Renal clearance: inulin, creatinine and PAH
Renal clearance is a bookkeeping volume, not a real flow. Use the same equation to estimate GFR with inulin and creatinine, effective renal plasma flow with PAH, and to read what the tubule did.
Chapters (10)
Use the clearance equation to estimate GFR (inulin, creatinine) and effective renal plasma flow (PAH), and interpret a clearance relative to GFR.
The short version
- Clearance = (urine concentration × urine flow) ÷ plasma concentration; it is a virtual plasma volume cleared per minute, not a real flow.
- Inulin is freely filtered, not reabsorbed and not secreted, so inulin clearance equals GFR; creatinine clearance runs roughly 10–20 percent above GFR in healthy people because the tubule secretes a little creatinine.
- PAH clearance gives the effective renal plasma flow, which underestimates the true flow by about 10 percent.
- Filtration fraction = GFR ÷ plasma flow (about 0.2); renal blood flow = plasma flow ÷ (1 − hematocrit).
- For a freely filtered substance, clearance above GFR means net secretion, below GFR means net reabsorption, and equal to GFR means no net handling.
Two renal clearance tests, two answers
One patient has two tests. A 24-hour urine collection gives a creatinine clearance of 96 mL/min. An inulin infusion gives a clearance of 80 mL/min. Same kidneys, two answers, and the gap is not an error.[2][6]
This lesson is about renal clearance: how the one clearance equation estimates GFR with inulin and creatinine, estimates renal plasma flow with PAH, and shows what the tubule did to any substance. Earlier in the renal series, the Starling forces set GFR.
The renal clearance equation: a virtual volume
Clearance is a virtual volume: the plasma volume completely cleared of a substance each minute. It is not a real flow, just bookkeeping.[5]
U = urine concentration; V = urine flow (mL/min); P = plasma concentration; C in mL/min
Inulin clearance equals GFR; creatinine clearance runs high
Inulin is the perfect marker. It is freely filtered, not reabsorbed, and not secreted, so everything filtered ends up in the urine and inulin clearance equals GFR.[1][6]
The catch is that the body does not make inulin, so it must be infused.
Creatinine comes from muscle at a steady rate, so no infusion is needed. But the tubule secretes a little creatinine, so creatinine clearance runs above the true GFR, roughly 10–20 percent in healthy people. The opening case, 96 against 80, fits that pattern.[2]
PAH clearance: effective renal plasma flow, filtration fraction and blood flow
PAH is filtered and also secreted. At low plasma levels, about 90 percent is removed in a single pass through the kidney, so PAH clearance gives the effective renal plasma flow. It is called effective because it underestimates the true flow by about 10 percent.[3][7]
| Quantity | Typical value |
|---|---|
| GFR | about 125 mL/min |
| Effective renal plasma flow | about 600–650 mL/min |
| Filtration fraction | about 0.2 |
FF is about 0.2 when GFR is about 125 and effective plasma flow is about 600–650 mL/min
Worked example: GFR, plasma flow, filtration fraction and blood flow
A volunteer makes 1.5 mL of urine per minute. Inulin: plasma 0.3 mg/mL, urine 24 mg/mL. PAH: plasma 0.02 mg/mL, urine 8 mg/mL. Hematocrit is 40 percent.[7]
- GFR = inulin clearance = (24 × 1.5) / 0.3 = 36 / 0.3 = 120 mL/min
- Effective plasma flow = PAH clearance = (8 × 1.5) / 0.02 = 12 / 0.02 = 600 mL/min
- Blood flow (from the effective plasma flow) = 600 / (1 − 0.40) = 600 / 0.6 = 1000 mL/min
- Filtration fraction = 120 / 600 = 0.2
The reading rule: compare any clearance with GFR
Compare a substance's clearance with the GFR. This holds for freely filtered substances.[6]
| Clearance compared with GFR | What the tubule did |
|---|---|
| Above GFR | The tubule added it: net secretion |
| Below GFR | The tubule took some back: net reabsorption |
| Equal to GFR | No net handling |
Practice question
Original · USMLE styleA 27-year-old healthy man volunteers for a renal clearance study. After an intravenous infusion of inulin and of substance X has reached a steady state, urine is collected over 60 minutes and plasma is sampled at the midpoint. Substance X is a small, unbound molecule that is freely filtered at the glomerulus. Results are shown:
| Inulin, plasma concentration | 0.4 mg/mL |
|---|---|
| Inulin, urine concentration | 20 mg/mL |
| Substance X, plasma concentration | 0.5 mg/mL |
| Substance X, urine concentration | 7.5 mg/mL |
| Urine flow rate | 2.0 mL/min |
Common questions
What is the renal clearance equation?
Clearance equals urine concentration times urine flow, divided by plasma concentration: C = (U × V) / P. It is a virtual volume of plasma completely cleared of a substance each minute, not a real flow.
Why does inulin clearance equal GFR?
Inulin is freely filtered, not reabsorbed and not secreted, so everything filtered ends up in the urine. The body does not make inulin, so it must be infused.
Why does creatinine clearance overestimate GFR?
The tubule secretes a little creatinine, so its clearance runs above the true GFR, roughly 10–20 percent in healthy people, and the gap tends to widen as GFR falls. Creatinine comes from muscle at a steady rate, so no infusion is needed.
What does PAH clearance measure?
The effective renal plasma flow. At low plasma levels about 90 percent of PAH is removed in a single pass, so the clearance underestimates the true plasma flow by about 10 percent.
How do you tell secretion from reabsorption using clearance?
For a freely filtered substance, a clearance above GFR means net secretion and a clearance below GFR means net reabsorption. Equal to GFR means no net handling.
Transcript
Read the full narration, by chapter632 words
0:00Two clearances, one patient
Two tests on the same patient. A 24 hour urine collection says creatinine clearance is 96 mL per minute. An inulin infusion says 80. Same kidneys, two answers, and the gap is not an error. Today: use the clearance equation to estimate GFR with inulin and creatinine, and renal plasma flow with PAH. Then read what any clearance says about the tubule.
0:27The mental model and the clearance equation
First, the mental model. Last time, the Starling forces set GFR. Clearance is a virtual volume: the plasma volume completely cleared of a substance each minute. It is not a real flow, just bookkeeping. The equation: clearance equals urine concentration times urine flow, divided by plasma concentration.
0:49Inulin: the perfect marker for GFR
In-yoo-lin is the perfect marker. It is freely filtered, not reabsorbed, and not secreted. So everything filtered ends up in urine, which makes inulin clearance equal to GFR. The catch: the body does not make inulin, so it must be infused.
1:08Creatinine: slightly secreted, slightly high
Creatinine comes from muscle at a steady rate, so no infusion is needed. But the tubule secretes a little creatinine, so its clearance runs above the true GFR, roughly 10 to 20% in healthy people.
1:22PAH: effective renal plasma flow
Now PAH. It is filtered, and it is also secreted. At low plasma levels, about 90% is removed in a single pass through the kidney. So PAH clearance gives the effective renal plasma flow. Effective, because it underestimates true flow by about 10%.
1:42Typical values, filtration fraction and renal blood flow
Typical values: GFR about 125, effective plasma flow about 600 to 650 mL per minute. GFR divided by plasma flow is the filtration fraction, about point two. And blood flow is plasma flow divided by one minus the hematocrit.
2:04Worked example: GFR 120, eRPF 600, FF 0.2, RBF 1000
Let's run it. A volunteer makes 1.5 mL of urine per minute. Inulin: plasma point three milligrams per millilitre, urine 24. Twenty-four times 1.5 is 36, divided by point three: 120. That is the GFR, in mL per minute. PAH: plasma point zero two, urine eight. Eight times 1.5 is 12, divided by point zero two: 600. Hematocrit 40%: blood flow is 600 over point six, 1000 mL per minute. Filtration fraction, point two.
2:46The reading rule: clearance versus GFR
Now the reading rule: compare a substance's clearance with the GFR. Clearance above GFR means the tubule added it: net secretion. Clearance below GFR means the tubule took some back: net reabsorption. Equal means no net handling. This holds for freely filtered substances.
3:08Practice question
Your turn. A 27-year-old healthy man volunteers for a clearance study at steady state. Urine flow is 2 mL per minute. Inulin: plasma point four milligrams per millilitre, urine 20. Substance X is small, unbound and freely filtered: plasma point five, urine 7.5. Which best describes how the kidney handles substance X? A, neither reabsorbed nor secreted. B, net tubular reabsorption. C, net tubular secretion. D, secretion with nearly complete removal from plasma in one pass. E, complete reabsorption of the filtered load. Pause here and decide. First the GFR: 20 times two, divided by point four, is 100. Now X: 7.5 times two, divided by point five, is 30. A is out: neither would give a clearance equal to the GFR, 100. C is out: secretion would push clearance above 100. D is out: complete removal in one pass would give a clearance near the plasma flow, five to 600. E is out: if all of X were reabsorbed, none would reach the urine and its clearance would be zero. So the answer is B: 30 is below 100, which means net reabsorption.
4:41High-yield recap and next lesson
Three things to keep. One: clearance is urine concentration times flow, over plasma concentration. In-yoo-lin gives GFR, creatinine slightly overestimates it, and PAH gives effective plasma flow. Two: filtration fraction is GFR over plasma flow, and blood flow is plasma flow over one minus hematocrit. Three: above GFR is secretion, below is reabsorption. Next up: the proximal tubule, its glucose transport maximum, and splay.
Sources
Numbers in the text, e.g. [1], point here- Physiology of Urine Formation (25.5). OpenStax Anatomy & Physiology 2e. openstax.org/books/anatomy-and-physiology-2e/pages/25-5-physiology-of-urine-formation
- Tubular secretion of creatinine and kidney function: an observational study. PMC7104490. pmc.ncbi.nlm.nih.gov/articles/PMC7104490/
- Glucosuria interferes with measurement of ERPF using PAH. PMC7609950. pmc.ncbi.nlm.nih.gov/articles/PMC7609950/
- Renal blood flow, clearance of PAH, extraction ratio, filtration fraction. greek.doctor, 74.
- Renal clearance. Wikipedia. en.wikipedia.org/wiki/Renal_clearance
- Physiology, Glomerular Filtration Rate. StatPearls (NCBI Bookshelf). ncbi.nlm.nih.gov/books/NBK500032/
- Physiology, Renal Blood Flow and Filtration. StatPearls (NCBI Bookshelf). ncbi.nlm.nih.gov/books/NBK482248/
The 10-20 percent creatinine overestimate and the 600-650 mL/min effective plasma flow are approximate teaching values.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Thumbnail illustration: Servier Medical Art (smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/); adapted (recoloured, outlined, resized): nephron. All video diagrams are original (code-drawn); no third-party artwork is used in the video. Example lab values are illustrative.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-REN-025.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01GFR and Starling forces at the glomerulus5:25
- EP 02Afferent vs efferent arteriole: GFR, RPF and filtration fraction5:16
- EP 03Renal clearance: inulin, creatinine and PAH5:20
- EP 04Glucose transport maximum, splay and SGLT24:55
- EP 05Countercurrent multiplier: how the kidney concentrates urine5:02
- EP 06Aldosterone and ADH in the distal nephron5:20
- EP 07Diuretics by nephron segment: loop, thiazide and more5:53
- EP 08RAAS explained: renin, angiotensin II and aldosterone5:23