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Important cautions about eGFR

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Source : [www.renal.org/]

It is only an esti­mate. A signi­fi­cant error is pos­sible. eGFR is most like­ly to be inac­cu­rate in people at extremes of body type, for example mal­nou­ri­shed, ampu­tees, etc. See Race, below. It is not valid in pre­gnant women or in chil­dren (see Age, below).

Confi­dence inter­vals : 90% confi­dence inter­vals are quite wide, e.g. 90% of patients will have a mea­su­red GFR within 30% of their esti­ma­ted GFR. 98% have mea­su­red values within 50% of the esti­ma­ted value. For an indi­vi­dual patient values will be much more consistent than this, just as crea­ti­nine values are – e.g. a 20% fall in eGFR is cer­tain to reflect an impor­tant change.

Race : Some racial groups may not fit the MDRD equa­tion well. It was ori­gi­nal­ly vali­da­ted for US white and black patients. For Afro Carib­bean black patients, eGFR was 21% higher for any given crea­ti­nine in the MDRD stu­dy. So if race was not inclu­ded in the esti­mate you have, it should be increa­sed by approx. 20% for a black patient. In the UK white popu­la­tion the equa­tion seems to work quite well. It may not per­form so well in all racial groups.

Not so good near nor­mal : The MDRD equa­tion tends to unde­res­ti­mate nor­mal or near-nor­mal func­tion, so slight­ly low values should not be over-inter­pre­ted. Fur­ther­more, labo­ra­to­ry dif­fe­rences in crea­ti­nine esti­ma­tions may make signi­fi­cant dif­fe­rences. Rou­tine repor­ting of eGFR values >90 is not recom­men­ded and many labs are now repor­ting all values over 60 as >60. Note howe­ver that a signi­fi­cant (e.g. 20%) rise in crea­ti­nine while eGFR is >60 may still be impor­tant as it will usual­ly reflect a real change in GFR.

Crea­ti­nine level must be stable : eGFR cal­cu­la­tions assume that the level of crea­ti­nine in the blood is stable over days or lon­ger. They are not valid if it is changing.

Age : The MDRD equa­tion is not valid for under-18s. Use the Cou­na­han-Bar­rat method for chil­dren – e.g. the pae­dia­tric cal­cu­la­tor from Steve Fadem at nephron.com

Which MDRD equa­tion ? in the UK, most labo­ra­to­ries cal­cu­late eGFR on all samples sent for crea­ti­nine mea­su­re­ment. Since 2006 the equa­tion they use should take into account local varia­tions in accu­ra­cy of crea­ti­nine assays, so eGFR values obtai­ned in this way should be a lit­tle more accu­rate than those gene­ra­ted by any of the online cal­cu­la­tors (inclu­ding ours). Tech­ni­cal­ly, this is the « 175 » equa­tion used with a cor­rec­tion fac­tor when neces­sa­ry, rather than the ori­gi­nal « 186 » equation.

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