# GFR Estimation: A Useful Estimate Has Assumptions

Small numerical differences may not be clinically meaningful. Explain uncertainty when the estimate is close to a treatment or classification boundary.

![Infographic: GFR Estimation: A Useful Estimate Has Assumptions](https://urinenephrology.org/visual-reference/images/mastery-gfr.png?v=20261003c)

## Markers have nonrenal influences

Creatinine reflects generation and clearance; cystatin C also has nonfiltration determinants. Neither is a perfect direct measurement of filtration.

## Recognize unreliable settings

AKI, low or changing muscle mass, unusual diet, medications, and selected systemic conditions can make an estimate less reliable.

## Confirm when uncertainty changes the dose

When low muscle mass makes creatinine unreliable, consider combined creatinine–cystatin C eGFR. Steroids, thyroid disease, and other non-GFR influences also affect cystatin C. If a narrow therapeutic window or major eligibility decision remains uncertain, measured GFR may be more useful than repeatedly recalculating estimates.

## Convert indexed to absolute units correctly

For a decision requiring absolute GFR, multiply indexed eGFR by the patient’s body-surface area ÷ 1.73. This yields mL/min rather than mL/min/1.73 m². Follow the drug’s specified method; a label using creatinine clearance is not automatically asking for deindexed eGFR.

## Follow context and trends

A change in muscle mass or medication can alter creatinine without equivalent change in filtration. Compare the estimate with urine findings and clinical course.

## Avoid false precision

Small numerical differences may not be clinically meaningful. Explain uncertainty when the estimate is close to a treatment or classification boundary.

## Supporting evidence

- [Supporting guideline or source](https://kdigo.org/wp-content/uploads/2024/03/KDIGO-2024-CKD-Guideline.pdf)

## Source lessons

- [gfr estimation](https://urinenephrology.org/mastery/ckd-advanced/gfr-estimation.html)

Read alongside the full lessons; the findings and decisions shown here require the stated clinical context.