Clinical Evaluation of Glomerular Filtration S.P. DiBartola, DVM D.J. Chew, DVM

Clinical Evaluation of Clinical Evaluation of Glomerular FiltrationGlomerular FiltrationClinical Evaluation of Clinical Evaluation of Glomerular FiltrationGlomerular Filtration

S.P. DiBartola, DVMS.P. DiBartola, DVM

D.J. Chew, DVMD.J. Chew, DVM

The ideal substance to measure GFR The ideal substance to measure GFR would …would …The ideal substance to measure GFR The ideal substance to measure GFR would …would …• Be freely filtered at the glomeruliBe freely filtered at the glomeruli• Not be bound to plasma proteinsNot be bound to plasma proteins• Not be metabolizedNot be metabolized• Be non-toxicBe non-toxic• Be excreted only by the kidneysBe excreted only by the kidneys• Be neither reabsorbed nor secreted by the Be neither reabsorbed nor secreted by the

renal tubulesrenal tubules• Be stable in blood and urineBe stable in blood and urine• Be easily measuredBe easily measured

Clinical Assessment of GFRClinical Assessment of GFRClinical Assessment of GFRClinical Assessment of GFR

• BUNBUN• Serum creatinineSerum creatinine• Creatinine clearance Creatinine clearance

(endogenous or exogenous)(endogenous or exogenous)• Sodium sulfanilateSodium sulfanilate• RadioisotopesRadioisotopes

Essential terminologyEssential terminologyEssential terminologyEssential terminology

• Renal diseaseRenal disease

• Renal failureRenal failure

• AzotemiaAzotemia

• UremiaUremia

Renal diseaseRenal diseaseRenal diseaseRenal disease

• Implies the presence of histologic Implies the presence of histologic lesions in the kidney but does not lesions in the kidney but does not specify any degree of renal specify any degree of renal dysfunctiondysfunction

Renal failureRenal failureRenal failureRenal failure

• Implies that 75% of the total Implies that 75% of the total nephron population has become nephron population has become non-functional but does not non-functional but does not necessarily imply underlying necessarily imply underlying histologic lesionshistologic lesions

AzotemiaAzotemiaAzotemiaAzotemia

• Increased concentration of non-Increased concentration of non-protein nitrogenous waste protein nitrogenous waste products (e.g. urea, creatinine) in products (e.g. urea, creatinine) in the bloodthe blood

AzotemiaAzotemiaAzotemiaAzotemia

• Pre-renalPre-renal: due to reduced renal : due to reduced renal perfusionperfusion

• RenalRenal: due to renal parenchymal : due to renal parenchymal diseasedisease

• Post-renalPost-renal: due to impaired : due to impaired elimination of urine from the bodyelimination of urine from the body

UremiaUremiaUremiaUremia

• The constellation of clinical and The constellation of clinical and biochemical abnormalities biochemical abnormalities associated with a critical loss of associated with a critical loss of functioning nephronsfunctioning nephrons

• Includes the extra-renal Includes the extra-renal manifesations of renal failuremanifesations of renal failure

Blood urea nitrogen (BUN)Blood urea nitrogen (BUN)Blood urea nitrogen (BUN)Blood urea nitrogen (BUN)

• Dogs: 8-25 mg/dLDogs: 8-25 mg/dL

• Cats: 15-35 mg/dLCats: 15-35 mg/dL

• Horses: 10-27 mg/dLHorses: 10-27 mg/dL

• Cattle: 5-23 mg/dLCattle: 5-23 mg/dL

Normal values may vary among laboratories

Production of ureaProduction of ureaProduction of ureaProduction of urea

• Synthesized in liver Synthesized in liver using NHusing NH33 derived from derived from amino acids of amino acids of endogenous (body) or endogenous (body) or exogenous (dietary) exogenous (dietary) proteinsproteins

• Not produced at a Not produced at a constant rate (affected constant rate (affected by protein intake)by protein intake)

NH4+ + CO2 + 3ATP + 2H2O + aspartate urea + 2ADP + 2Pi +

AMP + PPi + fumarate + 2H+

Distribution and excretion of ureaDistribution and excretion of ureaDistribution and excretion of ureaDistribution and excretion of urea

• Freely permeable and distributed Freely permeable and distributed throughout total body waterthroughout total body water

• Renal excretion most importantRenal excretion most important• Filtered by glomeruliFiltered by glomeruli• Passively reabsorbed in renal tubules Passively reabsorbed in renal tubules

depending on tubular flow ratedepending on tubular flow rate• Not secreted by renal tubulesNot secreted by renal tubules

• Not excreted at a constant rate (high Not excreted at a constant rate (high protein meal transiently increases GFR)protein meal transiently increases GFR)

Measurement of ureaMeasurement of ureaMeasurement of ureaMeasurement of urea

• Technically easy and reproducibleTechnically easy and reproducible• Measured by diacetylmonoxamine or Measured by diacetylmonoxamine or

urease methodologyurease methodology• Urease methodology most specific Urease methodology most specific

and accurate (used on Hitachi and accurate (used on Hitachi autoanalyzer)autoanalyzer)

• Dipstrip (e.g. Azostix) methods not Dipstrip (e.g. Azostix) methods not very accuratevery accurate

Abnormal BUN concentrationAbnormal BUN concentrationAbnormal BUN concentrationAbnormal BUN concentration

• Non-renal factorsNon-renal factors

• Renal factorsRenal factors• Pre-renal (e.g. dehydration, heart failure, Pre-renal (e.g. dehydration, heart failure,

shock)shock)

• Renal (e.g. parenchymal renal disease)Renal (e.g. parenchymal renal disease)

• Post-renal (e.g. urethral obstruction, Post-renal (e.g. urethral obstruction, ruptured bladder)ruptured bladder)

Abnormal BUN: Non-renal factorsAbnormal BUN: Non-renal factorsAbnormal BUN: Non-renal factorsAbnormal BUN: Non-renal factors

• Increased BUNIncreased BUN• High protein mealHigh protein meal• Hemorrhage into gastrointestinal tractHemorrhage into gastrointestinal tract• Increased catabolismIncreased catabolism• Drugs (glucocorticoids, azathioprine, tetracycline)Drugs (glucocorticoids, azathioprine, tetracycline)

• Decreased BUNDecreased BUN• Low protein dietLow protein diet• Severe liver disease or portosystemic shuntSevere liver disease or portosystemic shunt• Drugs (anabolic steroids)Drugs (anabolic steroids)

CreatinineCreatinineCreatinineCreatinine

• Dogs: 0.3-1.2 mg/dLDogs: 0.3-1.2 mg/dL

• Cats: 0.8-1.8 mg/dLCats: 0.8-1.8 mg/dL

• Horses: 1.0-1.8 mg/dLHorses: 1.0-1.8 mg/dL

• Cattle: 0.6-1.5 mg/dLCattle: 0.6-1.5 mg/dL

Normal values may vary among laboratories

Production of creatinineProduction of creatinineProduction of creatinineProduction of creatinine

• Non-enzymatic Non-enzymatic breakdown product of breakdown product of phosphocreatine in phosphocreatine in musclemuscle

• Produced at a Produced at a relatively constant rate relatively constant rate based on age, gender, based on age, gender, and muscle massand muscle mass

• Not affected by dietNot affected by diet

Distribution and excretion of creatinineDistribution and excretion of creatinineDistribution and excretion of creatinineDistribution and excretion of creatinine

• Freely permeable and distributed Freely permeable and distributed throughout total body waterthroughout total body water

• Renal excretion most importantRenal excretion most important• Filtered by glomeruliFiltered by glomeruli• Not reabsorbed by renal tubulesNot reabsorbed by renal tubules• Not secreted by renal tubulesNot secreted by renal tubules

• Excreted at a relatively constant rateExcreted at a relatively constant rate

Measurement of creatinineMeasurement of creatinineMeasurement of creatinineMeasurement of creatinine

• Usually measured by alkaline Usually measured by alkaline picrate method (used on Hitachi picrate method (used on Hitachi autoanalyzer)autoanalyzer)

• Measures creatinine and non-Measures creatinine and non-creatinine chromagenscreatinine chromagens

Measurement of creatinine: Measurement of creatinine: Non-creatinine chromagensNon-creatinine chromagens

Measurement of creatinine: Measurement of creatinine: Non-creatinine chromagensNon-creatinine chromagens

• May constitute up to 50% of measured May constitute up to 50% of measured “creatinine” at normal serum creatinine “creatinine” at normal serum creatinine concentrations (but progressively less concentrations (but progressively less as renal function declines)as renal function declines)

• Do not appear in urine (affects Do not appear in urine (affects clearance calculations)clearance calculations)

• Special techniques to circumvent them Special techniques to circumvent them are not in common use by clinical are not in common use by clinical laboratorieslaboratories

Abnormal serum creatinine Abnormal serum creatinine concentrationconcentration

Abnormal serum creatinine Abnormal serum creatinine concentrationconcentration

• Non-renal factors (usually transient)Non-renal factors (usually transient)

• Renal factorsRenal factors• Pre-renal (e.g. dehydration, heart Pre-renal (e.g. dehydration, heart

failure, shock)failure, shock)

• Renal (e.g. parenchymal renal disease)Renal (e.g. parenchymal renal disease)

• Post-renal (e.g. urethral obstruction, Post-renal (e.g. urethral obstruction, ruptured bladder)ruptured bladder)

Abnormal serum creatinine Abnormal serum creatinine concentration: Non-renal factorsconcentration: Non-renal factors

Abnormal serum creatinine Abnormal serum creatinine concentration: Non-renal factorsconcentration: Non-renal factors

• Increased creatinine (usually Increased creatinine (usually transient)transient)• Massive muscle necrosisMassive muscle necrosis• Prolonged strenuous exerciseProlonged strenuous exercise

• Decreased creatinineDecreased creatinine• Severe loss of muscle massSevere loss of muscle mass• Small body sizeSmall body size• Young ageYoung age

Relationship between BUN or Relationship between BUN or creatinine and % functional nephrons creatinine and % functional nephrons is a “rectangular hyperbola”is a “rectangular hyperbola”

Relationship between BUN or Relationship between BUN or creatinine and % functional nephrons creatinine and % functional nephrons is a “rectangular hyperbola”is a “rectangular hyperbola”

• Large changes in GFR Large changes in GFR “early” in renal disease “early” in renal disease cause small changes in cause small changes in BUN or creatinineBUN or creatinine

• Small changes in GFR Small changes in GFR late in renal disesae late in renal disesae cause big changes in cause big changes in BUN or serum creatinineBUN or serum creatinine

Implication of azotemiaImplication of azotemiaImplication of azotemiaImplication of azotemia

• In a “steady state” and when non-In a “steady state” and when non-renal factors have been renal factors have been eliminated from consideration, an eliminated from consideration, an increase of BUN or creatinine increase of BUN or creatinine above normal implies that at least above normal implies that at least 75% of the nephrons are not 75% of the nephrons are not functioningfunctioning

Magnitude of azotemia does Magnitude of azotemia does NOT …NOT …

Magnitude of azotemia does Magnitude of azotemia does NOT …NOT …

• Differentiate pre-renal, renal, and post-Differentiate pre-renal, renal, and post-renal processesrenal processes

• Differentiate acute from chronic Differentiate acute from chronic processesprocesses

• Differentiate reversible from irreversible Differentiate reversible from irreversible processesprocesses

• Differentiate progressive from non-Differentiate progressive from non-progressive processesprogressive processes

BUN vs serum creatinineBUN vs serum creatinineBUN vs serum creatinineBUN vs serum creatinine

• Both are relatively insensitive Both are relatively insensitive indicators of renal function (one is indicators of renal function (one is not more sensitive than the other)not more sensitive than the other)

• Serum creatinine is affected by fewer Serum creatinine is affected by fewer non-renal variablesnon-renal variables

• Creatinine is not affected by passive Creatinine is not affected by passive renal tubular reabsorptionrenal tubular reabsorption

BUN/creatinine ratioBUN/creatinine ratioBUN/creatinine ratioBUN/creatinine ratio

• May be increased in pre-renal May be increased in pre-renal azotemia (e.g. dehydration) due to azotemia (e.g. dehydration) due to increased tubular reabsorption of increased tubular reabsorption of urea at slower tubular flow ratesurea at slower tubular flow rates

• May be increased in post-renal May be increased in post-renal azotemia caused by ruptured bladder azotemia caused by ruptured bladder due to easier reabsorption of urea due to easier reabsorption of urea across peritoneal membranesacross peritoneal membranes

Localization of azotemiaLocalization of azotemiaLocalization of azotemiaLocalization of azotemia

• Must consider:Must consider:• HistoryHistory• Physical examination findingsPhysical examination findings• Urine specific gravity before fluids Urine specific gravity before fluids

or drugs that may interfere with or drugs that may interfere with concentrating abilityconcentrating ability• Patient’s response to fluid therapyPatient’s response to fluid therapy

Localization of azotemiaLocalization of azotemiaExample 1Example 1


• HxHx: Persistent vomiting: Persistent vomiting

• PEPE: 10% dehydrated: 10% dehydrated

• LabLab: BUN 70 mg/dL; USG 1.054: BUN 70 mg/dL; USG 1.054

• Response to fluidsResponse to fluids: BUN 20 mg/dL: BUN 20 mg/dL

• ConclusionConclusion: Pre-renal azotemia: Pre-renal azotemia



• HxHx: Weight loss, lethargy, anorexia, : Weight loss, lethargy, anorexia, vomitingvomiting

• PEPE: 10% dehydrated: 10% dehydrated

• LabLab: BUN 175 mg/dL; USG 1.013: BUN 175 mg/dL; USG 1.013

• Response to fluidsResponse to fluids: BUN 75 mg/dL: BUN 75 mg/dL

• ConclusionConclusion: Pre-renal and renal azotemia: Pre-renal and renal azotemia



• HxHx: Lethargy, vomiting: Lethargy, vomiting• PEPE: 10% dehydrated: 10% dehydrated• LabLab: BUN 70 mg/dL; USG 1.013: BUN 70 mg/dL; USG 1.013• Response to fluidsResponse to fluids: BUN 20 mg/dL: BUN 20 mg/dL• ConclusionConclusion: Pre-renal azotemia, : Pre-renal azotemia,

underlying renal diseaseunderlying renal disease

Concept of clearanceConcept of clearanceConcept of clearanceConcept of clearance

• Volume of plasma that would have to Volume of plasma that would have to be filtered by the glomeruli in one be filtered by the glomeruli in one minute to account for the amount of minute to account for the amount of that substance appearing in the urine that substance appearing in the urine each minute under steady state each minute under steady state conditionsconditions

• Volume of plasma that contains the Volume of plasma that contains the amount of the substance excreted in amount of the substance excreted in the urine in one minute under steady the urine in one minute under steady state conditionsstate conditions

Clearance = UClearance = UxxV/PV/PxxClearance = UClearance = UxxV/PV/Pxx

• Where,Where,• UUxx = urine concentration of x = urine concentration of x

(mg/dL)(mg/dL)

• PPxx = plasma concentration of x = plasma concentration of x (mg/dL)(mg/dL)

• V = urine output (mL/min)V = urine output (mL/min)

Relationship of clearance to GFRRelationship of clearance to GFRRelationship of clearance to GFRRelationship of clearance to GFR

• In a steady state, for a substance handled In a steady state, for a substance handled only be the kidneys that is neither only be the kidneys that is neither reabsorbed nor secreted:reabsorbed nor secreted:• Amount filtered = amount excretedAmount filtered = amount excreted• GFR GFR P Pxx = U = Uxx V V• GFR = UGFR = UxxV/PV/Pxx

• Thus, the clearance of a substance that is Thus, the clearance of a substance that is neither reabsorbed nor secreted is equal neither reabsorbed nor secreted is equal to GFRto GFR

Relationship of clearance to GFRRelationship of clearance to GFRRelationship of clearance to GFRRelationship of clearance to GFR

• If X is neither reabsorbed nor If X is neither reabsorbed nor secreted, clearance = GFRsecreted, clearance = GFR

• If X is reabsorbed, clearance < GFRIf X is reabsorbed, clearance < GFR

• If X is secreted, clearance > GFRIf X is secreted, clearance > GFR

Inulin clearanceInulin clearanceInulin clearanceInulin clearance

• Inulin is a polymer of fructose that Inulin is a polymer of fructose that meets all of the criteria for the ideal meets all of the criteria for the ideal substance to measure GFRsubstance to measure GFR

• Inulin clearance is the “gold Inulin clearance is the “gold standard” for GFR determinationstandard” for GFR determination

• Inulin must be continuously infused Inulin must be continuously infused into the animal to achieve a steady into the animal to achieve a steady state concentration in plasmastate concentration in plasma

Creatinine clearanceCreatinine clearanceCreatinine clearanceCreatinine clearance

• Creatinine is produced endogenously at a Creatinine is produced endogenously at a constant rateconstant rate

• It is not metabolizedIt is not metabolized• It is excreted by the kidneys by glomerular It is excreted by the kidneys by glomerular

filtrationfiltration• It is neither reabsorbed nor secreted by the It is neither reabsorbed nor secreted by the

renal tubulesrenal tubules• Creatinine clearance can be used to Creatinine clearance can be used to

estimate GFRestimate GFR

Endogenous creatinine Endogenous creatinine clearanceclearance

Endogenous creatinine Endogenous creatinine clearanceclearance

• Requirements:Requirements:• Accurately timed collection of urineAccurately timed collection of urine• Body weightBody weight• Serum and urine creatinine concentrationsSerum and urine creatinine concentrations

• Normal = 2 to 5 ml/min/kgNormal = 2 to 5 ml/min/kg• Underestimates GFR (compared to inulin Underestimates GFR (compared to inulin

clearance) due to non-creatinine clearance) due to non-creatinine chromagens in blood (Pchromagens in blood (Px x increased)increased)

Exogenous creatinine Exogenous creatinine clearanceclearance

Exogenous creatinine Exogenous creatinine clearanceclearance

• Serum creatinine increased 10-fold by Serum creatinine increased 10-fold by administration of creatinineadministration of creatinine

• Minimizes effect of non-creatinine Minimizes effect of non-creatinine chromagenschromagens

• More closely approximates inulin More closely approximates inulin clearanceclearance

• Technically more difficult than Technically more difficult than endogenous creatinine clearanceendogenous creatinine clearance

Indications for creatinine Indications for creatinine clearanceclearance

Indications for creatinine Indications for creatinine clearanceclearance

• Suspicion of renal Suspicion of renal disease in a non-disease in a non-aoztemic patient aoztemic patient with PU/PDwith PU/PD

Sodium sulfanilateSodium sulfanilateSodium sulfanilateSodium sulfanilate• Excreted solely by glomerular Excreted solely by glomerular

filtrationfiltration• Plasma half-life is an indicator of Plasma half-life is an indicator of

GFRGFR• Administered IV and heparinized Administered IV and heparinized

blood samples collected at 30, 60 and blood samples collected at 30, 60 and 90 min90 min

• Normal values: 30-80 min (depending Normal values: 30-80 min (depending on species)on species)

Sodium sulfanilateSodium sulfanilateSodium sulfanilateSodium sulfanilate

• Advantage: Urine samples not Advantage: Urine samples not requiredrequired

• Disadvantage: No numerical Disadvantage: No numerical value for GFR is obtainedvalue for GFR is obtained

Seldom used in clinical evaluation of renal function in domestic animals

RadioisotopesRadioisotopesRadioisotopesRadioisotopes• Used to determine glomerular filtration, Used to determine glomerular filtration,

renal plasma flow, and filtration fraction renal plasma flow, and filtration fraction in domestic animalsin domestic animals

• AdvantagesAdvantages• Do not require collection of urineDo not require collection of urine

• Not time consumingNot time consuming

• DisadvantagesDisadvantages• Use of radioactivityUse of radioactivity

• Require special equipment and expertiseRequire special equipment and expertise

RadioisotopesRadioisotopesRadioisotopesRadioisotopes

• Glomerular filtration rateGlomerular filtration rate• 125125I-iothalamateI-iothalamate• 5151Cr-ethylenediaminetetraacetic acid Cr-ethylenediaminetetraacetic acid

(EDTA)(EDTA)• 99m99mTc-diethylenetriaminepentaacetic Tc-diethylenetriaminepentaacetic

acid (DTPA)acid (DTPA)

• Renal plasma flowRenal plasma flow• 131131I-iodohippurateI-iodohippurate• 33H-tetraethylammonium bromideH-tetraethylammonium bromide

Documents

Clinical Evaluation of Glomerular Filtration S.P. DiBartola, DVM D.J. Chew, DVM