# Filtration Fraction Calculator

Determines the ratio of glomerular filtration rate (GFR) to renal plasma flow (RPF) to evaluate kidney function.

Refer to the text below the tool for more information about the parameters used and the result interpretation.

Filtration Fraction (FF) is the ratio of the glomerular filtration rate (GFR) to the renal plasma flow (RPF) and helps quantify the proportion of fluid that passes into renal tubules, thus giving a measure of how well the kidneys are functioning.

Where continuous renal replacement therapies (CRRT) are used, the filtration fraction helps determine the volume of plasma removed from the dialysed blood by ultrafiltration, known as “the ratio of ultrafiltration rate to plasma water flow rate”.

### Formulas

`Filtration Fraction (FF) % = GFR / RPF * 100`

`Filtration Fraction = Ultrafiltrate flow rate / [Blood flow rate x (1 – Hct) + Pre-dilution replacement flow rate]`

* The second equation is the formula employed in continuous renal replacement therapies (CRRT).

### Reference Ranges

Parameters |
Normal Low |
Normal High |

Glomerular Filtration Rate (GFR) | 90 mL/min | 131 mL/min |

Renal Plasma Flow (RPF) | 309 mL/min | 1,424 mL/min |

Filtration Fraction (FF) | 17% | 23% |

## Send Us Your Feedback

**Steps on how to print your input & results:**

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.

## Filtration Fraction as ratio of GFR to RPF

Filtration Fraction (FF) is the ratio of the glomerular filtration rate (GFR) to the renal plasma flow (RPF) and helps quantify the proportion of fluid that passes into renal tubules, thus giving a measure of how well the kidneys are functioning. FF normal values range between 17 and 23%.

The formula to use is: `Filtration Fraction % = GFR / RPF`

This can then be multiplied by 100 to retrieve the percentage FF.

Parameters |
Normal Low |
Normal High |

Glomerular Filtration Rate (GFR) | 90 mL/min | 131 mL/min |

Renal Plasma Flow (RPF) | 309 mL/min | 1,424 mL/min |

Filtration Fraction (FF) | 17% | 23% |

Increases in filtration fraction also determine an increase in the protein concentration of the peritubular capillaries which in turn leads to additional absorption in the proximal tubule. Severe haemorrhage will also result in an increased filtration fraction.

Decreases in filtration fraction determine a decrease in the amount of fluid being filtered across the glomerular filtration barrier per unit of time. The protein concentration downstream in the peritubular vessels also decreases. Loop diuretics and thiazide diuretics decrease filtration fraction.

With age, GFR was found to decrease with a rate of decline that accelerates after 50-60 years, as part of the normal physiologic process of cellular and organ senescence. This also impacts on the filtration fraction.

## Determining FF in CRRT

Where continuous renal replacement therapies (CRRT) are used, the filtration fraction helps determine the volume of plasma removed from the dialysed blood by ultrafiltration, known as “the ratio of ultrafiltration rate to plasma water flow rate”.

The FF formula employed in CRRT:

`Filtration Fraction = Ultrafiltrate flow rate / [Blood flow rate x (1 – Hct) + Pre-dilution replacement flow rate]`

This can then be multiplied by 100 to retrieve the percentage FF.

With CRRT, filtration fraction can be increased up to 30%, leading to a post-filter blood hematocrit of 0.40. A post-filter haematocrit which is too high will tend to degrade the life of the filter and promote clot formation, a common issue in CRRT.

## References

Neri, Mauro, et al. Nomenclature for renal replacement therapy in acute kidney injury: basic principles. Critical Care 20.1 (2016): 318.

Rajeev Dalal; Zachary S. Bruss; Jasjit S. Sehdev. Physiology, Renal Blood Flow and Filtration. Treasure Island (FL): StatPearls Publishing; 2020.

Andronico G, Romé M, Lo Cicero A, et al. Renal plasma flow, filtration fraction and microalbuminuria in hypertensive patients: effects of chronic smoking. Nephrology (Carlton). 2005; 10(5):483-486.

Glassock RJ, Winearls C. Ageing and the glomerular filtration rate: truths and consequences. Trans Am Clin Climatol Assoc. 2009; 120:419-28.

Specialty: Nephrology

System: Urinary

Article By: Denise Nedea

Published On: June 17, 2020

Last Checked: June 17, 2020

Next Review: June 17, 2025