Which of the following findings best distinguishes inulin clearance from creatinine clearance as a measure of glomerular filtration rate?

## Distinguishing Inulin from Creatinine Clearance ### Ideal GFR Marker vs. Clinical Marker **Key Point:** Inulin is the gold standard for measuring true GFR because it is freely filtered and undergoes no renal tubular handling, whereas creatinine, though commonly used clinically, is partially secreted by proximal tubule cells. ### Mechanism of Difference | Feature | Inulin | Creatinine | |---------|--------|------------| | **Glomerular Filtration** | Freely filtered | Freely filtered | | **Tubular Reabsorption** | None | None | | **Tubular Secretion** | None | **Yes** (10–40% of urinary excretion) | | **Clinical Use** | Gold standard, research | Routine clinical practice | | **GFR Accuracy** | Measures true GFR | Overestimates true GFR | **High-Yield:** The tubular secretion of creatinine is the key discriminator. This secretion causes the creatinine clearance to exceed the true GFR, making inulin clearance the reference standard. ### Clinical Pearl Because creatinine is secreted, creatinine clearance (measured from 24-hour urine collection) overestimates true GFR by approximately 10–20%. This is why eGFR equations (MDRD, CKD-EPI) apply correction factors to serum creatinine to approximate true GFR more accurately. **Mnemonic:** **FUSS** — Freely filtered, Unabsorbed, Secretion-free, Standard (inulin). Creatinine fails the "S" test (secretion occurs).