Glomerular filtration rate

The glomerular filtration rate (GFR) is the amount of primary urine formed by glomerular filtration per time (Herold 2022). The GFR is normally 120 ml/ min / 1.73 m² body surface or approx. 180 l / d (Silbernagl 2003).

In medical care, the assessment of glomerular filtration rate is very important (Musso 2016). However, the measurement itself is rarely performed due to its complexity (Ferguson 2015). In addition, GFR represents a dynamic variable that can change almost instantaneously due to stress responses, for example (Solomon 2017).

To accurately measure GFR, inulin, an indicator substance (Silbrenagl 2003), is used. The formula is:

GFR = UIN x VU / PIN ml / min.

UIN = urine concentration of inulin

VU = urine time volume

PIN = plasma concentration of inulin (Emminger 2005).

Therefore, over decades, enormous efforts have been made to develop the most accurate equations possible to estimate renal function such as the

- CKD- EPI- equation (Chronic Kidney Disease Epidemiology Collaboration).

This formula takes into account: serum creatinine, weight, height and age of the patient. This equation is now widely used to estimate eGFR.

Link to eGFR- calculator: https://nierenrechner.de/ (Weckmann 2019).

- MDRD- equation.

This equation is somewhat less precise than the CKD- EPI- equation, but is still recommended according to the guideline.

Age, gender, skin color, height, weight, creatinine, urea, protein and albumin in serum and urea in urine play a role here.

Link to the eGFR- value calculator: https://nierenrechner.de/index.php?page=egfr-mdrd (Weckmann 2019)

- EKFC ( European- Kidney Function Consortium).

The EKFC- equation uses either serum creatinine, cystatin C, or a combination of both to determine GFR (Schäffner 2023).

- DRA equation

- Gregori- Macías equation (Musso 2016)

Other freely available eGFR- calculators can be found at https://www.labor-limbach.de/medizinische-rechner/ (Weckmann 2019).

GFR can be disturbed by:

- prerenal

- renal

- postrenal causes (Renz 2009).

In addition, there is a physiological decrease in GFR of approximately 10 ml / l every 10 years from the age of 40 (Renz 2009).

Autoregulation of glomerular filtration is influenced by 3 main factors:

- 1. autonomic vasoreactive (myogenic) reflex in the afferent arteriole.

This is the first to become active during fluctuations in renal blood flow. Reflex constriction or dilation of the afferent arteriole protects the glomerular capillaries from sudden changes in systolic pressure (Kasper 2015).

- 2. tubuloglomerular feedback (TGF).

TGF alters both filtration rate and tubular flow through reflex vasoconstriction or dilation of the afferent arteriole (Kasper 2015).

- 3. by angiotensin- II.

Once decreased renal blood flow occurs, renin is released from the juxtaglomerular apparatus. Renin catalyzes the conversion of angiotensinogen to angiotensin I. Angiotensin I, in turn, is converted to angiotensin II by the enzyme ACE.

Angiotensin- II-mediated vasoconstriction of the efferent arteriole results in increased glomerular hydrostatic pressure, which raises the filtration rate to a normal level (Kasper 2015).

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