eGFR BSA Adjustment Calculator
eGFR BSA Adjustment Calculator
Convert an indexed eGFR (mL/min/1.73 m²) to an absolute clearance in mL/min for drug dosing.
eGFR BSA Adjustment
Indexed → absoluteIndexed eGFR 55 mL/min/1.73 m², BSA 2.1 m²
Formula
- indexed eGFR
- mL/min/1.73 m², as reported by the laboratory (CKD-EPI or similar)
- BSA
- body surface area in m², calculated from the patient's actual height and weight
- 1.73
- the reference body surface area the indexed eGFR is normalised to
Worked example
Indexed eGFR 55 mL/min/1.73 m², BSA 2.1 m²
55 × 2.1 = 115.5
115.5 ÷ 1.73 = 66.8 mL/min
60 or above → within the conventional absolute-clearance dosing threshold
Absolute clearance and dosing
| Absolute clearance | Category |
|---|---|
| < 15 | Very severely reduced |
| 15 – 29 | Severely reduced |
| 30 – 59 | Moderately reduced |
| ≥ 60 | At or above conventional threshold |
Two different numbers doing two different jobs
This is arguably the single most under-appreciated issue in renal drug dosing. Laboratory eGFR is reported per 1.73 m² so that kidney function can be compared fairly across people of different sizes — it is a normalised value built for staging chronic kidney disease and for population comparisons, where body size should not distort the number. Drug clearance, however, happens in an actual kidney of an actual size, and the amount of drug removed per unit time depends on that patient’s real renal mass and blood flow, not on a standardised 1.73 m² reference body.
For most adults of average build the two values are close enough that the distinction rarely matters. It matters a great deal at the extremes. A large patient has more functioning nephron mass than the indexed eGFR implies, so their true drug clearance is higher than the reported number suggests; a very small or frail patient has less. Using the indexed value directly for dosing in either case introduces a systematic error in the wrong direction — under-dosing a large patient, or over-dosing a small one.
The scale of the error can be substantial. A 120 kg patient with a BSA of 2.4 m² has an absolute clearance nearly 40% higher than their reported indexed eGFR — dosing to the indexed number alone would meaningfully under-treat them. This conversion is most relevant for narrow-therapeutic-index drugs with significant renal clearance, and it should be used alongside, not instead of, drug-specific dosing references.
Frequently asked questions
Why isn't laboratory eGFR the right number for drug dosing?
Laboratory eGFR is normalised to a standard body surface area of 1.73 m² for CKD staging and population comparisons. Actual drug clearance depends on the patient’s real kidney size and blood flow, which the indexed number does not represent for anyone whose BSA differs from 1.73 m².
When does the difference between indexed and absolute eGFR matter most?
In obese and very large patients (where absolute clearance is higher than the indexed value suggests) and in very small or frail patients (where it is lower), particularly for narrow-therapeutic-index drugs cleared renally.
Which value should be used for CKD staging?
Always the indexed eGFR (mL/min/1.73 m²). Absolute clearance is for dosing calculations only and should never be used to stage chronic kidney disease.
How much can the two values differ?
Substantially at extremes of body size — a 120 kg patient with a BSA of 2.4 m² has an absolute clearance nearly 40% higher than their indexed eGFR, which is large enough to change a dosing decision.
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References
- Nyman HA et al. Comparative evaluation of the Cockcroft-Gault equation and the MDRD Study equation for drug dosing: an opinion of the Nephrology Practice and Research Network of the ACCP. Pharmacotherapy. 2011;31(11):1130–44.
- Levey AS, Inker LA. GFR as the gold standard for the measurement of kidney function. Am J Kidney Dis. 2016;67(1):9–12.