Estimating Kidney Function using Basic Metabolic Panel

Introduction to eGFR

The Kidney Disease: Improving Global Outcomes (KDIGO) guidelines recommend using eGFR based on serum creatinine (eGFRcr) as the initial test for evaluation of kidney function in routine clinical practice 1, 2

A Basic Metabolic Panel (BMP) routinely includes serum creatinine measurement, which is one of the standard components of this panel along with electrolytes, glucose, and blood urea nitrogen (BUN) 2
Clinical laboratories are directed to automatically report eGFR in addition to serum creatinine concentrations using validated equations, as this is a laboratory implementation standard established by KDIGO guidelines 1

The eGFR should be reported rounded to the nearest whole number and relative to a body surface area of 1.73 m² using the units mL/min per 1.73 m², and any eGFR values below 60 mL/min per 1.73 m² should be flagged as being low 1
The eGFR is calculated using validated equations that incorporate the serum creatinine value from your BMP along with demographic variables (age, sex, and race), without requiring any additional testing or urine collection 1, 3

The eGFR serves as the best available index of kidney function and is used to diagnose chronic kidney disease, classify its stages, and monitor disease progression 1, 4
Normal eGFR is typically >60 mL/min/1.73 m², and values below this threshold may indicate chronic kidney disease according to KDIGO classification 2

eGFR may be less accurate in patients with extremes of muscle mass, very elderly patients, or those with extremes of body size, as creatinine has significant non-GFR variability from muscle mass that is not adequately accounted for in eGFR equations 1, 4, 3
The laboratory must use standardized creatinine assays with calibration traceable to international standards (coefficient of variation <2.3%, bias <3.7%) to ensure accurate eGFR calculation, preferably using enzymatic methods 1
When eGFR is thought to be inaccurate, consider measured creatinine clearance with timed urine collection or measured GFR using exogenous markers, particularly when treatment decisions depend on accurate GFR assessment 1, 3

The National Kidney Foundation recommends using the standard eGFR, normalized to 1.73 m², for universal comparison across all patients, allowing for accurate kidney function assessment regardless of body size 5, 6
The American Journal of Kidney Diseases suggests that for routine kidney disease screening and CKD staging, the standard eGFR should be used without adjustment, with values <60 mL/min per 1.73 m² indicating potential chronic kidney disease 5, 7

The Kidney International guideline recommends adjusting the standardized eGFR to the patient's actual BSA for medication dosing decisions, particularly with renally-cleared drugs, to provide a more accurate kidney function assessment 5
The American Journal of Kidney Diseases guideline suggests that adjusting the reported eGFR by multiplying: *eGFR = eGFR × (patient's BSA / 1.73) can help determine appropriate medication doses for patients with extreme body sizes 7