Kt/V for haemodialysis adequacy (Daugirdas)

Clinical background

Kt/V is the accepted measure of the delivered dialysis dose in intermittent haemodialysis and expresses the fraction of the body's urea distribution volume cleared during a session. The measure was developed to quantify and compare the dialysis dose between patients, centres and treatment periods, and to ensure that a minimum acceptable dose is delivered. Without a standardised calculation, the actual dialysis dose risks being underestimated because of sampling error, overestimated because of urea rebound, or overlooked altogether in favour of the urea reduction ratio (URR), which does not capture the effect of fluid removal and treatment time.

The decision the tool serves is concrete: is the delivered dose adequate against the current benchmark, or must the prescription be adjusted? On a thrice-weekly schedule the target is a single-pool Kt/V ≥ 1.2 per session, a benchmark resting on observational data and expert consensus [5,6].

Calculating Kt/V (Daugirdas)

The calculator computes the single-pool Kt/V with the Daugirdas second-generation formula:

$$\text{Kt/V} = -\ln(R - 0{.}008 \times t) + (4 - 3{.}5 \times R) \times \frac{\text{UF}}{W}$$

where:

• $R = \text{post-dialysis BUN} / \text{pre-dialysis BUN}$ (the ratio of post- to pre-dialysis urea nitrogen)
• $t$ = the duration of dialysis in hours
• $\text{UF}$ = the ultrafiltration volume in litres
• $W$ = the post-dialysis weight in kg

The first term, $-\ln(R - 0{.}008 \times t)$, captures the urea reduction corrected for urea generation during dialysis (0.008 per hour). The second term, $(4 - 3{.}5 \times R) \times \text{UF}/W$, corrects for the effect of volume reduction on the fall in concentration and hence on apparently increased clearance.

The formula was derived by Daugirdas in 1993 as a second-generation version of the original logarithmic estimate [1]. Its predecessor, $Kt/V = -\ln(R - 0{.}008 \times t - \text{UF}/W)$, tended to overestimate Kt/V when the value exceeded 1.3, which had become a problem as delivered doses rose. Daugirdas analysed the error by solving variable-volume equations for simulated haemodialysis situations with a Kt/V from 0.6 to 2.6 and then validated the new formula against 500 modelling sessions in which the Kt/V ranged from 0.7 to 2.1 [1]. The second-generation formula eliminated the overestimation in the high range and reduced the total error (absolute percentage error ± 2 SD) compared with its predecessor [1].

Interpretation in practice

A value above 1.4 does not automatically mean the dose should be reduced. The HEMO study found no mortality benefit from raising the dose from standard (spKt/V 1.32) to high (spKt/V 1.71), but no increased harm was seen either [3]. KDOQI 2015 emphasises volume and blood pressure control over pushing the Kt/V well above target [5].

The single-pool Kt/V overestimates the actual, equilibrated clearance by approximately 0.2 units, because urea rebound after dialysis is not taken into account [2]. This is recognised and accepted in routine practice: the single-pool target of 1.2 corresponds to approximately an eKt/V of 1.0, which is the value to which the observational data originally related prognosis.

Validation and performance

The Daugirdas second-generation formula was validated internally in the derivation work against 500 modelling sessions and showed improved accuracy compared with its predecessor across the whole range from 0.7 to 2.1 [1]. The formula has since been used as the reference standard in several large studies, among them the HEMO study, where it formed the basis of dose determination [3].

In the pilot phase of the HEMO study, different methods of estimating the equilibrated Kt/V (eKt/V) from single-pool values were compared [2]. The so-called rate method, $eKt/V = spKt/V - 0{.}6 \times K/V + 0{.}03$, correlated well with the reference method (a 30-minute post-dialysis sample) with r = 0.85 and reduced the median absolute error to 0.061 Kt/V units in an independent comparison [2,4]. This confirms that the single-pool Kt/V is a robust starting point, but that it lies systematically approximately 0.2 units above the eKt/V.

The HEMO study randomised 1,846 patients to a standard dose (spKt/V 1.32 ± 0.09) or a high dose (spKt/V 1.71 ± 0.11) with three treatments per week [3]. The higher dose produced no significant reduction in mortality (RR 0.96, 95% CI 0.84–1.10, P = 0.53) [3]. Observational studies, including DOPPS, have shown an association between a higher Kt/V and lower mortality, but these data are subject to selection bias and confounding, and the randomised trial could not confirm a causal effect of increasing the dose beyond established benchmarks [6].

Limitations

Sampling technique is decisive. The post-dialysis BUN must be taken with a slow-flow or stop-pump technique to avoid contamination with recirculated blood from the access. A post-dialysis sample taken too early or incorrectly can markedly overestimate clearance and give an apparently satisfactory Kt/V that conceals an inadequate dose. Access recirculation, particularly with central venous catheters, worsens the problem.

The formula applies only to intermittent haemodialysis three times a week. With other schedules, such as short daily haemodialysis (5–6 times a week) or slow nocturnal haemodialysis, the per-session Kt/V target is different and not directly comparable. KDOQI 2015 gives specific recommendations for high-frequency schedules [5].

The single-pool Kt/V does not capture urea rebound. The value overestimates the equilibrated clearance by approximately 0.2 units [2]. This is built into how the target of 1.2 was set and is not in itself an error, but the difference must be borne in mind when comparing with eKt/V-based studies.

Kt/V measures only small-solute clearance (urea). It says nothing about the clearance of middle molecules such as β₂-microglobulin, nor about volume status, blood pressure control or mineral metabolism, all of which are independent determinants of prognosis. A satisfactory Kt/V does not exclude inadequate dialysis in other respects.

The magnitude of fluid removal affects the result. With very high ultrafiltration, the volume-correcting term can make a substantial positive contribution, giving a higher Kt/V without urea clearance actually being better. This should be borne in mind in interpretation, particularly in patients with large interdialytic weight changes.

References

1. Daugirdas JT. Second generation logarithmic estimates of single-pool variable volume Kt/V: an analysis of error. J Am Soc Nephrol 1993;4(5):1205–13. PMID: 8305648
2. Daugirdas JT et al. Comparison of methods to predict equilibrated Kt/V in the HEMO Pilot Study. Kidney Int 1997;52(5):1395–405. PMID: 9350665
3. Eknoyan G et al. Effect of dialysis dose and membrane flux in maintenance hemodialysis. N Engl J Med 2002;347(25):2010–9. PMID: 12490682
4. Depner TA et al. Imprecision of the hemodialysis dose when measured directly from urea removal. Hemodialysis Study Group. Kidney Int 1999;55(2):635–47. PMID: 9987088
5. National Kidney Foundation. KDOQI Clinical Practice Guideline for Hemodialysis Adequacy: 2015 update. Am J Kidney Dis 2015;66(5):884–930. PMID: 26498416
6. Saran R et al. Dose of dialysis: key lessons from major observational studies and clinical trials. Am J Kidney Dis 2004;44(5 Suppl 2):47–53. PMID: 15486874