Handbook of Dialysis Fifth Edition by Daugirdas, John T., Blake, Peter G. and Ing, Todd S.. Philadelphia, PA: Lippincott Williams & Wilkins. Handbook of dialysis / [edited by] John T. Daugirdas, Peter G. Blake, Todd S. Ing. — Fifth edition. p. ; cm. Includes bibliographical references and index. Request PDF on ResearchGate | On Feb 5, , John T Daugirdas and others published Handbook of Dialysis 5th Edition.
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For ex- ample, a serum creatinine of 1. In the United States and many other countries, laboratories are now normal- izing their measurement methods to IDMS, and the nor- malized values tend to be lower than those obtained using other methods.
Creatinine clearance by timed urine collection.
A timed usu- ally 24 hours urine collection of creatinine excretion can be used to calculate the creatinine clearance CCr , which is defined as the volume of serum cleared of creatinine per minute. Patients are instructed to urinate Daugirdasch Next they are to pass all of their urine into a container during the ensuing day and night.
The following morning, the patients are to urinate into the container one last time, and to note this time as the end of the collection period.
By dividing the amount of creati- nine in the collected urine by the number of minutes in the collection period start time to finish time , the laboratory can calculate the per minute rate of creatinine excretion. A sample of blood must be drawn at some point during the urine collection period in which the serum creatinine level is measured.
To calculate creatinine clearance, one simply divides the per minute creatinine excretion rate by the se- rum value.
For example, if the per minute creatinine excretion rate is 1. Despite the technical challenge of collecting urine properly, timed urine collections are a very useful means of estimating kidney function in cachec- tic patients, including those with cirrhosis and ascites, as well as markedly obese patients.
The completeness of the urine collection for creatinine can be estimated by com- paring the amount of creatinine recovered per day based on the expected daily creatinine excretion rate for a given patient based on sex and body weight.
A more exact estimate of daily creatinine excretion rate can be obtained from the use of an equation incorpo- rating body weight, gender, age, and race, such as that de- veloped by Ix , and which is detailed as a nomogram in Appendix A.
A creatinine excretion rate that is signifi- cantly less than expected usually indicates an incomplete urine collection. Because creatinine is cleared by the renal tubules in addition to being filtered at the glomerulus, the creatinine clearance is greater than GFR. To get a more reliable estimate of GFR when GFR is low, one can measure the amounts of both creatinine and urea in the timed urine sample, and measure the serum urea level as well as the creatinine level during the collection period.
The per minute clearance of urea is calculated in the same way as for creatinine. Urea is filtered at the glomerulus, but then some urea is reabsorbed by the renal tubules, so with urea, the situation is opposite to that with creatinine; due to tubular reabsorption, the urea clearance will be less than Daugirdasch Estimated creatinine clearance.
To avoid the inaccuracies and inconvenience of timed urine collections, creatinine clear- ance CCr can be estimated by using equations that esti- mate the per minute creatinine excretion rate based on age, body size, gender, and in some equations, race.
This equation provides a quick and reasonably accurate estimate of renal function at the bedside. The more recently developed Ix equation Ix, , described in Appendix A, also can be used. The Ix equation was developed and validated in a much larger sample of individuals, including blacks, and was based on modern, IDMS-calibrated laboratory measures of creati- nine. Neither equation is very accurate in markedly obese or cachectic patients. Some have suggested that the ac- curacy of the Cockcroft—Gault equation can be increased by using actual body weight for cachectic patients, ideal body weight for normal weight patients, and adjusted body weight for markedly obese patients Brown, Estimated GFR a.
First, it was developed from data that measured GFR by iothalamate, a substance which is not secreted by the re- nal tubules, and so it predicts GFR rather than creatinine clearance. All else being equal, the MDRD equation will give a lower value for renal function GFR than creatinine clearance, which includes the tubular secretion component of renal function.
Creatinine clearance, whether obtained from a timed urine sample or from the Ix or Cockcroft—Gault equation, is raw renal creatinine clearance that has not been adjusted for body size. This is similar to the MDRD equation, but this newer equation was validated in a larger group of subjects, particularly those with only mild degrees of renal impairment.
The differences between the two equations are usually not of clinical importance, as they occur primarily in patients with GFR levels greater than 60, where the impact of knowing the precise level of renal function is not particularly large. Cystatin C equations.
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