A central issue in the management of patients undergoing maintenance hemodialysis is the assessment of adequacy of dialysis. Simply following the blood urea nitrogen (BUN) is insufficient because a low BUN can reflect inadequate nutrition rather than sufficient dialytic urea removal. Monitoring the patient's symptoms alone is also insufficient, since the combination of dialysis plus erythropoietin to correct anemia can eliminate most uremic symptoms although the patient may be underdialyzed. (See "Erythropoietin for the anemia of chronic kidney disease among predialysis and peritoneal dialysis patients" and "Erythropoietin for the anemia of chronic kidney disease in hemodialysis patients".) Thus, in addition to symptoms, patient nutrition and survival appear to best reflect dialysis adequacy.
The National Cooperative Dialysis Study (NCDS) established that the timed average urea concentration and the protein catabolic rate (PCR) were important determinants of morbidity and mortality in hemodialysis patients [1,2]. (See "Protein catabolic rate in maintenance dialysis".) In particular, well-nourished patients with a more intensive dialysis prescription and a lower timed average BUN had a better outcome. In a patient with little or no urine output, the PCR (in g/day) is equal to the sum of the dialysis and stool losses of urea, protein, and amino acids. These losses (the PCR) are roughly equal to protein intake when a patient is in a steady state with a relatively constant predialysis BUN.
Gotch later used a mechanistic analysis of these data and showed that the Kt/V of urea (see below) was an important measure of clinical outcome . Urea was chosen by the NCDS as the clearance marker for the Kt/V since it is a reflection both of dietary protein intake and of the efficiency of removal of small uremic toxins. (See "Uremic toxins".)
Individualizing the hemodialysis prescription to a particular patient's needs using Kt/V can be a useful tool in providing a safe and cost-effective dialysis treatment. This can be accomplished with urea kinetic modeling, which allows for variations in dialysis time, use of larger, high efficiency, high-flux dialyzers, and optimization of dietary protein need. Urea kinetic modeling is a method for verifying that the amount of dialysis prescribed (the prescribed Kt/V) equals the amount of dialysis delivered (the effective Kt/V). Kinetic modeling also quantifies the amount of urea generated, which is a marker of the protein catabolic rate and therefore of protein intake.
DEFINITION AND CALCULATION OF KT/V
Kt/V is defined as the dialyzer clearance of urea (K, obtained from the manufacturer in mL/min, and periodically measured and verified by the dialysis team) multiplied by the duration of the dialysis treatment (t, in minutes) divided by the volume of distribution of urea in the body (V, in mL), which is approximately equal to the total body water. The correction of total urea removal (Kt) for volume of distribution is important because, in a large patient, a given degree of urea loss represents a lower rate of removal of the total body burden of urea (and presumably of other small uremic toxins).