Creatinine clearance

Creatinine clearance and glomerular filtration rate[edit]

The creatinine clearance approximates the glomerular filtration rate. However, the creatinine clearance systematically overestimates the GFR due to excretion creatinine by the renal tubules. The correction factor is below:^[1]

{\mbox{Glomerlular filtration rate}}={\mbox{0.81}}\ \times \ {\mbox{Creatinine clearance}}

Food and Drug Administration provides guidance on the labeling of prescription medications to guide dosing for patients with impaired renal function.^[2] Although this categorization uses estimated creatinine clearance, using estimated glomerular filtration yields similar recommendations for dosing adjustments.^[3]

Direct measurement of the creatinine clearance[edit]

By measuring the amount of creatinine excreted in the urine over one day, the creatinine clearance may be calculated. Creatinine is an endogenous molecule, synthesized in the body, which is freely filtered by the glomerulus (but also secreted by the renal tubules in very small amounts). Creatinine clearance is therefore a close approximation of the glomerular filtration rate. The formula for the creatinine clearance is:^[1]

{\mbox{Creatinine clearance }}={\frac {\frac {\mbox{ Amount of creatinine in urine (mg)}}{\mbox{Duration of the urine collection (minutes)}}}{{\mbox{Plasma creatinine concentration (mg}}/{\mbox{ml)}}}}

Example: A person has a plasma creatinine concentration of 0.01 mg/ml and in 1 hour he excretes 75 mg of creatinine in the urine. The GFR is calculated as M/P (where M is the mass of creatinine excreted per unit time and P is the plasma concentration of creatinine).

{\mbox{Creatinine clearance }}={\frac {\frac {75{\mbox{ mg}}}{60{\mbox{ mins}}}}{0.01{\mbox{ mg}}/{\mbox{ml}}}}=125{\mbox{ ml}}/{\mbox{min}}

Estimating the creatinine clearance[edit]

The Cockcroft-Gault formula may be used to estimate the creatinine clearance without having to collect urine over a period of time.^[4] However, it does not correlate as strongly with the GFR as do the MRDR formula.^[5]

{\mbox{Creatinine clearance}}={\frac {{\mbox{(140 - Age)}}\times {\mbox{Mass (in kilograms)}}}{{\mbox{72}}\times {\mbox{Plasma Creatinine (in mg/dl)}}}}\times {\mbox{0.85 if female}}

The Cockcroft-Gault formula may use the ideal body weight (IBW) instead of actual body weight(ACT).^[3]^[6] If the ideal body weight is used, "if ACT was less than IBW, then ACT was used or if ACT exceeded IBW by > 30%, ABW was used. The adjusted body weight (ABW) is calculated:

{\text{ABW}}={\text{IBW}}+(0.4\times ({\text{ACT}}-{\text{IBW}})

The Estimated Creatinine Clearance then estimates GFR:^[1]

{\mbox{Glomerlular filtration rate}}={\mbox{0.84}}\ \times \ {\mbox{Cockcroft-Gault formula}}

References[edit]

↑ ^1.0 ^1.1 ^1.2 Levey AS, Bosch JP, Lewis JB, Greene T, Rogers N, Roth D (1999). "A more accurate method to estimate glomerular filtration rate from serum creatinine: a new prediction equation. Modification of Diet in Renal Disease Study Group". Ann. Intern. Med. 130 (6): 461–70. PMID 10075613. ^[e]
↑ Food and Drug Administration, Guidance for Industry: Pharmacokinetics in Patients With Impaired Renal Function—Study Design, Data Analysis, and Impact on Dosing and Labeling, US Department of Health and Human Services, Rockville, MD (1998) May.
↑ ^3.0 ^3.1 Stevens LA, Nolin TD, Richardson MM, Feldman HI, Lewis JB, Rodby R et al. (2009). "Comparison of drug dosing recommendations based on measured GFR and kidney function estimating equations.". Am J Kidney Dis 54 (1): 33-42. DOI:10.1053/j.ajkd.2009.03.008. PMID 19446939. PMC PMC2756662. Research Blogging.
↑ GFR Calculator at cato.at - Cockcroft-Gault - GFR calculation (Cockcroft-Gault formula)
↑ Levey AS, Coresh J, Greene T, et al (2006). "Using standardized serum creatinine values in the modification of diet in renal disease study equation for estimating glomerular filtration rate". Ann. Intern. Med. 145 (4): 247–54. PMID 16908915. ^[e]
↑ Spinler SA, Nawarskas JJ, Boyce EG, Connors JE, Charland SL, Goldfarb S (1998). "Predictive performance of ten equations for estimating creatinine clearance in cardiac patients. Iohexol Cooperative Study Group.". Ann Pharmacother 32 (12): 1275-83. PMID 9876806.

[pmid10075613-1] 1.0 ^1.1 ^1.2 Levey AS, Bosch JP, Lewis JB, Greene T, Rogers N, Roth D (1999). "A more accurate method to estimate glomerular filtration rate from serum creatinine: a new prediction equation. Modification of Diet in Renal Disease Study Group". Ann. Intern. Med. 130 (6): 461–70. PMID 10075613. ^[e]

[fda-renalcategories-2] Food and Drug Administration, Guidance for Industry: Pharmacokinetics in Patients With Impaired Renal Function—Study Design, Data Analysis, and Impact on Dosing and Labeling, US Department of Health and Human Services, Rockville, MD (1998) May.

[pmid19446939-3] 3.0 ^3.1 Stevens LA, Nolin TD, Richardson MM, Feldman HI, Lewis JB, Rodby R et al. (2009). "Comparison of drug dosing recommendations based on measured GFR and kidney function estimating equations.". Am J Kidney Dis 54 (1): 33-42. DOI:10.1053/j.ajkd.2009.03.008. PMID 19446939. PMC PMC2756662. Research Blogging.

[4] GFR Calculator at cato.at - Cockcroft-Gault - GFR calculation (Cockcroft-Gault formula)

[pmid16908915-5] Levey AS, Coresh J, Greene T, et al (2006). "Using standardized serum creatinine values in the modification of diet in renal disease study equation for estimating glomerular filtration rate". Ann. Intern. Med. 145 (4): 247–54. PMID 16908915. ^[e]

[pmid9876806-6] Spinler SA, Nawarskas JJ, Boyce EG, Connors JE, Charland SL, Goldfarb S (1998). "Predictive performance of ten equations for estimating creatinine clearance in cardiac patients. Iohexol Cooperative Study Group.". Ann Pharmacother 32 (12): 1275-83. PMID 9876806.

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