Direct Calculation
Kt/V =
= (liters of plasma cleared of urea per hour) * (hours dialysis performed) / (urea distribution volume)
where:
• urea distribution volume is approximately equal to total body water
Equations Estimating Kt/V from Dialysis Data
linear estimate of Kt/V =
= 2.2 - (3.3 * (R - 0.03 - ((UF) / W)))
logarithmic estimate #1 of Kt/V =
= ((-1) * LN(R - (0.008 * (hours of dialysis)) - (0.75 * (UF) / W)))
logarithmic estimate #2 of Kt/V =
= ((-1) * LN(R - (0.008 * (hours of dialysis)))) + ((4 - (3.5 * R)) * (UF) / W)
estimate of Kt/V if dialysis data lacking =
= 1.18 * (-1) * LN (R)
where
• R = (postdialysis urea nitrogen) / (predialysis urea nitrogen)
• The post-dialysis urea nitrogen specimen should be drawn from an arteriovenous blood access at least 1 minute and preferably 2 minutes after completion of dialysis. A source of error is post-dialysis rebound in plasma urea which occurs during the first 30 minutes after dialysis is complete.
• UF = ultrafiltrate volume removed, in liters
• W = postdialysis body weight in kilograms
In most cases the logarithmic equations can be approximated by:
logarithmic estimate #1 of Kt/V =
= ((-1) * LN(R - 0.03 - (0.75 * (UF) / W)))
logarithmic estimate #2 of Kt/V =
= ((-1) * LN(R - 0.03)) + ((4 - (3.5 * R)) * (UF) / W)
Comparison of equations
(1) The linear equation underestimates Kt/V if R is less than 0.25 or greater than 0.45.
(2) The third logarithmic equation is necessary if Kt/V is > 1.7 or R is < 0.25.
Interpretation:
• During hemodialysis a Kt/V > 1.0 is considered adequate, with many preferring a value > 1.2-1.4
