TOPIC DISCUSSION: URR and Kt/V

Measure of adequacy of dialysis:
Relationship between URR and spKt/V ( single pool Kt/V):
URR (Urea Reduction ratio)= predialysis BUN-postdialysis BUN/ predialysis BUN X 100
spKt/V dimensionless unit- refers to amount of plasma cleared of urea corrected for volume of distribution of urea. K (dialyzer clearence L/hr) X t (duration of dialysis hr)/V (Volume of distribution of urea L).
Relationship between URR and spKt/V:
spKt/V=-ln(1-URR)
so based on above equation, for Kt/V of 1.0 URR should be 0.63 ( i.e. 63% urea extraction) however this equation does not account for urea generation during dialysis ( will increase postdialysis BUN) and ultrafiltration( which will remove urea by convection) so corrected equation is:
spKt/V = -ln(R – 0.008 x t) + (4 - 3.5 x R) x 0.55 UF/V
(where R = 1 – URR, V = postdialysis volume distribution, UF=volume of fluid removed)
0.008 x t is correction for urea generation which is dependent on time and 0.55UF/V is correction for ultrafiltration. Therefore, for Kt/V of 1.0, URR actually is 60% (if there is no ultrafiltration) instead of 63%.
Remember that URR we see, does not account for urea generation and ultrafiltration which can change the amount of dialysis a patient is actually getting.
eKT/V which is equillibrated Kt/V takes into account for rebound. Rebound is seen because of sequestration of urea in muscle which is rich in water and hence urea but has poor blood flow, thereby limiting the extration of urea into blood. At the end of dialysis, this urea is released from the tissues because of concentration gradient causing rebound. Hence eKt/V is lower than spKt/V by 0.21 for each dialysis.
eKt/V=spKt/V-rebound.