Understanding eGFR vs *eGFR (Asterisk eGFR)
*The asterisk (eGFR) indicates that the reported eGFR value has been adjusted for the patient's actual body surface area (BSA) rather than being standardized to 1.73 m², which is the default reporting standard for routine eGFR values. 1, 2
Standard eGFR Reporting
- Standard eGFR is automatically calculated and reported by clinical laboratories using serum creatinine from your basic metabolic panel, normalized to a body surface area of 1.73 m² (the average adult BSA). 1
- The value is reported as mL/min per 1.73 m² and represents kidney function as if every patient had the same body size. 1
- This standardization allows for universal comparison across all patients regardless of their actual body size. 3
When *eGFR (Adjusted eGFR) Matters
- *For medication dosing decisions, particularly with renally-cleared drugs, adjusting the standardized eGFR to the patient's actual BSA (creating eGFR) provides more accurate kidney function assessment when BSA differs significantly from 1.73 m². 2
- Patients with BSA >1.73 m² (larger individuals) will have a higher *eGFR than their reported eGFR, while those with BSA <1.73 m² (smaller individuals) will have a lower *eGFR. 2
- This adjustment is particularly critical for obesity class III (BMI >40 kg/m²) or very small patients where body size extremes affect the accuracy of medication dosing calculations. 4, 2
Clinical Application Algorithm
For routine kidney disease screening and CKD staging:
- Use the standard eGFR (normalized to 1.73 m²) without adjustment. 3
- Values <60 mL/min per 1.73 m² should be flagged as low and may indicate chronic kidney disease. 1
For medication-related decisions (drug dosing, chemotherapy):
- Calculate the patient's actual BSA using height and weight. 2
- Adjust the reported eGFR by multiplying: *eGFR = eGFR × (patient's BSA / 1.73). 2
- Use this *eGFR value for determining appropriate medication doses, particularly for renally-cleared drugs. 2
Common Pitfalls to Avoid
- Do not use the standardized eGFR (1.73 m²) for medication dosing in patients with extreme body sizes without adjusting for actual BSA, as this can lead to significant under- or overdosing. 2
- The asterisk notation may vary between laboratories—some facilities may use different symbols or terminology to indicate BSA-adjusted values, so verify your laboratory's specific reporting conventions. 1
- Remember that both eGFR and *eGFR share the same fundamental limitations related to muscle mass extremes, dietary protein intake, and conditions affecting creatinine generation independent of kidney function. 4