Creatinine clearance is reported as an absolute rate in mL/min, while eGFR is reported per 1.73m² of body surface area (mL/min/1.73m²). They measure related quantities on different scales, so a like-for-like comparison needs one to be converted. This tool moves a value between the two using the patient’s body surface area.
The Two Conversion Formulas
Everything turns on the body surface area (BSA) and the reference of 1.73m²:
Indexed eGFR (mL/min/1.73m²) = absolute clearance × 1.73 ÷ BSA
Absolute clearance (mL/min) = indexed eGFR × BSA ÷ 1.73
Compute BSA from height and weight with the BSA calculator (Mosteller: √((cm × kg) ÷ 3600)). When BSA equals the 1.73m² reference exactly, the two numbers are identical — the correction only bites as a patient moves away from average size.
Worked Example
Consider a large adult with a BSA of 2.10 m² whose CKD-EPI eGFR reads 72 mL/min/1.73m². De-index to find the absolute clearance the kidneys actually deliver: 72 × 2.10 ÷ 1.73 = 87 mL/min. The indexed figure understated the real filtration by about 15 mL/min — enough to move a borderline drug-dosing decision. For a small adult with a BSA of 1.40 m² and the same indexed 72, the absolute clearance is 72 × 1.40 ÷ 1.73 = 58 mL/min, so the indexed value overstated true clearance.
How Much Does BSA Change the Number?
| Patient BSA | Absolute clearance (mL/min) | Direction of error if uncorrected |
|---|---|---|
| 1.40 m² | 58 | Indexed value overstates clearance |
| 1.73 m² | 72 | No correction needed (equals reference) |
| 2.00 m² | 83 | Indexed value understates clearance |
| 2.10 m² | 87 | Indexed value understates clearance |
When You Need This
For very large or very small patients, the BSA correction matters — a 90 mL/min CrCl is not the same as a 90 mL/min/1.73m² eGFR. Use the absolute mL/min figure for renal drug dosing, where thresholds were validated against Cockcroft–Gault, and use the indexed mL/min/1.73m² figure for CKD staging. See CrCl vs eGFR for the full decision and the BSA calculator to get the body surface area.
Why Two Different Scales Exist
The two units answer two different clinical questions. An absolute clearance in mL/min tells you how much blood the kidneys are actually clearing in this particular body — the figure you want when a drug dose depends on how fast the patient eliminates it. An indexed rate in mL/min/1.73m² strips out body size so that two people of different builds can be compared on one fair scale — the figure you want when staging chronic kidney disease across a whole population. Reporting eGFR per 1.73m² is what lets a guideline say “below 60 is stage 3” and have it mean the same thing for a small older woman and a large young man.
The reference of 1.73m² is simply the average body surface area of a young adult from the era when GFR indexing was standardised. It is a convention, not a target — nobody needs to have a BSA of 1.73m². Its only job is to provide a common denominator so indexed values are comparable.
Choosing Which Number to Use
| Task | Use this unit | Why |
|---|---|---|
| CKD staging (G1–G5) | mL/min/1.73m² (indexed) | Comparable across body sizes |
| Renal drug dosing | mL/min (absolute) | Matches Cockcroft–Gault-validated labels |
| Comparing CrCl with eGFR | mL/min (absolute) | CrCl is already absolute |
| Tracking CKD over time | mL/min/1.73m² (indexed) | Consistent reference across visits |
The two only diverge meaningfully when a patient is far from the 1.73m² reference. For an average-sized adult, the indexed and absolute numbers are within a few mL/min of each other, and either can be used without much consequence. The further the body surface area from 1.73m², the more the correction matters — which is exactly when a drug-dosing decision sitting on a threshold boundary deserves the de-indexed value.
A Common Source of Confusion
Many people assume an eGFR of 90 and a CrCl of 90 describe the same kidney, but they carry different units and only coincide when the patient’s BSA happens to equal 1.73m². Treating them as identical in a large or small patient is a genuine error: it can place someone in the wrong CKD stage or, worse, on the wrong side of a drug-dosing threshold. The fix is mechanical — decide which question you are answering, then express the value in the matching unit using this converter. The number itself does not lie; the unit it is wearing simply has to be the right one for the job.
A Second Worked Example
Take a petite adult with a BSA of 1.50 m² whose CrCl from the Cockcroft–Gault calculator is 55 mL/min (an absolute value). To compare it with a staged eGFR, index it: 55 × 1.73 ÷ 1.50 = 63 mL/min/1.73m². The indexed number is higher than the absolute one because this person is smaller than the reference. The same patient’s drug dose, however, should be driven by the absolute 55 mL/min — the indexing step is only to place them on the CKD staging scale, not to change the dose.
Limitations and Notes
- The correction is only as good as the BSA estimate; an error in height or weight propagates directly into the converted value.
- Indexing assumes filtration scales linearly with body surface area, which holds well across the usual adult range but less so at the extremes.
- This converter changes units only. It does not re-estimate GFR — start from a value produced by a validated equation.
- Pediatric eGFR uses its own equations and reference; do not apply adult normalisation to children without checking the method.