Urea clearance

A 3.5 h treatment of a 70 kg patient (V = 40.6 liters) with a urea clearance of 200 ml,/min should result in a 64% reduction in urea concentration or a value of 0.36 for the ratio d (f this parameter almost always falls between 0.30 and 0.45. The increase in urea concentration between hemodialysis treatments is obtained from equation 13, again assuming a constant V, where (f is the urea concentration in the patient s blood at the end of the hemodialysis, and d the concentration at time t during the intradialytic interval. 

Increased clearance of larger solutes, which may explain good clinical status in spite of lower urea clearance. 

Because pharmacokinetics comprises the first few chapters of this book and figures prominently in subsequent chapters, we will pause here to introduce the clinically most important concept in pharmacokinetics the concept of clearance. In 1929, Moller et al. (22) observed that, above a urine flow rate of 2 mL/min, the rate of urea excretion by the kidneys is proportional to the amount of urea in a constant volume of blood. They introduced the term clearance to describe this constant and defined urea clearance as the volume of blood that one minute s excretion serves to clear of urea. Since then, creatinine clearance has... 

Blood urea nitrogen concentration and urea clearance. 

A series of clinical outcome reports have demonstrated that measures of PD solute removal correlate with patient status and outcome. In particular, a multicenter prospective cohort study of 680 incident CAPD patients [Canada-United States (CANUSA) Study] showed that a decrease of 0.1 in weekly urea clearance (defined by Kt/V ,ea) was associated with a 5% increase in the relative risk of death. Similarly a decrease of 5L/wk/1.73m of total creatinine clearance (Cc,) was associated with a 7% increase in the risk for death. As a consequence of these studies, national guidefines from the United Kingdom, Australia, and the United States " have set standards of dialysis adequacy in terms of small solute removal. An estimate of adequacy is performed in all patients withm 6 to 8 weeks of commencement of dialysis. Further studies should be performed at least annually. ... 

A deficiency in delivered hemodialysis therapy may be related to patient compliance with the dialysis prescription (ending dialysis early). Other causes for a low Kt/V include low blood flow rates due to access stenosis or thrombosis, or due to the use of catheters. Adequate dialysis may not be achieved in some patients despite compliance and sufficient blood flow. Eor these patients there are really only two options to increase urea clearance use a larger membrane or increase the treatment time. 

As in hemodialysis, the clearance of urea, a product of protein catabolism, is measnred with Kt/V. Kt/V is a unitless value that correlates the patient s peritoneal membrane urea clearance (K) with the dnration of dialysis (t) and the volume of distribution (P) of urea. Calculation of Kt/V for PD requires that the total volume of drained effluent per day be determined (this value is the volume instilled plus volume of water ultraflltered). The dialysate to plasma (D/P) urea concentration is determined, and Kt is estimated as ... 

Blood urea nitrogen concentration (BUN) and urea clearance (Curea)... 

Fig. 7.24 Urea clearance in a microchannel dialyzer with a nominal area of 0.35 m (solid line and data points), devised by Oregon State University, compared with urea clearance in a typical hollow fiber commercial dialyzer (dashed line) [285].

Urea is filtered at the glomerulus and about 40% is reabsorbed by passive diffusion from the renal tubules. Estimation of urea clearance therefore represents about 60% of the glomerular filtration rate. In most laboratories, however, creatinine clearance measurement is preferred to urea clearance. 

Kt/Vis defined as K = urea clearance, t = treatment time, and V = urea distribution volume. Using a simple mass balance, a single pool Kt/V can be calculated as... 

The direct measurement of clearances normalized to body surface area and body water volume, expressed on a weekly basis, is crucial in the assessment of treatment adequacy. Creatinine and urea clearance are the most widely used adequacy indices in PD and APD. In a steady state, the calculation of clearance, that is, the ratio between dialytic (and renal) solute removal and blood solute concentration, requires quantitation of solute removed by total collection of drained dialysate. Blood and dialysate solute concentrations are measured by standard assays. The ratio of dialysate to blood solute concentration multiplied by the dialysate volume equals the clearance (Van Stone 1989). In APD, the intermittency or variable intensity of the therapy causes a modest compartmental disequilibrium with fluctuation of plasma concentrations between the predialytic (evening) and postdialytic (morning) values (Newman 1995, Calconi 1998). This difference is more marked for urea than for creatinine (Newman 1995). 

---