Urea reduction ratio
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The urea reduction ratio (URR), is a dimensionless number used to quantify hemodialysis treatment adequacy.
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[edit] Definition
<math>URR = \frac{U_{pre}-U_{post}}{U_{pre}} \times 100\% </math>
Where:
- Upre is the pre-dialysis urea level
- Upost is the post-dialysis urea level
[edit] Standard dose
In the standard thrice hemodialysis schedule a URR of 65% is considered the minimum acceptable dose.
[edit] URR in frequent dialysis
The URR decreases if dialysis is done more frequently, as the mass removed is spread over more dialysis sessions. A lower URR under these conditions does not necessarily mean that the dialysis is inadequate.
[edit] Relation to Kt/V
Kt/V is another measure of dialysis. This was based on the assumption that removal of urea is from a single space called (V), similar in dimensions to the total body water. This is approximately 50% of the body weight in women and 55% in men. K is the clearance of urea during a dialysis session, and can be expressed in either ml per min or L per hr. t is the length of the dialysis session, measured either in min or hr. So <math>K \cdot t</math> is in units of Volume, either <math>ml/min \cdot min = ml</math>, or <math>L/hr \cdot hr = L</math>. Because V is also a volume, measured in either ml or L, the ratio of <math>K \cdot t</math> to <math>V</math> has dimensions of ml/ml or L/L, that is, it is a dimensionless ratio.
In a model of urea removal from a fixed volume, <math> \frac{K \cdot t}{V}</math> is related to URR by the following relationship:
<math> \frac{K \cdot t}{V} = -ln (1-URR)</math>
In actual fact, this relationship is made a bit more complex by the fact that fluid is removed during dialysis, so the removal space (V) shrinks, and some urea is generated during dialysis, at a rate (g).
For this reason, a slightly more complex equation that takes into account the way Kt/V is usually calculated using a single-pool, variable-volume model, is as follows:
<math>\frac{K \cdot t}{V} = -ln ((1-URR) - 0.008 \cdot t) + (4-3.5 (1-URR)) \cdot \frac {0.55 \cdot UF}{V}</math>
The <math>(0.008 \cdot t)</math> term is a function of the dialysis session length (t), and adjusts for additional urea generated during the dialysis session. The second term, <math>(4-3.5 (1-URR)) \cdot \frac {0.55 \cdot UF}{V}</math> adjusts for the additional urea that is cleared from the body through volume contraction.
Because <math>\frac {0.55 \cdot UF}{V}</math> can be approximated by <math>\frac{UF}{W}</math>, where UF = ultrafiltrate removed during dialysis (estimated as the weight lost during the treatment) and W = postdialysis body weight, and because dialysis sessions given 3 times per week are usually about 3.5 hours long, the above equation can be simplified to:
<math>\frac{K \cdot t}{V} = -ln ((1-URR) - 0.03) + (4-3.5 (1-URR)) \cdot \frac {UF}{W}</math>
Since the URR and Kt/V are related their predictive power in terms of outcome is similar. However, use of Kt/V and urea modeling in general allows for comparing expected with predicted results of dialysis, and also calculation of the urea generation rate, which can give clues about a patient's protein intake.
[edit] History
The URR was introduced by Lowrie and Lew in 1991.<ref>Shinaberger JH. Quantitation of dialysis: historical perspective. Semin Dial. 2001 Jul-Aug;14(4):238-45. PMID 11489196.</ref>
[edit] References
<ref>
Daugirdas JT. Second generation logarithmic estimates of single-pool variable volume Kt/V: an analysis of error. J Am Soc Nephrol. 1993 Nov;4(5):1205-13. PMID: 8305648</ref>
<references/>
[edit] See also
[edit] External links
- Hemodialysis Dose and Adequacy - a description of URR and Kt/V from the Kidney and Urologic Diseases Clearinghouse.
[edit] Calculators
- Kt/V calculator - medindia.com
- TAC calculator - medindia.com
- Nephrology Calculator - does various renal calculations.
- Kt/V calculator - hdcn.com
