemo concentration like in center hemodialysis olhdf is usually prescribed thrice weekly for four hours when starting the patients on hemodiafiltration dialysate and blood flow rates type of dialyzer needle size filtration fraction volume of substitution fluid and anticoagulation should be considered the patient should also have a functioning vascular access such as an arterial venous fistula or graft typical blood flow rate qb and dialysate flow rate qd range from 300 to 450 milliliters per minute and 400 to 500 milliliters per minute respectively filtration fraction is the ratio or percentage of convective flow to blood flow and is usually between 20 to 30 percent of the achieved blood flow rate a typical high volume post dilution hdf treatment uses a substitution fluid infusion rate of 100 milliliters per minute or 24 liters for a four-hour session pre-dilution hdf often requires double the substitution infusion flow rate to 200 milliliters per minute or 48 liters in a four hour session needle size should match qb and 15 to 16 gauge have been used successfully in hdf treatments high flux dialyzers with surface areas of 1.6 to 2.5 meter squared can provide efficient removal of middle molecules while minimizing albumin loss anticoagulation usually involves heparin importantly the transition from hemodialysis to hdf must be done gradually and modified according to the individual patient's characteristics to avoid any potential complications you