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Encyclopedia > Urea reduction ratio

For the Scottish river see: Urr Water Urr Water is a river of southwest Scotland. ...

The urea reduction ratio (URR), is a dimensionless number used to quantify hemodialysis treatment adequacy. In dimensional analysis, a dimensionless number (or more precisely, a number with the dimensions of 1) is a pure number without any physical units. ... It has been suggested that Artificial kidney be merged into this article or section. ...



URR = frac{U_{pre}-U_{post}}{U_{pre}} times 100%


  • Upre is the pre-dialysis urea level
  • Upost is the post-dialysis urea level

Whereas the URR is formally defined as the urea reduction "ratio", in practice it is informally multiplied by 100% as shown in the formula above, and expressed as a percent. Urea is an organic compound with the chemical formula (NH2)2CO. Urea is also known as carbamide, especially in the recommended International Nonproprietary Names (rINN) in use in Europe. ...

History and Overview

The URR was first popularized by Lowrie and Lew in 1991 as a method of measuring amount of dialysis that correlated with patient outcome.[1] This method is very useful because of its simplicity. It permits easy monitoring of the amount of dialysis therapy delivered to individual patients, as well as across dialysis units, groups of units, states, regions, or countries, because monthly predialysis and postdialysis urea nitrogen values are routinely measured. It also permits quality control and improvement initiatives and regulatory oversight. The United States Renal Data Systems (USRDS) publishes annual data regarding the URR values being delivered to dialysis patients across the United States. The ESRD networks monitor therapy across groups of states. The European Renal Association (ERA-EDTA) Registry covers most European countries, and DOPPS (Dialysis Outcomes Practice Patterns Study) records and analyzes URR and other data from selected dialysis units located in countries across the world.

Relation to Kt/V

Mathematically, the URR is closely related to Kt/V, and the two quantities can be derived from another with more or less precision, depending on the amount of additional information available about a given dialysis session. In medicine, Kt/V is a number used to quantify hemodialysis and peritoneal dialysis treatment adequacy. ... In medicine, Kt/V is a number used to quantify hemodialysis and peritoneal dialysis treatment adequacy. ...

Kt/V is one of the reference methods by which the amount of dialysis given is measured. Kt/V, like the URR, focuses on urea as the target solute, and is based on the assumption that removal of urea is from a single space - urea distribution volume, or V , similar in capacity to the total body water. The urea distribution volume V ,, although traditionally thought of as 60% of body weight, may actually be closer to 50% of the body weight in women and 55% in men with stage V (GFR < 15 ml/min) chronic kidney disease. The clearance of urea during the dialysis session K , can be expressed in either frac {ml}{min} or frac {L}{hr}.
Time or t , is the duration of the dialysis session, measured either in minutes or hours. So K cdot t is also a volume, either frac {ml}{min} cdot min = ml, or  frac {L}{hr} cdot hr = L, and represents the volume of blood (in ml or L) cleared of urea during the dialysis session. Because V , is also a volume, the ratio of frac {K cdot t}{V} has dimensions of frac{ml}{ml} or frac{L}{L}, making it a "dimensionless" ratio.

In a simplified model of urea removal from a fixed volume with no urea generation,  frac{K cdot t}{V} is related to URR , by the following relationship:

 frac{K cdot t}{V} = -ln (1-URR)

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 because a small amount of urea is generated during the dialysis session. Both of these factors make the actual post-dialysis serum urea level higher than expected, and the URR lower than expected, when the extremely simplified equation above, is used.

A more accurate relationship between URR and Kt/V can be derived by single-pool, variable volume urea kinetic modeling. A simplified estimating equation also can be used [2]. This gives results that are quite similar to formal urea modeling as long as dialysis treatments of 2-6 hours in duration are given, and Kt/V is between 0.7 and 2.0.

frac{K cdot t}{V} = -ln ((1-URR) - 0.008 cdot t) + (4-3.5 (1-URR)) cdot frac {0.55 cdot UF}{V}

The (0.008 cdot t) term is a function of the dialysis session duration (t), and adjusts for the amount of urea generated during the dialysis session. The second term, (4-3.5 (1-URR)) cdot frac {0.55 cdot UF}{V} adjusts for the additional urea that is cleared from the body through volume contraction.

Because frac {0.55 cdot UF}{V} can be approximated by frac{UF}{W}, 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:

frac{K cdot t}{V} = -ln ((1-URR) - 0.03) + (4-3.5 (1-URR)) cdot frac {UF}{W}

Nomogram relating Kt/V and URR

Instead of equations, a nomogram can be used to easily estimate Kt/V from the URR in clinical practice. To use the nomogram, one needs to know the postdialysis weight (W) as well as the amount of weight (fluid) loss during the dialysis session (UF). First, find the URR on the vertical axis, then move over to the proper isopleth (curved line) depending on the amount of weight lost during dialysis (UF/W). Then drop down to the horizontal axis to read off the Kt/V value: Image File history File links Urr_ktv. ... Image File history File links Urr_ktv. ...

Limitations of URR vs. Kt/V

The URR is designed to measure the amount of dialysis given when the dialysis clearance of urea greatly exceeds the urea generation rate. In continuous hemodialysis or in peritoneal dialysis, for example, a considerable amount of dialysis is delivered, but the urea level remains roughly constant after the initial treatment of uremia, so the URR is essentially zero. In long slow overnight dialysis, if simplified equations are used, the URR also underestimates the amount of dialysis due to urea generation during the long dialytic session. For this reason, the kinetically modeled Kt/V is always recommended as the best measure of dialysis adequacy. The Kt/V, even that derived by formal modeling, is primarily based on the URR, and so it contains little additional information in terms of the amount of dialysis that was delivered. Since the URR and Kt/V are so closely related, their predictive power in terms of patient outcome is similar. However, use of Kt/V and urea modeling in general allows for comparing expected with predicted dose of dialysis, which can be used to analyze dialysis treatments and dialyzer clearances and in troubleshooting and quality control activities. Also, Kt/V permits calculation of the urea generation rate, which can give clues about a patient's protein intake.

Minimally adequate dose in terms of URR

In the standard 3x/week hemodialysis schedule a URR of 65% is considered the minimum acceptable dose, corresponding to a minimum Kt/V of 1.2 [3] When dialysis is given more frequently than three times a week, the minimum acceptable URR is lower; because more dialysis treatments are given over the week, the dose of dialysis for each treatment does not need to be as large. Also minimally acceptable values for URR (and Kt/V) can be reduced in patients who have substantial amounts of residual renal function [4].

See also

In medicine, the clearance, also renal clearance or renal plasma clearance (when referring to the function of the kidney), of a substance is the inverse of the time constant that describes its removal rate from the body divided by its volume of distribution (or total body water). ... It has been suggested that Artificial kidney be merged into this article or section. ...

External links

  • Hemodialysis Dose and Adequacy - a description of URR and Kt/V from the Kidney and Urologic Diseases Clearinghouse.


It has been suggested that this article or section be merged into Excretory system. ... Human Physiology is the science of the mechanical, physical, and biochemical functions of humans in good health, their organs, and the cells of which they are composed. ... This illustration demonstrates the normal kidney physiology. ... Acid-base physiology is the study of the acids, bases and their reactions in the body. ... Look up filtration in Wiktionary, the free dictionary. ... Ultrafiltration (UF) is a variety of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane. ... Countercurrent exchange is a mechanism used to transfer some component of a fluid from one flowing current of fluid to another across a permeable barrier between them. ... Arginine vasopressin (AVP), also known as argipressin or antidiuretic hormone (ADH), is a human hormone that is released when the body is low on water; it causes the kidneys to conserve water, but not salt, by concentrating the urine and reducing urine volume. ... Aldosterone is a steroid hormone (mineralocorticoid family) produced by the outer-section (zona glomerulosa) of the adrenal cortex in the adrenal gland to regulate sodium and potassium balance in the blood. ... Atrial natriuretic factor (ANF), atrial natriuretic peptide (ANP) or atriopeptin, is a polypeptide hormone involved in the homeostatic control of body water and sodium. ... The endocrine system is a control system of ductless endocrine glands that secrete chemical messengers called hormones that circulate within the body via the bloodstream to affect distant organs. ... Not to be confused with rennin, the active enzyme in rennet. ... Erythropoietin (IPA pronunciation: , alternative pronunciations: ) or EPO is a glycoprotein hormone that is a cytokine for erythrocyte (red blood cell) precursors in the bone marrow. ... Vitamin D is a fat soluble vitamin that contributes to the maintenance of normal levels of calcium and phosphorus in the bloodstream. ... E1 - Alprostadil I2 - Prostacyclin A prostaglandin is any member of a group of lipid compounds that are derived enzymatically from fatty acids and have important functions in the animal body. ... In medicine (nephrology) renal function is an indication of the state of the kidney and its role in physiology. ... In nephrology, dialysis adequacy is the measurement of renal dialysis for the purpose of determining dialysis treatment regime and to better understand the pathophysiology of renal dialysis. ... Glomerular filtration rate (GFR) is the volume of fluid filtered from the renal (kidney) glomerular capillaries into the Bowmans capsule per unit time. ... Creatinine clearance is a method that estimates the glomerular filtration rate (GFR) of the kidneys. ... The renal clearance ratio is found with the following equation: X is the analyte substance Cx is the renal plasma clearance of X Cin is the renal plasma clearance of inulin. ... In medicine, Kt/V is a number used to quantify hemodialysis and peritoneal dialysis treatment adequacy. ... Standardized Kt/V, also std Kt/V, is a way of measuring (renal) dialysis adequacy. ... Hemodialysis product (HDP) - is a number used to quantify hemodialysis and peritoneal dialysis treatment adequacy. ... Acid-base physiology is the study of the acids, bases and their reactions in the body. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... A Darrow Yannet diagram is a schematic used in physiology to identify how the volumes of extracellular fluid and intracellular fluid alter in response to conditions such as adrenal insufficiency and SIADH. It was developed in 1935. ... A significant fraction of the human body is water. ... Interstitial fluid (or tissue fluid, or intercellular fluid) is a solution which bathes and surrounds the cells of multicellular animals. ... In some animals, including mammals, the two types of extracellular fluids are interstitial fluid and blood plasma. ... The cytosol (cf. ... Blood plasma is the liquid component of blood, in which the blood cells are suspended. ... Transcellular fluid is the portion of total body water contained within epithelial lined spaces. ... In human physiology, the base excess (see: base) excess refers to the amount of acid required to return the blood pH of an individual to the normal value. ... In acid base physiology, the Davenport Diagram is a graphical tool, developed by Horace Davenport, that allows a clinician or investigator to describe blood bicarbonate concentrations and blood pH following a respiratory and/or metabolic acid-base disturbance. ... The anion gap is used to aid in the differential diagnosis of metabolic acidosis. ... The Bicarbonate buffering system is the most important buffer for mantaining a relatively constant pH in the plasma. ... Respiratory compensation is a mechanism by which plasma pH can be altered by varying the respiratory rate. ... Renal compensation is a mechanism by which the kidneys can regulate the plasma pH. It is slower than respiratory compensation, but has a greater ability to restore normal values. ...


  1. ^ Owen WF Jr, Lew NL, Liu Y, Lowrie EG, Lazarus JM. The urea reduction ratio and serum albumin concentration as predictors of mortality in patients undergoing hemodialysis. N Engl J Med. 1993 Sep 30;329(14):1001-6. PMID 8366899
  2. ^ 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
  3. ^ KDOQI 2006 Hemodialysis Adequacy Guidelines. Guideline 4. [1].
  4. ^ KDOQI 2006 Hemodialysis Adequacy Guidelines. CPR (Clinical Practice Recommendation) #4. [2]

  Results from FactBites:
Urea reduction ratio - Wikipedia, the free encyclopedia (209 words)
The urea reduction ratio (URR), is a dimensionless number used to quantify hemodialysis treatment adequacy.
The numerator of the URR definition is the removal of urea in the dialysis session, if the urea produced during the dialysis is ignored.
The URR was introduced by Lowrie and Lew in 1991.
Blood urea nitrogen - Wikipedia, the free encyclopedia (593 words)
Urea is a substance secreted by the liver, and removed from the blood by the kidneys.
Impaired renal excretion of urea may be due to temporary conditions such as dehydration or shock, or may be due to either acute or chronic disease of the kidneys themselves.
Elevated BUN in the setting of a relatively normal creatinine may reflect a physiological response to a relative decrease of blood flow to the kidney (as seen in heart failure or dehydration) without indicating any true injury to the kidney.
  More results at FactBites »



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