Making Sense of GFR
• Jamal Salameh, M.D.
• First Coast Nephrology
Making Sense of GFR
• GFR-True measure of Renal Function
• The clearance of any substance in ml/min
• Since there is NO IDEAL endogenous substrate we use
External Compounds
Iothalamate/Iohexol/DTPA-Tc99/Inulin
Laborious
Not Readily Available
Making Sense of GFR
• eGFR-Formulaic attempt at Spot GFR measure
Easy and Fast
Based on a Few Variables (CRAS)
MDRD Equation
Cockroft and Gault Equation
Making Sense of GFR
• Modified Diet in Renal Disease (MDRD)
-GFR = 175 x SerumCr-1.154 * age-0.203 * 1.212 (if
patient is black) * 0.742 (if female)
-4 or 6 Variable Formula for derivation of GFR
-Creatinine, Race, Age and Sex Variable
-Muscle Mass changes the GFR (M>F, AA>W)
-Corrected for Body Surface Area-1.73 m2
-Underestimates GFR by 10-30% in GFR>60
Making Sense of GFR
MDRD
• 1070 pts f/u 2.3 yrs
• Moderate to Severe Dz (GFR 13-55)
• Males Cr > 1.4
• Females Cr > 1.2
• Average Age 51 y.o.
• White 88%
• Male 60%
Making Sense of GFR
MDRD-limitations
• No IDDM (Insulin Dependent)
• No Healthy patients
• No Pregnant patients
• Urine P/Cr < 10
• Age < 70 (originally but now <85 and > 18 yo)
• No one < 80% or > 160% of IBW
Making Sense of GFR
• Since the original study this Formula has been
validated in African Americans, Asians &
Europeans.
• Diabetic and Non Diabetic pts c and s CKD
• Transplant Donors
• Transplant Recipients
• Most Common Formula used for eGFR calc
Making Sense of GFR
• MDRD
-This eqn standardizes for BSA of 1.73 m2
-If your pt is > or < than this-consider
correction
-GFR ml/min/1.73m2 x BSA pt/1.73 m2
-In large pts this will inc GFR and vice versa
-Elderly pts tend to shrink in Ht/Wt thus
affecting BSA-Cr Normal or close to it.
Making Sense of GFR
• Cockroft and Gault
(140-Age) x weight (Kg)/Pcr x 72 *0.85 if F
Does not correct for BSA but does for Weight
Issues re IBW and Real Body weight arise
Study of 249 Men c Cr Cl of 30-130 ml/min
Developed in 1973
Making Sense of GFR
• Creatinine Clearance (Cr Cl)
Requires a 24 Hr Urine Sample and Pcr
Laborious
High likelihood of error b/c long collection
Repeated measurements may help this error
Overestimates Cr Cl by approx 10%-Secreted
Increased Secretion c AKI/CKD up to Cr of 2
Making Sense of GFR
• Limitations in Estimating Equations
Rely on Serum Creatinine
Decreased accuracy at GFR > 60 ml/min/BSA
If Non Steady State they cannot be trusted
If Creatinine inc by 1 g per day, GFR prob nil
Making Sense of GFR
Making Sense of GFR
• Creatinine Variables
Secreted
Increased Generation (Catabolic/Protein XS)
Non-Standard Assay (false Increases can
falsely lower GFR)-0.1 to 0.3 decline with
Standardization-Jaffe method
Wide Range of Normals limits our GFR
assesment until about 50% of Normal
Ketoacids/Glucose/Proteins all Falsely Increase
Creatinine
Making Sense of GFR
22 yo AAM
58 yo WM
80 yo WF
Cr
1.2 mg/dl
1.2
1.2
GFR
98 ml/min
66
48
Maybe
CKD 3
Disease
No
Making Sense of GFR
• When is it necessary to obtain a Real GFR
-Extremes of Age
-Extremes of Body Size
-Kidney Donation asssesment
-Toxic Drugs w/ Low TI (Therap Index)
-Malnutrition/Obesity
-Pregnancy
-Para/Quadriplegic/Skeletal M dz
Making Sense of GFR
• In these patients you can obtain
Iothalamate Clearance
Iohexol Clearance
DTPA-Tc99 Clearance
Inulin Clearance
Making Sense of GFR
• GFR and Age
-Declines with Age (0.7 ml/min/yr p 30-40)
-Is it a normal process of Aging?
-Elderly pts w Low GFR have independent
risk of Cardiovascular dz and Death
-Most of these patients have a NORMAL Cr
-Caution w Drug Dosing in this age
-Young pts have > GFR than old b/c of
Lower Muscle mass and Nephron Mass in
elderly
Making Sense of GFR
Age (Yr)
20-29
30-39
40-49
50-59
60-69
> or = 70
GFR (ml/min)
116
107
99
93
85
75
Making Sense of GFR
• Stages of CKD
1-GFR >90, Evidence of Renal Dz, >3 mos
2-GFR 60-89, Evidence of Renal Dz, > 3mos
3-GFR 30-59, 3a 45-59, 3b 30-44, Diff dzs
4-GFR 15-29
5-GFR <15, HD and Non HD pts
Making Sense of GFR
Making Sense of GFR
• Caveat-The most common question beginning
new patient visits is “How can my function
be so low, I see the Doctor Regularly?”
-I educate pts and inform them of the
limitations of the tests, but there can be
some confusion and early referral may limit
this.
Making Sense of GFR
• In Conclusion
The Goal of All of this is to Accurately
Measure Renal function and Thus more
Accurately asses an Individual patients Risk of
Progressive Renal Dysfunction as well as
Comorbidities and Prognosis.