Calculator Inputs
Enter either a direct urine flow rate or provide collected urine volume with collection time. When both methods are entered, direct flow rate is used.
Formula Used
Renal drug clearance: CLR = (Cu × V) / Cp
Filtered clearance: CLfiltered = fu × GFR
Net secretory clearance: max(0, CLR − CLfiltered)
Net reabsorptive component: max(0, CLfiltered − CLR)
Urinary excretion rate: (Cu × V) / 1000
Here, Cu is urine drug concentration, V is urine flow rate, Cp is plasma drug concentration, fu is fraction unbound, and GFR is glomerular filtration rate.
When total renal clearance exceeds fu × GFR, secretion is likely contributing. When it falls below fu × GFR, reabsorption may be reducing net elimination.
How to Use This Calculator
- Enter urine and plasma drug concentrations using matching concentration units.
- Add direct urine flow rate, or provide urine volume plus collection time.
- Enter fraction unbound and GFR to compare measured clearance with filtration.
- Optionally enter dose and body weight for extra pharmacokinetic outputs.
- Press the calculate button to view results above the form.
- Review the interpretation, graph, and downloadable CSV or PDF summary.
Example Data Table
| Input or Output | Example Value | Unit |
|---|---|---|
| Urine drug concentration | 120 | mg/L |
| Plasma drug concentration | 2.0 | mg/L |
| Urine flow rate | 1.2 | mL/min |
| Fraction unbound | 0.25 | ratio |
| GFR | 110 | mL/min |
| Calculated renal clearance | 72.00 | mL/min |
| Filtered clearance | 27.50 | mL/min |
| Interpretation | Net tubular secretion suggested | - |
Frequently Asked Questions
1. What does renal drug clearance describe?
It describes the volume of plasma effectively cleared of a drug by the kidneys per unit time. It helps summarize renal elimination performance from measured urine and plasma data.
2. Why compare renal clearance with fu × GFR?
That comparison helps distinguish filtration from other tubular processes. Higher measured clearance suggests secretion, while lower measured clearance suggests net reabsorption.
3. Why is fraction unbound included?
Only unbound drug is freely filtered at the glomerulus. Using fu improves the estimate of how much clearance could be explained by filtration alone.
4. Can clearance be greater than GFR?
Yes. A drug can show clearance above filtration capacity when active tubular secretion adds extra renal elimination beyond glomerular filtration.
5. What does a low clearance relative to filtration suggest?
It may suggest net tubular reabsorption, meaning some filtered drug returns from tubular fluid back into the bloodstream before excretion.
6. Should I enter direct urine flow or collected volume?
Either approach works. If you know direct flow, enter it. If not, provide total urine volume and collection time so the calculator derives flow automatically.
7. Does this calculator replace clinical dosing guidance?
No. It is an educational and analytical tool. Clinical decisions should also consider diagnosis, renal function trends, assay timing, formulation, and specialist judgment.
8. Which units should be used?
Use matching concentration units for urine and plasma, such as mg/L for both. Flow should be in mL/min, GFR in mL/min, dose in mg, and body weight in kg.