What happens to dialyzer urea clearance (K) with changes in dialyzer size (KoA), Qb and Qd?
- 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.
- 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.
- 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.
- Meyer TW,Sirich TL,Fong KD et al. Kt/Vurea and non urea small solute levels in the hemodialysis study. JASN 2016;27:3469
- 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.
Daugirdas mathematical formula to calculate:
Kt/V = -ln (R – 0.03) + [(4 – 3.5R) x (UF ÷ W)]
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.
|(high diffusive clearance)
||(high convective clearance)
More on High Flux Dialysis
- HEMO study1 did not show a mortality benefit between high flux and low flux dialysis.
- However, the HEMO study, MPO2 trial, and EGE3 trial showed a survival benefit (especially cardiovascular) in subset of patients with albumin≤4mg/dl, dialysis vintage ≥3.7 years, diabetes or those with an AV fistula.
- KDOQI Adequacy Work group recommends use of high flux dialyzers routinely as long as appropriate water treatment is available.
- 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)
- Locatelli, martin-Malo, Hannedouche et al. Membrane permeability Outcome (MPO) study group. Effect of membrane permeability on survival of hemodialysis patients. JASN 2009;20(3):645
- Asci, Tz, Ozkahya et al. EGE study group. The impact of Membrane permeability and dialysate purity on cardiovascular outcomes.JASN 2013 May;24(6):1014-23