CREATININE, SERUM

With effect from 02/09/2024, all serum creatinine results are reported together with an eGFR calculated using the 2009 CKD-EPI creatinine equation (see below). The eGFR calculation uses the patient’s serum creatinine, age, sex and race to estimate the patient’s GFR.* 

For individuals less than 18 years of age, no eGFR will be calculated. 

2009 CKD-EPI creatinine equation (Levey AS et al. Ann Intern Med 2009;150:604-12):

where:

Scr is serum creatinine in µmol/L,

κ is 61.9 for females and 79.6 for males,

α is -0.329 for females and -0.411 for males,

min indicates the minimum of Scr /κ or 1, and

max indicates the maximum of Scr /κ or 1

Estimating equations are derived from large populations of patients and provide the best estimate of a patient’s GFR and not the patient’s actual GFR. These equations are designed for use in patients with stable renal function. 

Creatinine-based eGFR calculation may be less reliable in situations where renal function is rapidly changing/unstable or body weight/muscle mass deviates significantly from normal. These include:

• Acute kidney injury

• Critically ill

• Renal Dialysis

• Extremes of age (i.e. elderly)

• Extremes of muscle mass (i.e. bodybuilders, amputation)

• Malnourishment 

• Muscle wasting disorders

• Pregnancy

In situations where creatinine-based eGFR is unreliable, one may consider the use of cystatin-based eGFR or measured GFR to assess renal function.

Glomerular filtration rate (GFR) is traditionally indexed to body surface area (BSA) to allow comparison among individuals of different body size. Definitions of chronic kidney disease are based on GFR indexed to BSA. Modern GFR estimating equations report estimated GFR (eGFR) indexed to BSA of 1.73m²

For drug dosing purposes, please consult a pharmacist on the use of eGFR (vs estimated creatinine clearance). 

Drug clearance is only related to GFR and not to body size or body surface area (BSA). In patients whose body sizes are very different from average, use of non-indexed eGFR (ml/min) may be preferrable. 

Conversion of indexed eGFR (ml/min/1.73m2) to non-indexed eGFR (ml/min) can be performed by multiplying the indexed GFR result by the patient’s BSA (e.g. BSA = (Ht(in cm) x Wt(in kg)/3600)^0.5) and dividing by 1.73m².