4.34: Renal Function - Creatinine Clearance Calculation

A technologist is asked to calculate a patient’s 24 hour creatinine clearance. The urine sample is an aliquot from the total sample reported on the laboratory slip to be 800 mL. The plasma creatinine level is observed to be 14 mg/L. The urine creatinine concentration is found to be 900 mg/L.

QUESTIONS

1. How is a creatinine clearance calculated?
2. The technologist calculates the creatinine clearance and reports the value in the LIS as 36 mL/min. That afternoon, a nephrologist calls the laboratory, questioning the accuracy of the result and asks for a reanalysis of the specimens.
   
   The technologist repeats both the creatinine analyses and the calculation and comes up with essentially the same result. The technologist brings the problem to the laboratory’s clinical chemist. Together they review the patient’s demographics in the laboratory computer and find out that the patient is a 23 year old, 225 lb., 6' 3", white male. Does this information help resolve the question of the validity of the results?

Questions to Consider

1. What are the units for creatinine clearance?
2. What is the formula for the calculation of creatinine clearance, and what additional data are needed?
3. What is the usual volume of urine output for a healthy male? What is the usual amount of creatinine excreted in urine of such individuals?
4. Will the correction of the creatinine clearance for body surface area give a more valid result?
5. What is the formula for correcting creatinine clearance for body surface area?
6. What additional data are needed to calculate a corrected creatinine clearance?
7. What is the corrected value for creatinine clearance for this patient?
8. What is the reference range for serum creatinine and creatinine clearance?

Answer

1. The creatinine clearance is calculated from the formula given in Answer #2. It is important to remember that only the urine and plasma creatinine values were directly measured by the technologist. The 24-hour creatinine clearance is calculated as follows: $$\begin{split} C_{cr} &= \frac{UV}{P} \\ &= \frac{(900\; mg/L) (\frac{800\; mL}{24\; hrs})}{(14\; mg/L)(\frac{1440\; min}{L \cdot 24\; hours})} \\ &= 36\; mL/min \end{split}$$
2. No! The ca1culations are all correct, but the final result is obviously not appropriate for an individual whose serum creatinine is 14 mg/L. Since the creatinine measurements were probably accurate one must assume the urine volume of 800 mL did not reflect the actual 24-hour urine excretion. This supposition is supported by the very low urine output (approximately one-half to one-third that of a normal male) and by the total creatinine output.
   
   A proper 24-hour urine sample is one of the most difficult specimens to obtain. The laboratory should request a new 24-hour urine sample after consulting with the patient’s nephrologist.

Answers to Questions to Consider

1. The units for creatinine clearance are mL/min. This is the theoretical equivalent amount of plasma from which creatinine is cleared by the kidneys per minute. See p 487, Chapter 26.
2. The equation for creatinine clearance described on Chapter 26 p 487 is: $$\text{Creatinine clearance (mL/min)} = \frac{UV}{P}$$which requires the term “urine volume per minute” (V) as well as the urine (U) and plasma (P) creatinine concentrations. The additional information which is needed to perform the calculation is the time over which the sample was collected. This allows the calculation of the rate of plasma clearance in mL/min.
   
   The creatinine clearance is a way of estimating a patient's glomerular filtration rate (GFR). Many now argue (See Web site: www.kidney.org/professionals/...elineindex.cfm) that, given the difficulties and errors associated with a timed (i.e., a 24-hour urine collection) urine sample, that it is actually better to estimate the GFR directly from the measured serum creatinine value. One of the popular equations is the Cockcroft & Gault equation also listed on p 487).
3. The usual urine volume output for an adult is approximately 1.5 L per day (Chapter 26). The usual urinary output of creatinine per day is 1 g to 2 g for a healthy male (See Method Creatinine on CD-ROM).
4. The creatinine output is dependent upon a body’s muscle mass and, of course, renal function. Thus normalizing the calculated creatinine clearance to an “average” body size is usually performed to correct for differences in muscle mass.
5. The formula for correction for body surface area is the calculated clearance (Cl) times the ratio of the ideal body surface (1.73 m²)/patient body surface. $$Corrected\; C_{cr} = Cl \times \frac{1.73\; m^{2}}{\text{Patient body surface}}$$
6. The additional data needed are the patient’s height and weight. These will allow calculation of the body surface from the nomogram of Appendix I.
7. The calculation of the normalized value is as follows: $$37\; mL/min \times \frac{1.73\; m^{2}}{2.30\; m^{2}} = 28\; mL/min$$
8. The reference range for uncorrected creatinine clearance is 97-137 mL/min. While the plasma creatinine level reference range is 6.4-10.4 mg/L. The creatinine level is only a little outside the reference range. See CREATININE metehod on CD-ROM.