{"id":148,"date":"2018-04-23T11:38:38","date_gmt":"2018-04-23T11:38:38","guid":{"rendered":"http:\/\/edren.org\/ren\/?page_id=148"},"modified":"2025-01-27T09:54:46","modified_gmt":"2025-01-27T09:54:46","slug":"gfr-estimation","status":"publish","type":"page","link":"https:\/\/edren.org\/ren\/handbook\/unithdbk\/ckd\/gfr-estimation\/","title":{"rendered":"GFR estimation"},"content":{"rendered":"<p>Glomerular filtration rate (GFR) is the usual method for measuring renal excretory function. Most commonly it is estimated (eGFR), or sometimes surrogates are used such as creatinine clearance (CrCl). Occasionally it is formally measured.<\/p>\n<h5><strong><span style=\"color: #993300;\">Normal values for GFR<\/span><\/strong><\/h5>\n<p><strong><span style=\"color: #993300;\">120\u00b125 mls\/min<\/span><\/strong> (95th centiles. Males approx 5mls higher, females approx 5mls lower). Therefore values &gt;90 are normal for all. Converting for average surface area (per 1.73m<sup>2<\/sup>) removes the sex difference.<\/p>\n<p>With age, GFR tends to fall (to approx 100mls\/min\/1.73m<sup>2<\/sup> at age 70), although serum creatinine does not rise much in healthy individuals (Fliser et al, Kidney Int<i> <\/i>51:1196-1204, 1997). This fall is mostly due to subclinical pathology.<b><\/b><\/p>\n<h5><strong><span style=\"color: #993300;\">Estimating GFR<\/span><\/strong><\/h5>\n<p>A variety of methods have been used. All are based on serum creatinine estimations. Although the <a href=\"#The_Cockcroft-Gault_equation\">Cockcroft-Gault<\/a> formula is widely used, it was developed to permit estimation of CrCl, not GFR. The best validated method until recently came from the <strong><span style=\"color: #993300;\">MDRD study<\/span><\/strong> data (<i>Ann Int Med<\/i>130:461-70, 877-84, 1999). The study derived a number of equations. The abbreviated, or four-variable equation includes age, sex, creatinine, and race (black or not black). Adding more variables (albumin, urea) adds little to accuracy.<\/p>\n<p>The CKD-EPI equation (ref ) is more accurate at higher GFRs than the MDRD equation.<\/p>\n<p>It is important to be aware of the limitations of these equations (<a href=\"http:\/\/www.renal.org\/information-resources\/the-uk-eckd-guide#sthash.5Tk9CLNE.dpbs\">&gt;more info on eGFR from the UK CKD eGuide<\/a>):<\/p>\n<ul style=\"list-style-type: disc;\">\n<li><strong><span style=\"color: #993300;\">Accuracy <\/span><\/strong>&#8211; the confidence limits are wide. For MDRD 90% of values are within 30% of the true value; 98% within 50%.<\/li>\n<li><strong><span style=\"color: #993300;\">Extremes<\/span><\/strong> &#8211; none of the methods for estimating GFR without actual measurements of it are likely to be accurate in extreme examples of low muscle mass, or other unusual circumstances.<\/li>\n<li><strong><span style=\"color: #993300;\">Stability<\/span><\/strong> &#8211; for all methods, [creat] must not be changing quickly.<\/li>\n<li><strong><span style=\"color: #993300;\">Systematic errors<\/span><\/strong> &#8211; accuracy is better at low GFRs.<\/li>\n<li><strong><span style=\"color: #993300;\">Age<\/span><\/strong> &#8211; the Schwartz or Counahan Barrat equation (requires height) should be used in children.<\/li>\n<li><strong><span style=\"color: #993300;\">Race, different equations<\/span><\/strong> &#8211; <a href=\"http:\/\/www.renal.org\/information-resources\/the-uk-eckd-guide#sthash.5Tk9CLNE.dpbs\">more info<\/a><\/li>\n<\/ul>\n<p>Most UK labs now report eGFR when returning creatinine values. When available, these values should be more accurate than calculations performed by you, as they should incorporate lab-specific correction factors. If not, here are some resources:<\/p>\n<ul style=\"list-style-type: disc;\">\n<li><a href=\"https:\/\/patient.info\/doctor\/estimated-glomerular-filtration-rate-gfr-calculator\" target=\"_blank\" rel=\"noopener noreferrer\">Online MDRD calculator from Patient.info<\/a> (SI units only, nice and simple)<\/li>\n<li><a href=\"http:\/\/kidney.org.au\/health-professionals\/detect\/calculator-and-tools\">CKD-EPI calculator from Kidney Health Australia <\/a>(SI units only, nice and simple)<\/li>\n<li><a href=\"http:\/\/nephron.com\/mdrd\/default.html\">Online calculator from nephron.com<\/a> &#8211; option of SI units or quaint mg\/dl; shows results of alternative calculations including CKD-EPI and MDRD; even pointless Cystatin C variants. Take care to set units for [creat] correctly.<\/li>\n<li>more about eGFR and the stages of CKD &#8211; small print, caveats, etc, from the <a href=\"https:\/\/renal.org\/information-resources\/the-uk-eckd-guide\/\">UK CKD guidelines<\/a><\/li>\n<\/ul>\n<p>In NHS Lothian, the labs report eGFR calculated using a three-variable MDRD equation (age, sex, Cr).\u00a0 VitalData eGFR is calculated using a 6-variable equation.\u00a0 (We will probably move to CKD-EPI in 2024.)<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>eGFR is an estimate of GFR this using serum creatinine, age and sex. For patients of black African racial origin, this should be included too (but won&#8217;t be in reported values from laboratories). <a href=\"http:\/\/www.renal.org\/pages\/pages\/other-info\/ckd\/about-egfr.php\">More info about eGFR from the UK CKD guidelines<\/a>.<\/p>\n<p><strong><span style=\"color: #993300;\">I know the patient&#8217;s (or my) eGFR:<\/span><\/strong> choose one of these &#8211;<\/p>\n<table style=\"width: 90%;\" border=\"1\" width=\"90%\" cellspacing=\"0\" cellpadding=\"2\">\n<tbody>\n<tr>\n<td style=\"width: 28.4313%; text-align: left;\" bgcolor=\"white\" width=\"45%\">\n<div align=\"left\">GFR&gt;90<\/div>\n<\/td>\n<td style=\"width: 26.5687%;\" bgcolor=\"white\" width=\"10%\">normal or stage 1 CKD<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 28.4313%; text-align: left;\" bgcolor=\"#ffcccc\" width=\"45%\">\n<div align=\"left\">GFR 60-89<\/div>\n<\/td>\n<td style=\"width: 26.5687%;\" bgcolor=\"#ffcccc\" width=\"10%\">normal or stage 2 CKD<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 28.4313%; text-align: left;\" bgcolor=\"#ff9999\" width=\"45%\">\n<div align=\"left\">GFR 30-59<\/div>\n<\/td>\n<td style=\"width: 26.5687%;\" bgcolor=\"#ff9999\" width=\"10%\">stage 3 CKD<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 28.4313%; text-align: left;\" bgcolor=\"#ff6699\" width=\"45%\">\n<div align=\"left\">GFR&lt;30<\/div>\n<\/td>\n<td style=\"width: 26.5687%;\" bgcolor=\"#ff6699\" width=\"10%\">stages 4+5 CKD<\/td>\n<\/tr>\n<tr>\n<td style=\"background-color: #cbf2ef; text-align: center; width: 55%;\" colspan=\"2\"><a href=\"https:\/\/edren.org\/ren\/handbook\/unithdbk\/ckd\/ckd-summary-overview\/\" data-cke-saved-href=\"http:\/\/renux.dmed.ed.ac.uk\/EdREN\/Unitbits\/CKDmanagement.html#Anchor-47857\">About CKD stages<\/a> (this site) | <a href=\"http:\/\/www.renal.org\/eGFR\/about.html\" data-cke-saved-href=\"http:\/\/www.renal.org\/eGFR\/about.html\">About eGFR <\/a>(UK CKD)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h5><\/h5>\n<h5><strong><span style=\"color: #993300;\">Measuring GFR<\/span><\/strong><\/h5>\n<p>For direct measurement of GFR, isotope tests are most commonly used. Like inulin, or the radiographic contrast agent iohexol, these markers (eg 51Cr-EDTA, 99Tc-DTPA) are cleared almost entirely by glomerular filtration, and measures of their disappearance rate from the circulation, or appearance in the urine, can be used to estimate GFR.<\/p>\n<h5><\/h5>\n<h5><strong><span style=\"color: #993300;\">Cystatin C eGFR<\/span><\/strong><\/h5>\n<p>Serum Cystatin C is an alternative to serum creatinine as a marker of GFR.\u00a0 It gives a more accurate eGFR in individuals who have a muscle bulk that is atypical for their age and sex.\u00a0 It is not routinely used in NHS Lothian, not least for cost reasons.\u00a0 It is available for selected patients.\u00a0 The request should be submitted on a paper form, stating a named Renal Consultant who has generated or sanctioned the request.\u00a0 Requires a serum (brown-top) tube.<\/p>\n<p>&nbsp;<\/p>\n<h5><strong><span style=\"color: #993300;\">GFR and Reciprocal of creatinine plots<\/span><\/strong><\/h5>\n<p>Plots of GFR: time, or of the reciprocal of creatinine are useful as they can be used to give an approximate prediction of the date of ESRF for many patients, and to identify changes in the rate of progression. The rate of creatinine production in an individual changes slowly if at all. Note that linear plots of GFR\/time are almost identical to reciprocal creatinine plots.<\/p>\n<table border=\"0\" width=\"85%\" cellspacing=\"0\" cellpadding=\"7\" align=\"center\">\n<tbody>\n<tr bgcolor=\"#ffffcc\">\n<td><b>Download <\/b>a <a href=\"http:\/\/edren.org\/ren\/wp-content\/uploads\/2018\/10\/recipcreatfig.pdf\">blank reciprocal creatinine plot<\/a> (pdf file) but note that plots of GFR vs time should be linear and in the future will probably replace 1\/creatinine.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>An example of a reciprocal creatinine plot for a patient with an original diagnosis of small vessel vasculitis:<\/p>\n<p><center><a href=\"http:\/\/edren.org\/ren\/wp-content\/uploads\/2018\/10\/PC-graph.gif\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-1832\" src=\"http:\/\/edren.org\/ren\/wp-content\/uploads\/2018\/10\/PC-graph.gif\" alt=\"\" width=\"750\" height=\"500\" \/><\/a><\/center><\/p>\n<h5><strong><span style=\"color: #993300;\">Creatinine Clearance<br \/>\n<\/span><\/strong><\/h5>\n<p><strong><span style=\"color: #993300;\">Creatinine clearance<\/span><\/strong> over estimates GFR in a variety of circumstances, because of tubular secretion. This is particularly important at low levels of GFR. To calculate it, pay close attention to units and remember that there are 1440 minutes in 24h (<a href=\"#The_Cockcroft-Gault_equation\">details of calculations below<\/a>). However 24 hour urine collections are error-prone and have been shown to be less accurate for estimating GFR than deductions from plasma creatinine.<\/p>\n<p><strong><span style=\"color: #993300;\">Calculations:<\/span><\/strong> everyone knows it&#8217;s UV\/P but the units get a little confusing. This formula shows SI units.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-medium wp-image-3885\" src=\"http:\/\/edren.org\/ren\/wp-content\/uploads\/2020\/03\/CrCl-300x61.gif\" alt=\"\" width=\"300\" height=\"61\" srcset=\"https:\/\/edren.org\/ren\/wp-content\/uploads\/2020\/03\/CrCl-300x61.gif 300w, https:\/\/edren.org\/ren\/wp-content\/uploads\/2020\/03\/CrCl-150x30.gif 150w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/p>\n<table border=\"0\" width=\"85%\" cellspacing=\"0\" cellpadding=\"7\" align=\"center\">\n<tbody>\n<tr bgcolor=\"#ffffcc\">\n<td style=\"background-color: #e6f7ed;\"><strong><span style=\"color: #993300;\">UV<\/span><\/strong><\/td>\n<td style=\"background-color: #e6f7ed;\">Amount of creatinine in 24h of urine is reported by the lab in mmol\/24h. x1000 to convert to micromols<\/td>\n<\/tr>\n<tr bgcolor=\"#ffffcc\">\n<td style=\"background-color: #e6f7ed;\"><strong><span style=\"color: #993300;\">P<\/span><\/strong><\/td>\n<td style=\"background-color: #e6f7ed;\">Serum creatinine is reported in micromols\/l. x1000 to do calculation for mls<\/td>\n<\/tr>\n<tr bgcolor=\"#ffffcc\">\n<td style=\"background-color: #e6f7ed;\"><strong><span style=\"color: #993300;\">1440<\/span><\/strong><\/td>\n<td style=\"background-color: #e6f7ed;\">Number of minutes in 24h<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h5><strong><span style=\"color: #993300;\">Estimating CrCl<\/span><\/strong><\/h5>\n<p>The <strong><span style=\"color: #993300;\">Cockcroft-Gault equation<\/span><\/strong> aims to predict creatinine clearance from knowledge of serum creatinine, age and weight. See <a href=\"http:\/\/nephron.com\/mdrd\/default.html\" target=\"_blank\" rel=\"noopener noreferrer\">Calculator 2<\/a><b> <\/b>(mentioned above) or do it the hard way:<\/p>\n<div align=\"center\">\n<p><strong><span style=\"color: #993300;\"><u><span style=\"color: #993300;\">(140-age) x weight (kg) x 1.23 x (0.85 if female)<\/span><\/u><br \/>\nCreat [micromol\/l]<\/span><\/strong><\/p>\n<\/div>\n<p>Weight should be lean body mass, but attempting to calculate that makes the equation unreasonably complicated. Estimate lean body mass for extremes of size, or use the MDRD equation instead, or best of all, measure it.<\/p>\n<p><strong><span style=\"color: #993300;\">Note that these estimates\/measures of CrCl are not normalised for body size, if you want to compare with eGFR you should adjust according to calculated surface area.<\/span><\/strong><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: left;\"><span style=\"color: #808080;\"><span style=\"font-size: 12pt;\"><strong>Acknowledgements: <\/strong><\/span><span style=\"font-size: 12pt;\"> Neil Turner was the main author for this page.<\/span> <\/span><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Glomerular filtration rate (GFR) is the usual method for measuring renal excretory function. Most commonly it is estimated (eGFR), or sometimes surrogates are used such as creatinine clearance (CrCl). Occasionally it is formally measured. Normal values for GFR 120\u00b125 mls\/min (95th centiles. Males approx 5mls higher, females approx 5mls lower).\u2026<\/p>\n<p> <a class=\"continue-reading-link\" href=\"https:\/\/edren.org\/ren\/handbook\/unithdbk\/ckd\/gfr-estimation\/\"><span>Continue reading<\/span><i class=\"crycon-right-dir\"><\/i><\/a> <\/p>\n","protected":false},"author":2,"featured_media":0,"parent":5365,"menu_order":6,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_lmt_disableupdate":"no","_lmt_disable":"","footnotes":""},"class_list":["post-148","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/edren.org\/ren\/wp-json\/wp\/v2\/pages\/148","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/edren.org\/ren\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/edren.org\/ren\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/edren.org\/ren\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/edren.org\/ren\/wp-json\/wp\/v2\/comments?post=148"}],"version-history":[{"count":30,"href":"https:\/\/edren.org\/ren\/wp-json\/wp\/v2\/pages\/148\/revisions"}],"predecessor-version":[{"id":6577,"href":"https:\/\/edren.org\/ren\/wp-json\/wp\/v2\/pages\/148\/revisions\/6577"}],"up":[{"embeddable":true,"href":"https:\/\/edren.org\/ren\/wp-json\/wp\/v2\/pages\/5365"}],"wp:attachment":[{"href":"https:\/\/edren.org\/ren\/wp-json\/wp\/v2\/media?parent=148"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}