Glomerular+Filtration

Objectives

 * To explain how knowing a patient's GFR will impact the treatment they receive
 * To know the methods of calculating GFR
 * To know the strengths and weaknesses of the methods of calculating GFR
 * To develop a working knowledge of commonly used GFR formulae

Normal Flow rate 100-120ml/min/1.73m^2

Properties of a good GFR Marker:
 * Freely filtered
 * Not altered
 * not absorbed, secreted, synthesized, or metabolized
 * does not alter renal function

Estimation of GFR Inuin clearance 125I-iothalamate, 99Tc-DTPA
 * urea nitrogen clearance
 * creatinine
 * Cockroft-Gault Formula
 * "Levey" (MDRD) formula
 * Schwartz formla
 * BUN
 * Serum Creatinine
 * Serum Cystatin C

Urea: 6.25g protein catabolism generates 1g urea nitrogen. Freely filtered, but partially reabsorbed

Creatinine: Production by muscle at predictable rate... 10mg/kg/day older female 15 young female or older male 20-25 young male

normal renal function, 980% excreted by GFR, 10%TUBULAR SECRETION 1/S. creat ~ GFR

GFR usually expressed as mL/min =U*V/P per time U is urine concentraiton V is urine volume P is plasma concentration

Cockroft-Gault FOrmula
(140-age) * wt (in kg) / 72 * s. creat (mg/dl) multiply by 0.85 for females.

Levey formulae
GFRa=170* Pcr^(-.9999)*age^(-0.176)*female?0.762:1*black?1.18:1*BUN^(-0.170) * albumin^(0.318)

GFRb=186.3 * Pcr^(-1.154) * age^(-0.203) *female?0.76$2:1*black?1.21:1 Baseline characteristics age 61 __0 13 y GFR 40 _0 21 Ml 80 +- 17kg serum creatinine 2.3 mg/dL 88% white 60% male 6% diabetic

When is eGFR wrong? acute renal failure Morbid obesity VOlume overload CHF, ascites GFR> 69 ml/min liver disease organ tarnsplant peds, geriatrics

Cystatin C Low MW...13 kDa made bya ll nucleated cells... not affected by muscle mass, race, or sex constant production rate btewteen age 25, 70 freely filtered, completely catabolized, no extrarenal excretion normal values: 40-.03 mg/L

=SchwartzFOrmla== ediatric larance estimate clearance=(k*height) / s. creat k = 0.33 LBW 0.45, term 0.55 child 0.7 adolescent male