Editing Assessment of kidney function (section)

== Blood tests ==
Blood tests are also used to assess kidney function. These include tests that are intended to directly measure the function of the kidneys, as well as tests that assess the function of the kidneys by looking for evidence of problems associated with abnormal function. One of the measures of kidney function is the glomerular filtration rate (GFR). Other tests that can assess the function of the kidneys include assessment of [[electrolyte]] levels such as [[potassium]] and [[phosphate]], assessment of acid-base status by the measurement of [[bicarbonate]] levels from a vein, and assessment of the [[full blood count]] for [[anaemia]].{{citation needed|date=July 2020}}

===Glomerular filtration rate===
{{main|Glomerular filtration rate}}
The glomerular filtration rate (GFR) describes the volume of fluid filtered from the [[kidney|renal]] (kidney) [[Glomerulus (kidney)|glomerular]] capillaries into the [[Bowman's capsule]] per unit time.<ref>{{cite book| title= Essentials of Human Physiology| first= Thomas M. |last= Nosek| chapter=Section 7/7ch04/7ch04p11 |chapter-url=http://humanphysiology.tuars.com/program/section7/7ch04/7ch04p11.htm |archive-url=https://web.archive.org/web/20160324124828/http://humanphysiology.tuars.com/program/section7/7ch04/7ch04p11.htm|archivedate=2016-03-24}} – "Glomerular Filtration Rate"</ref> Creatinine clearance (C<sub>Cr</sub>) is the volume of [[blood plasma]] that is cleared of [[creatinine]] per unit time and is a useful measure for approximating the GFR. Creatinine clearance exceeds GFR due to creatinine secretion,<ref>{{cite book|last=Ganong|title=Review of Medical Physiology, 25th ed.|publisher=McGraw-Hill Education|year=2016|isbn=978-0-07-184897-8|page=677|chapter=Renal Function & Micturition}}</ref> which can be blocked by [[cimetidine]].   Both GFR and C<sub>Cr</sub> may be accurately calculated by comparative measurements of substances in the blood and urine, or estimated by formulas using just a blood test result ('''eGFR''' and '''eC<sub>Cr</sub>''') The results of these tests are used to assess the excretory function of the kidneys. [[Chronic kidney disease#Stages|Staging]] of [[chronic kidney disease]] is based on categories of GFR as well as [[albuminuria]] and cause of [[kidney disease]].<ref name=Stevens2013>{{Cite journal|last1=Stevens|first1=Paul E.|last2=Levin|first2=Adeera|authorlink2=Adeera Levin|date=Jun 4, 2013|title=Evaluation and management of chronic kidney disease: synopsis of the kidney disease: improving global outcomes 2012 clinical practice guideline|journal=Annals of Internal Medicine|volume=158|issue=11|pages=825–830|doi=10.7326/0003-4819-158-11-201306040-00007|issn=1539-3704|pmid=23732715|doi-access=}}</ref>

Central to the physiologic maintenance of GFR is the differential basal tone of the afferent and efferent arterioles (see diagram). In other words, the filtration rate is dependent on the difference between the higher blood pressure created by vasoconstriction of the input or afferent arteriole versus the lower blood pressure created by lesser vasoconstriction of the output or efferent arteriole.

GFR is equal to the [[renal clearance ratio]] when any solute is freely filtered and is neither reabsorbed nor secreted by the kidneys. The rate therefore measured is the quantity of the substance in the urine that originated from a calculable volume of blood. Relating this principle to the below equation – for the substance used, the product of urine concentration and urine flow equals the mass of substance excreted during the time that urine has been collected. This mass equals the mass filtered at the glomerulus as nothing is added or removed in the nephron. Dividing this mass by the plasma concentration gives the volume of plasma which the mass must have originally come from, and thus the volume of plasma fluid that has entered Bowman's capsule within the aforementioned period of time. The GFR is typically recorded in units of ''volume per time'', e.g., milliliters per minute ([[millilitre|mL]]/[[minute|min]]). Compare to [[filtration fraction]].{{citation needed|date=July 2020}}

:<math>GFR = \frac { \mbox{Urine Concentration} \times \mbox{Urine Flow} }{ \mbox{Plasma Concentration} }</math>

There are several different techniques used to calculate or estimate the glomerular filtration rate (GFR or eGFR). The above formula only applies for GFR calculation when it is equal to the clearance rate.

The normal range of GFR, adjusted for [[body surface area]], is 100–130 average 125 (mL/min)/(1.73 m<sup>2</sup>) in men and 90–120 (mL/min)/(1.73 m<sup>2</sup>) in women younger than the age of 40. In children, GFR measured by inulin clearance is 110 (mL/min)/(1.73 m<sup>2</sup>) until 2 years of age in both sexes, and then it progressively decreases. After age 40, GFR decreases progressively with age, by 0.4–1.2 mL/min per year.{{Citation needed|date=January 2013}}

Estimated GFR (eGFR) is now recommended by clinical practice guidelines and regulatory agencies for routine evaluation of GFR whereas measured GFR (mGFR) is recommended as a confirmatory test when more accurate assessment is required.<ref name="naturerev">{{Cite journal |author1=Levey AS |author2=Coresh J |author3=Tighiouart H |author4=Greene T |author5=Inker LA |s2cid=202573933 |date=2020 |title=Measured and estimated glomerular filtration rate: current status and future directions. |journal=Nat Rev Nephrol |volume=16 |issue=1 |pages=51–64 |doi=10.1038/s41581-019-0191-y |pmid=31527790}}</ref>