Creatinine clearance overestimates the true glomerular filtration rate by approximately 10–20%. Which mechanism is primarily responsible for this overestimation?

## Why Creatinine Clearance Overestimates GFR **Key Point:** Creatinine clearance exceeds true GFR because creatinine is not only filtered at the glomerulus but also actively secreted by proximal tubule cells, increasing its urinary excretion. ### Mechanism of Overestimation ```mermaid flowchart LR A[Plasma Creatinine]:::outcome --> B[Glomerular Filtration]:::action A --> C[Proximal Tubule]:::action C --> D[Active Secretion via OAT]:::action B --> E[Urinary Creatinine]:::outcome D --> E E --> F[Measured Clearance > True GFR]:::urgent ``` **High-Yield:** The measured creatinine clearance includes both: 1. **Filtered creatinine** (represents true GFR) 2. **Secreted creatinine** (additional contribution, ~10–20% of total urinary creatinine) ### Comparison: Filtered vs. Secreted Creatinine | Component | Source | Percentage of Urinary Creatinine | |-----------|--------|----------------------------------| | Filtered at glomerulus | GFR × plasma creatinine | 80–90% | | Secreted by proximal tubule | Active transport (OAT3) | 10–20% | | **Total urinary creatinine** | Filtration + secretion | 100% | **Clinical Pearl:** This overestimation is clinically insignificant in patients with normal renal function but becomes problematic in advanced chronic kidney disease (CKD stage 4–5), where tubular secretion may increase further and creatinine clearance becomes a poor estimate of GFR. In these cases, cystatin C or eGFR equations (MDRD, CKD-EPI) are preferred. **Mnemonic:** **CREST** — **C**reatinine **R**eabsorbed? No. **E**xcretion includes **S**ecretion. **T**rue GFR < measured clearance.