Creatinine clearance may be standardized based on lean body mass (LBM). Hallynck et al have found that this can be used to derive a clearance value independent of a patient's age and gender, something that may be missing when the clearance is standardized to body surface area. This clearance can be useful when evaluating drug elimination by the kidney. The authors are from several institutions in Belgium and Switzerland.
creatinine clearance in mL per min per 50 kg LBM =
= (excretion creatinine) / (serum creatinine) = E / S =
= (urinary creatinine excretion rate in mg per 100 min per 50 kg LBM) / (serum creatinine in mg/dl)
where:
• If the units are calculated out (mg per 100 min per 50 kg LBM divided by mg per 100 mL), the output value is in mL per min per 50 kg LBM.
The urinary excretion of creatinine in mg per 100 min per 50 kg LBM can be obtained by:
(1) use of the nomogram shown in Figure 1, page 416 (Hallynck et al, Clin Pharmacol Therap, 1981).
(2) testing a 24 hour urine
Data from the nomogram in Figure 1 based on age (does not include data for an age < 1 year of age)
|
Age in Years
|
urinary creatinine excretion in mg per 100 min per 50 kg LBM
|
|
1
|
52
|
|
3
|
68
|
|
4
|
76
|
|
5
|
81
|
|
10
|
92.5
|
|
15
|
100.5
|
|
17
|
101
|
|
20
|
98
|
|
25
|
97
|
|
30
|
97.5
|
|
36
|
98
|
|
40
|
94
|
|
55
|
85
|
|
60
|
76
|
|
70
|
65.5
|
|
80
|
57.5
|
|
100
|
42.5
|
data read from graph in Figure 1
Equations for this data derived using JMP:
urinary creatinine excretion in mg per 100 min per 50 kg LBM for ages 1 to 17 years =
= (-0.001931 * ((age in years)^4)) + (0.0883451 * ((age in years)^3)) – (1.553784 * ((age in years)^2)) + (14.02078 * (age in years)) + 39.093745
urinary creatinine excretion in mg per 100 min per 50 kg LBM for ages 17 to 36 years =
= (0.0001797 * ((age in years)^4)) - (0.02192 * ((age in years)^3)) + (0.9955104 * ((age in years)^2)) - (19.86012 * (age in years)) + 243.60103
urinary creatinine excretion in mg per 100 min per 50 kg LBM for ages 36 to 100 years =
= 130.21774 – (0.891211 * (age in years))
To calculate the urinary excretion from a 24 hour urine:
urinary excretion creatinine in mg per 100 mL from 24 hour urine =
= (urinary concentration creatinine in mg/dL) * (10 * (24 hours urine volume in L)) * 100 / 1440
This can be normalized to 50 kg LBM by:
urinary excretion creatinine in mg per 100 mL per 50 kg LBM =
= (urinary excretion of creatinine in mg per 100 mL) * 50 / (patient's lean body mass in kg)
Limitations:
• I am not sure if the method has been validated in patients at the extremes of body weight (cachexia, morbid obesity).
• While the calculation is claimed to be independent of age, it would seem that the urinary excretion of creatinine is not.
• Variations in creatinine measurements based on different methods and laboratory techniques could affect the accuracy of the readings.
