 ### Description

Kt/V is the fractional clearance of urea and is used to monitor urea removal during dialysis.

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