Quality Control in Haemodialysis Delivery
various body functions is largely abolished. While these solutes constitute a heterogeneous group of substances that accumulate in uraemia,6
the
accomplishment of adequate solute removal by dialysis is generally judged from the pre- to post-dialysis reduction in the blood level of urea alone. The urea reduction ratio (URR) is frequently converted to a urea Kt/V number, reflecting the total volume cleared in relation to the patient’s urea distribution volume. For a standard thrice weekly dialysis schedule Kt/V is given either as single-pool (spKt/V) or equilibrated Kt/V (eKt/V), the latter taking into account the rebound that appears when the dialysis-induced disequilibrium between fluid compartments in the body is resolved as dialysis is stopped. Several formulas are available to facilitate the conversion from URR to spKt/V or eKt/V.7
Target Dose
Several observational studies have shown that low urea spKt/V values are associated with worse survival rates.8,9
The potential benefit
of increasingly higher spKt/V is more difficult to resolve from observational studies in which the association between Kt/V and outcome is confounded by body size10
and where the higher Kt/V
groups typically contains a larger fraction of high-risk patients with low body mass index (BMI).11
The randomised controlled HEMO trial,
challenging the target Kt/V dose, found similar survival rates with a high and a standard dose (target eKt/V 1.45 and 1.05 respectively).12 However, like some observational studies (e.g. Port, 2004), the HEMO study data indicated a gender effect, with female patients benefitting from a higher Kt/V dose target.13
Clinical guideline groups have defined the proper dialysis dose for anuric patients on a standard thrice weekly HD schedule, based on the available evidence. The European Renal Best Practice (ERBP) guidelines state that the prescribed target eKt/V should be at least 1.2 and that a higher dose (up to 1.4) should be considered for females and patients with high co-morbidity.14
The American Kidney
Disease Outcomes Quality Initiative (KDOQI) guidelines stipulate a target spKt/V of 1.4 to ensure that the delivered dose stays above a minimally adequate spKt/V of 1.2, recognising that there may be considerable dosing variability between treatments.15
Dose Delivery in Clinical Practice Compliance to Target Dose
Although information on residual renal function in patients on low Kt/Vs is lacking, the data clearly indicate that a significant portion of contemporary HD patients receives inadequate dose of dialysis. As for the large portion of patients on a spKt/V greater than 1.6, for which there is no evidence of an outcome benefit, the high dose may constitute a by-product of dialysis time, needed to manage middle molecule or fluid removal. However, it may also reflect a poorly controlled process and inefficient use of clinical resources and patient time.
Intrapatient Variability in Dose Delivery
Blood urea Kt/V monitoring is at best performed on a monthly basis. This gives a scarcity of longitudinal data in individual patients and may explain why intrapatient variability in dose delivery is rarely reported on. In stable patients on a stable prescription, the mean coefficient of variation (CV) in delivered dose has been reported as 5.4 % for monthly URR,17
4.5 to 11 % for Kt/V,18,19 Kt/V.19,20 and 5.1 to 13 % for daily assessment of ionic In all studies there were considerable differences between EUROPEAN NEPHROLOGY
For countries participating in DOPPS, data indicate that only 48 % of dialysis patients receive a spKt/V dose between 1.2 and 1.6, a spKt/V range that may be considered the appropriate target dose window (see Table 1).16
Table 1: Distribution of Delivered Kt/V in Various Countries According to Dialysis Outcomes and Practice Patterns Study 2009 Annual Report16
Australia/NZ Belgium Canada France
Germany Italy
Japan Spain
Sweden UK US
NZ = New Zealand.
patients in the dose variability. A considerable intrapatient variability in dose delivery was also found in an extensive treatment mapping study.21
Factors Predictive of High Variability
Several factors may contribute to the inability to consistently achieve the dose target. Inadequate access for the prescribed blood flow, catheter use, treatment time less than planned, failure to achieve blood flow rate ≥90 % of prescribed value, heparin-free dialysis and younger age (<61 years), have all been found to be predictive of not reaching the dose target.22,23
The more of these
factors that are present, the higher the risk of not achieving the target dose.
Variable volume of blood processed (composite of blood flow rate and treatment time) and catheter use are also significant predictive factors of high intrapatient variability in the delivered dose.17,24
Human
errors, like inadvertent reversal of the arterial and venous needles, also contribute to variability and failure to achieve the target dose;22 more stable dose delivery has been seen with self-care as compared to full nursing care.17
A greater intrapatient variation mostly results
in a lower mean Kt/V than intended; it also means that more measurements are needed to derive at an accurate estimate of the mean dose delivered.
Impact of Inadequate Dose
In a prospective observational study covering 22 dialysis units, higher hospitalisation rate and more hospital days were significantly associated with a delivered Kt/V <1.2.25
A 0.1 decrease in Kt/V was associated with
an 11 % increase in number of hospitalisations, a 12 % increase in hospital days and a significant increase in the cost for inpatient care. The pattern remained after excluding patients on catheter access or only looking at non-access-related hospitalisations.
Several observational studies have documented lower survival rates in patients getting a lower dialysis dose, in particular when Kt/V <1.2 or URR <65 %.8,9,26
Kt/V <1.2 7.4 %
20.9 % 17.3 % 14.4 % 22.9 % 22.7 % 30.3 % 12.9 % 10.1 % 19.5 % 8.7 %
Percentage of Patients Receiving Kt/V 1.2–1.59
44.2 % 47.9 % 48.2 % 45.1 % 52.3 % 49.1 % 52.8 % 51.5 % 42.0 % 45.0 % 43.2 %
Kt/V ≥1.6 48.5 % 31.2 % 34.4 % 40.4 % 24.9 % 28.2 % 16.9 % 35.5 % 47.9 % 35.5 % 48.1 %
The variability in dose delivery is another factor of major importance; analysis of a large patient database revealed that greater intrapatient variation in delivered Kt/V and greater fraction of inadequate treatment sessions (Kt/V <1.2) were both significantly associated with shorter survival time.27
These data clearly suggest that
efforts should be made to minimise the variation in dose delivery, a task that practically requires that every treatment is controlled on the effective dose delivered.
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