Table 2.
“Magic number” definitions and limits: hemodialysis (HD) and hemodiafiltration (HDF) prescriptions.
| Item | Number Definition | Advantages of the Definition | Disadvantages/Limits of Standardization |
|---|---|---|---|
| Permeability | Usually defined as high, medium, or low with respect to middle-molecule depuration; different cut-points available, no fully agreed definition. | Clear and easy definition; all types of membranes can be used in HD, and only high-permeability membranes in HDF. Back-filtration in HD is proportional to permeability. | Differences are less sharp for new membranes; research to improve selectivity, differences between membranes in the same category may be relevant. |
| Membrane size | In square meters: usually related to body surface (lower/higher/equal). | Clear and easy; several surfaces usually available for each membrane type. | Membrane size is related to membrane type and anticoagulation; effect of size on depuration depends on membrane performance. |
| Blood flow | No fully agreed standard; European reference 300–350 mL/min; in other settings target flow may be as high as 450 mL/min. | Clear and easy definition; good blood flow is also a marker of correct functioning of the vascular access. | Target may vary according to vascular access and type of treatment (lower in long-hour dialysis). Highly dependent on vascular access. |
| Dialysate flow | No fully agreed standard; European reference 500 mL/min., may be as high as 800 mL/min. | Clear and easy definition; agreed international standard. | Prescription can be adjusted (higher in HDF, lower in some types of daily dialysis). |
| Reinfusion (HDF) | No fully agreed standard; European reference 24 L/session on HDF. | Clear relationship between exchanges and middle-molecule depuration. | Standards are different across the world; pre-/post-dilution protocols are different; loss of albumin may increase with high exchanges. |
| Number of dialysis sessions | Thrice-weekly; incremental: 1–2 per week with progressive increase; intensive: 4–7 per week. “daily dialysis” at least 5 per week. | Clear, simple, validated. | All frequencies that differ from thrice-weekly are less validated, protocols are highly center-dependent. |
| Dialysis duration | Standard: 4 h thrice-weekly; shorter in “short” daily dialysis; various combinations of 2–8 h and 1–7 sessions. | Clear, simple, validated. | All durations that differ from 4 h are less validated, protocols are highly center-dependent. |