Category Archives: KT/V

Limitations of Kt/V

  • Kt/V overestimates dialysis adequacy in thin, malnourished patients and elderly females due to sarcopenia and a low ‘V’. This may lead to under-dosing of dialysis. Kt corrected for body surface area may be a more accurate measure of dialysis dose in these patients.
  • There is a high index of error related to inaccurate post-dialysis BUN measurement.
  • Single treatment may not represent other treatments ( missed or shortened)
  • spKt/V or eKt/V is not applicable to daily or nocturnal dialysis. It is useful only for conventional 3 times a week intermittent hemodialysis.
  • Kt/V is a marker only of urea clearance and not of other toxic/uremic solutes that exist.
  • Other uremic toxins such as phosphate, β2 microglobulin, guanidino compounds etc. do not follow the same hemodialysis kinetics as urea and therefore their clearances are not accurately reflected by Kt/Vurea.1
  • A high Kt/V(>1.4) has not been shown to predict better survival in any randomized controlled studies (e.g. HEMO study2).This implies that there are other determinants of poor survival in hemodialysis patients besides urea clearance and that despite a high dialysis urea clearance (Kt/V), patients may retain other toxic solutes that ultimately are equally or more important than urea in influencing prognosis on dialysis.3
  • Despite these controversies surrounding KT/Vurea, it remains the most frequently used parameter for determining dialysis adequacy and its routine monitoring may help to identify problems of dialysis delivery such as access recirculation etc.

Sources:

  1. Comparing the urea reduction ratio and the urea product as outcome-based measures of hemodialysis dose. AU Li Z, Lew NL, Lazarus JM, Lowrie EG SOAm J Kidney Dis. 2000;35(4):598.
  2. Eknoyan G, Beck, GJ Cheung AK, et al. Effect of dialysis dose and membrane flux in maintenance hemodialysis. N Engl J Med 2002; 347:2010.
  3. Meyer TW,Sirich TL,Fong KD et al. Kt/Vurea and non urea small solute levels in the hemodialysis study. JASN 2016;27:3469

Troubleshooting Low Kt/V

When Kt/V drops:

  • 40% of the cases occur due to decreased dialysis time or inadequate dialyzer blood flow (Qb)
  • 25% of cases are due to significant access recirculation

Troubleshooting:

  • Repeat measurement to confirm the drop using standardized measurement technique as described in the video “How to measure Kt/V in the HD unit
  • Confirm accuracy of post BUN measurement
  • Confirm adequate Qb and time on dialysis
  • Check access integrity
  • Optimize Qb and Treatment time, resolve access issues
  • Use a higher efficiency dialyzer (higher KoA)
  • Can increase dialysate flow (Qd) although gain above 800ml/min is minimal

Why Care About Kt/V?

  • Urea is considered a surrogate to other small molecular weight uremic toxins and is easy to measure in the blood pre and post hemodialysis.
  • Although its use has limitations, it is an easily measured and useful marker of hemodialysis adequacy.
  • Kt/Vurea has helped standardize dialysis dosage based on urea clearance. It provides a tool to avoid grossly inadequate dialysis.
  • Its routine monitoring may help to identify problems of dialysis delivery such as access recirculation. 
  • Urea reduction ratio (URR= 1-postdialysis BUN/predialysis BUN) is a less optimal measure of dialysis adequacy as it does not account for ultrafiltration during dialysis and assumes that V is constant during dialysis.

What is the goal Kt/V?

  • This applies to conventional intermittent 3 times a week hemodialysis
    • Per KDOQI 2006 guidelines:  For patients with minimal residual renal function (<2ml/min per 1.73m2)
      • Minimally adequate dose should be a spKt/V of 1.2
      • Target recommended dose should be a spKt/V of 1.4 (or eKT/V of 1.2)
  • There is a lack of randomized studies to support the minimum dialysis dose of spKt/V ≥1.2.
  • However, retrospective studies suggest that a Kt/V <1.0 is associated with poor outcomes and that a Kt/V ≥1.2 is associated with better survival123.
  • The HEMO study4 showed that targeting a spKt/V higher than 1.4 did not improve survival or reduce hospitalization rates over 2.8 years.
CLINICAL PEARLS:

  • Based on large retrospective studies, a Kt/V <1.0 is associated with poor outcomes. Clinical practice guidelines, therefore, recommend a minimum delivered spKt/V of 1.2.
  • Delivered Kt/V is often lower than the prescribed Kt/V. This could be related to multiple reasons such as interrupted treatment due to machine alarms, access recirculation, blood or dialysate pump calibration errors that leads to decreased delivered Qb or Qd etc.
  • Therefore, to achieve a delivered spKt/V of 1.2, a target spKt/V of 1.4 is recommended. This equates to an eKt/V of 1.2

Sources:

  1. Owen WF Jr, Lew NL, Liu Y, et al. The urea reduction ratio and serum albumin concentration as predictors of mortality in patients undergoing hemodialysis. N Engl J Med. 1993;329(14):1001.
  2. Gotch FA, Levin NW, Port FK et al. Clinical outcome relative to the dose of dialysis is not what you think: the fallacy of the mean. Am J Kidney Dis. 1997;30(1):1.
  3. Held PJ, Port FK, Wolfe RA et al. The dose of hemodialysis and patient mortality. Kidney Int. 1996;50(2):550.
  4. Eknoyan G, Beck, GJ Cheung AK, et al. Effect of dialysis dose and membrane flux in maintenance hemodialysis. N Engl J Med 2002; 347:2010. Hemodialysis study (HEMO)

How to Measure Kt/V?

Daugirdas mathematical formula to calculate:
Kt/V  = -ln (R – 0.03) + [(4 – 3.5R) x (UF ÷ W)]

Where…
R= post HD BUN/preHD BUN
UF= UF volume in L
W=  post dialysis weight in kg

Online calculators and normograms are available to calculate Kt/V using formula above.

The quick and dirty method:

  • If the dialyzer’s clearance (K) is known (based on the packet insert provide by the manufacturer at a Qb of 400ml/min) and is 250 ml/min and the dialysis session time is 240 minutes (4 hours)
    • then Kt (dialysis dose)= 250 x 240 = 60,000ml or 60 liters
  • If the patient weighs 70kg. His TBW is 60% of 70kg
    • then V = 70 kg multiplied by .60 = 42 liters
  • So the ratio – K multiplied by t to V, or Kt/V  compares the amount of water that passes through the dialyzer and is cleared of urea to the amount of water in the patient’s body.
    • KT/V for this patient will be= 60/42 = 1.42

Caveat: If this same patient has 3 kg of  edema fluid (EDW of 70kg)  then V = 60% (70kg)+ 100%(3kg)= 45L and not 42L. This is because edema fluid adds to the TBW in its entirety as urea distributes evenly across body water.

What is Kt/Vurea?

Kt/Vurea

As you go through this section of the module consider the following scenario:

67 year old African American male with history of diabetes, hypertension, coronary artery disease, and end stage renal disease secondary to diabetic nephropathy is on hemodialysis three times a week for 4 hours via a left arm brachiocephalic AV fistula. You are the rounding nephrologist in the dialysis unit and a concerned nurse reports that the patient’s most recent Kt/V is 1.0 down from previous levels of 1.4-1.6.

  1. What is Kt/Vurea?
  2. How do we measure Kt/V?
  3. What is the goal Kt/V?
  4. Why do we care about Kt/V?
  5. Troubleshooting a drop in Kt/V
  6. What are the limitations of Kt/V?
  7. Playground! Play around with variations of Qb (access blood flow rate), Qd (dialysate flow rate) and KoA to see  how it impacts K and Kt/V
    1. Playground Debrief