Filtration Fraction
Filtration fraction is the proportion of renal plasma flow that becomes filtrate at the glomerulus. A filtration fraction calculator helps show whether the kidney is filtering a relatively small, typical, or large share of incoming plasma by comparing glomerular filtration rate with renal plasma flow.
This quantity is especially useful in renal hemodynamics because it connects filtration to plasma delivery, not just to filtration alone. The calculator reports the result as both a decimal and a percentage so the physiologic meaning is easier to interpret.
Core definitions and formulas
The main relationship is the ratio of glomerular filtration rate to renal plasma flow:
\[
\begin{aligned}
\text{FF} &= \frac{\text{GFR}}{\text{RPF}}
\end{aligned}
\]
To express the same value as a percentage:
\[
\begin{aligned}
\text{FF}_{\%} &= \text{FF} \cdot 100
\end{aligned}
\]
Here, GFR is glomerular filtration rate and RPF is renal plasma flow. Both quantities must be in compatible flow units before the ratio is calculated, such as mL/min.
How to interpret the result
A larger filtration fraction means a greater share of incoming plasma is being filtered. A smaller filtration fraction means a smaller share is being filtered. In many teaching settings, a value near 0.20, or about 20%, is used as a typical reference point.
If filtration fraction rises, one possible explanation is that renal plasma flow has fallen more than filtration, or that glomerular pressure has increased, such as with efferent constriction. If filtration fraction falls, filtration may be reduced relative to plasma flow, which can happen when glomerular pressure decreases, such as with afferent constriction.
Common pitfalls
- Using GFR and RPF in different flow units without converting them first.
- Confusing filtration fraction with GFR itself.
- Entering a percentage like 20 instead of the decimal 0.20 when the input expects a ratio.
- Interpreting the value without considering the separate behavior of GFR and plasma flow.
Micro example: if GFR = 125 mL/min and RPF = 625 mL/min, then
\[
\begin{aligned}
\text{FF} &= \frac{125}{625} = 0.20 = 20\%
\end{aligned}
\]
This tool is useful for renal physiology teaching, clearance relationships, and hemodynamic comparisons between states. It should not replace full renal assessment; the next step is usually to relate filtration fraction to GFR, renal plasma flow, clearance, and tubular handling.