The clearance concept
The clearance of a substance is the volume of plasma completely cleared of it per unit time. Choosing the right marker lets us measure different aspects of renal function.
Measuring GFR
- Inulin clearance is the gold standard: inulin is freely filtered and neither reabsorbed nor secreted, so its clearance equals the GFR
- Creatinine clearance is a practical estimate but overestimates GFR slightly, because creatinine is filtered and secreted by the tubule
- eGFR equations estimate GFR from serum creatinine, age and sex (see the CKD classification note)
Measuring renal plasma flow
- PAH (para-aminohippuric acid) clearance measures renal plasma flow: PAH is filtered and almost completely secreted, so nearly all is removed in one pass
- Renal blood flow = renal plasma flow ÷ (1 − haematocrit)
Filtration fraction
Filtration fraction = GFR ÷ renal plasma flow (normally around 20%) — the proportion of plasma reaching the kidney that is filtered.
MRCP-specific traps
- Inulin = GFR (filtered only); PAH = renal plasma flow (filtered + secreted).
- Creatinine clearance overestimates GFR because of tubular secretion.
- Drugs like trimethoprim block creatinine secretion, raising serum creatinine without changing true GFR.
Summary
Clearance measures how much plasma is cleared of a marker per unit time. Inulin clearance gives GFR (freely filtered, not secreted/reabsorbed); PAH clearance gives renal plasma flow (filtered and fully secreted); creatinine clearance overestimates GFR because of tubular secretion. Filtration fraction = GFR/RPF (~20%).
nephrologyphysiologymrcp part 2