Бүйрек диализінде Kt/V көрсеткішінің маңызы мен есептелуі

In medicine, Kt/V is a number used to quantify hemodialysis and peritoneal dialysis treatment adequacy. K – dialyzer clearance of urea
t – dialysis time
V – volume of distribution of urea, approximately equal to patient's total body water

In the context of hemodialysis, Kt/V is a pseudo dimensionless number; it is dependent on the pre and post dialysis concentration (see below). It is not the product of K and t divided by V, as would be the case in a true dimensionless number. In peritoneal dialysis, it isn't dimensionless at all. It was developed by Frank Gotch and John Sargent as a way for measuring the dose of dialysis when they analyzed the data from the National Cooperative Dialysis Study. In hemodialysis the US National Kidney Foundation Kt/V target is ≥ 1.3, so that one can be sure that the delivered dose is at least 1.2. In peritoneal dialysis the target is ≥ 1.7/week. In some dialysis machines, the urea clearance during dialysis is estimated by testing the ability of the dialyzer to remove a small salt load that is added to the dialysate during dialysis.

The URR or Urea reduction ratio is simply the fractional reduction of urea during dialysis. So by definition, URR = 1 − C/C0. So 1−URR = C/C0. So by algebra, substituting into equation above, since 1=ln C/C0 = − ln C0/C, we get:

The above analysis assumes that urea is removed from a single compartment during dialysis. In fact, this Kt/V is usually called the "single pool" Kt/V. Due to the multiple compartments in the human body, a significant concentration rebound occurs following hemodialysis. Usually rebound lowers the Kt/V by about 15%. The amount of rebound depends on the rate of dialysis (K) in relation to the size of the patient (V). Equations have been devised to predict the amount of rebound based on the ratio of K/V, but usually this is not necessary in clinical practice. One can use such equations to calculate an "equilibrated Kt/V" or a "double pool Kt/V", and some think that this should be used as a measure of dialysis adequacy, but this is not widely done in the United States, and the KDOQI guidelines (see below) recommend using the regular single pool Kt/V for simplicity.

On a practical level, in peritoneal dialysis the calculation of Kt/V is often relatively easy because the fluid drained is usually close to 100% saturated with urea, i. e. the dialysate has equilibriated with the body. Therefore, the daily amount of plasma cleared is simply the drain volume divided by an estimate of the patient's volume of distribution. As an example, if someone is infusing four 2 liter exchanges a day, and drains out a total of 9 liters per day, then they drain 9 × 7 = 63 liters per week. If the patient has an estimated total body water volume V of about 35 liters, then the weekly Kt/V would be 63/35, or about 1.8. The above calculation is limited by the fact that the serum concentration of urea is changing during dialysis. So ideally this should not be used as it has not taken in account the urea level in dialysate or serum so it cannot be labelled as urea clearance
In automated PD this change cannot be ignored; thus, blood samples are usually measured at some time point in the day and assumed to be representative of an average value. The clearance is then calculated using this measurement.

Kt/V has been widely adopted because it was correlated with survival. Before Kt/V nephrologists measured the serum urea concentration (specifically the
time averaged concentration of urea (TAC of urea)), which was found not to be correlated with survival (due to its strong dependence on protein intake) and thus deemed an unreliable marker of dialysis adequacy.

It is complex and tedious to calculate. Many nephrologists have difficulty understanding it. Urea is not associated with toxicity. Kt/V only measures a change in the concentration of urea and implicitly assumes the clearance of urea is comparable to other toxins. (It ignores molecules larger than urea having diffusion limited transport so called middle molecules). Kt/V does not take into account the role of ultrafiltration. It ignores the mass transfer between body compartments and across the plasma membrane (i. e. intracellular to extracellular transport), which has been shown to be important for the clearance of molecules such as phosphate. Practical use of Kt/V requires adjustment for rebound of the urea concentration due to the multi compartmental nature of the body. Kt/V may disadvantage women and smaller patients in terms of the amount of dialysis received. Normal kidney function may be modeled as optimal Glomerular filtration rate or GFR. GFR is usually normalized in people to body surface area. A man and a woman of similar body surface areas will have markedly different levels of total body water (which corresponds to V). Also, smaller people of either sex will have markedly lower levels of V, but only slightly lower levels of body surface area. For this reason, any dialysis dosing system that is based on V may tend to underdose smaller patients and women. Some investigators have proposed dosing based on surface area (S) instead of V, but clinicians usually measure the URR and then calculate Kt/V. One can "adjust" the Kt/V, to calculate a "surface area normalized" or "SAN" Kt/V as well as a "SAN" standard Kt/V. This puts a wrapper around Kt/V and normalizes it to body surface area.

Kt/V has been criticized because quite high levels can be achieved, particularly in smaller patients, during relatively short dialysis sessions. This is especially true for small people, where "adequate" levels of Kt/V often can be achieved over 2 to 2.5 hours. One important part of dialysis adequacy has to do with adequate removal of salt and water, and also of solutes other than urea, especially larger molecular weight substances and phosphorus. Phosphorus and similar molecular weights remain elusive to filtration of any degree. A number of studies suggest that a longer amount of time on dialysis, or more frequent dialysis sessions, lead to better results. There have been various alternative methods of measuring dialysis adequacy, most of which have proposed some number based on Kt/V and number of dialysis sessions per week, e. g., the standardized Kt/V, or simply number of dialysis sessions per week squared multiplied by the hours on dialysis per session; e. g. the hemodialysis product by Scribner and Oreopoulos It is not practical to give long dialysis sessions (greater than 4.5 hours) thrice a week in a dialysis center during the day. Longer sessions can be practically delivered if dialysis is done at home. Most experience has been gained with such long dialysis sessions given at night. Some centers are offering every other night or thrice a week nocturnal dialysis. The benefits of giving more frequent dialysis sessions is also an area of active study, and new easy to use machines are permitting easier use of home dialysis, where 2–3+ hour sessions can be given 4–7 days per week.

One question in terms of Kt/V is, how much is enough? The answer has been based on observational studies, and the NIH funded HEMO trial done in the United States, and also, on kinetic analysis. For a US perspective, see the KDOQI clinical practice guidelines and for a United Kingdom perspective see: U. K. Renal Association clinical practice guidelines According to the US guidelines, for thrice a week dialysis a Kt/V (without rebound) should be 1.2 at a minimum with a target value of 1.4 (15% above the minimum values). However, there is suggestive evidence that larger amounts may need to be given to women, smaller patients, malnourished patients, and patients with clinical problems. The recommended minimum Kt/V value changes depending on how many sessions per week are given, and is reduced for patients who have a substantial degree of residual renal function.

For a US perspective, see: For the United States, the minimum weekly Kt/V target used to be 2.0. This was lowered to 1.7 in view of the results of a large randomized trial done in Mexico, the ADEMEX trial, and also from reanalysis of previous observational study results from the perspective of residual kidney function. For a United Kingdom perspective see: This is still in draft form.

Hemodialysis Dose and Adequacy – a description of URR and Kt/V from the Kidney and Urologic Diseases Clearinghouse. Kt/V and the adequacy of hemodialysis – UpToDate. com