. 2021 Apr 2;31(2):97–110. doi: 10.4103/ijn.IJN_245_19

Box 4.

Example- calculate total weekly Kt/V in a patient receiving CAPD

A 54-year old man is on CAPD for 2 years. He is on 3 exchanges per day with night time dwell. His weight is 65 Kg and height 168 cm. 24-hour peritoneal dialysate drain volume is 10 L, pooled dialysate urea is 42 mg/dL, and plasma urea is 48 mg/dl. Calculate his Kt/V peritoneal dialysis. If his 24-hour urine output is 450 ml, and urine urea concentration is 550 mg/dL, then calculate his residual kidney function, and total weekly Kt/V. Is he receiving adequate CAPD based on your result?

Answer:

Step 1: Calculating daily peritoneal urea clearance or Kt

Kt in Liters = (D_urea/P_urea) x V_D (measured in Liters)

=(42/48) x 10=0.875 x 10=8.75 L

Step 2: Calculating V (volume of distribution of urea, that is total body water, by Watson Equation)

Watson: Male TBW or Volume of distribution of urea in Liters=2.447 - (0.09156 x age) + (0.1074 x height) + (0.3362 x weight)

= 2.447 - (0.09156 x 54) + (0.1074 x 168) + (0.3362 x 65)

=2.447-4.9442+18.0432+21.853=37.399 L

Step 3: Calculating daily peritoneal dialysis achieved (Kt/V_urea)

=8.75/37.399=0.234

Weekly peritoneal dialysis=daily peritoneal dialysis x 7=0.234 x 7=1.638

Step 4: Calculating residual kidney function K_rt/V:

Calculate K_r, t, and V (K_r is urea clearance per week, t is time=1 week, and V is volume measured as total body water).

K_r is calculated in liters per week so we need to convert urea clearance obtained above from ml/min to liters/week.

K_r (L/week)= Urea clearance (ml/min) X [60 X 24 X 7/1000] = Urea clearance (ml/min) X 10.08

V (total body water or volume of distribution of urea) is already calculated above (37.399L)

Urine volume in 24 hours is 450 ml.

Urine volume per minute=450/(24 x 60) = 450/1440=0.3125 ml/min

Urea clearance=Urea_urine X V_urine (in ml/min)/Urea_plasma=550 x 0.3125/48=3.5807 ml/min

K_r in Liters/week=urea clearance (ml/min) x 10.08=3.5807 x 10.08=36.0935

K_rt/V = (36.0935 x 1)/37.399=0.9651

Step 5: Total Kt/V (Peritoneal dialysis+Residual kidney function) = Kt/V peritoneal + K_rt/V=1.638+0.9651=2.6031

Based on this result (total Kt/V=2.6031), this patient is receiving adequate peritoneal dialysis [minimum Kt/V urea should be more than 1.7 as per guidelines].