What are the methods for measuring the dose of dialysis?
The dose of dialysis can be expressed as the urea reduction ratio (URR), the single pool Kt/V, the double pool or equilibrated Kt/V, or the weekly or standard Kt/V. The first three expressions can be used only for patients who receive hemodialysis three times per week, whereas the standard or weekly Kt/V can be used to estimate the dose of dialysis regardless of the number of times per week that the patient receives hemodialysis.
- 1. The URR is expressed as follows:
where C t and C o represent postdialysis and predialysis serum urea levels. Because the URR equation does not account for volume removal during a dialysis treatment, it is considered less accurate than any of the Kt/V formulas.
- 2. Single pool Kt/Vurea (spKt/Vurea) is a unitless parameter that can be used to estimate the dose of dialysis provided to the patient, where K is the dialyzer blood water urea clearance (L/hour), t is the dialysis session length (hours), and V is the volume of distribution of urea (L). Because both K and V are difficult to measure accurately in vivo, several regression equations have been developed to estimate spKt/V. The most commonly used equation is the Daugirdas II equation:
where R is the postdialysis over predialysis serum urea level (C t /C o ), t is the time of dialysis (in hours), UF is ultrafiltration volume in liters (amount of fluid removed by dialysis), and V is the body water volume in liters. The first part of the equation represents the effects of urea generation during dialysis, whereas the second part of the equation represents the additional urea removed with volume removal during dialysis. A modification of the Daugirdas formula can be used to estimate spKt/V for dialysis frequencies other than three times per week (see link under #4 below).
3. Equilibrated Kt/Vurea (eKt/Vurea) values accounts for urea release from sequestered tissue sites into the blood that occurs in the first 30 to 60 minutes after dialysis. The eKt/V is usually about 0.2 units less than the spKt/V. The formula for eKt/V depends on whether the patient is using a catheter for hemodialysis access:
4. Standard or weekly Kt/V can be used to estimate the dose of dialysis regardless of the number of days per week that the patient receives dialysis. The formula for the weekly Kt/V is complex; calculators are available that can provide the value for the weekly Kt/V (including HYPERLINK “ http://www.hdcn.com/ukm/ ” http://www.hdcn.com/ukm/ ).
