Clinical Evaluation of GFR Flashcards
What does Azotemia mean to the practitioner?
High BUN and High Creatinine
What are the 3 types of Azotemia?
Pre-renal
Renal
Post-renal
Decreased renal perfusion (dehydration, shock, Congestive Heart Failure)
Pre-Renal
Secondary to intrinsic renal parenchymal disease
Renal
a kidney disorder where numerous cysts enlarge in both kidneys reducing the amount of normal kidney tissue and thereby the kidney’s ability to function normally, leading to kidney failure
Renal Parenchymal Disease (PKD)
Impaired elimination from the body (obstruction, ruptured bladder)
Post-Renal
Clinical and biological abnormalities that accompany a critical loss of functioning nephrons
Uremia
How do we measure kidney function?
Clearance
The volume of plasma that would have to be filtered by the glomeruli in 1 min. to produce a certain amount in the urine each min.
Clearance
Formula for Clearance
C = U V/P
Ux = Urine concentration of x (mg/dl)
Px = Plasma concentration of x (mg/dl)
V = Urine output (ml/min)
What if x is resorbed by the tubules?
Cx < GFR, because ↓Ux and ↑Px
What if x is secreted by the tubules?
Cx > GFR, because ↑Ux and ↓Px
How are GFR and Clearance Related?
Creatinine clearance measurements will consistently be 10 to 20 percent higher than GFR in patients with a normal GFR and progressively higher as the GFR falls
When do we actually need to calculate GFR?
- Patient who is PU/PD WITHOUT azotemia
- Patient who may have another disease which causes PU/PD
(hyperadrenocorticism, hyperthyroidism, diabetes mellitus, etc.) - Patient that may need a nephrectomy (need to know if other kidney
is functioning)
What substance (x) do we use?
- Freely filtered at glomerulus
- Not plasma protein bound
- Not metabolized
- Non-toxic
- Excreted only by kidneys
- Neither reabsorbed nor secreted by the renal tubules
- Stable in blood and urine and easily measured
Under these circumstances
CLEARANCE = GFR
Which substance meets all criteria for a good substance to measure Clearance so it = GFR, but difficult to use because must be infused IV over a period of time with constant urine collection?
Inulin
(Gold-standard for GFR determination)
Uses patients’ naturally made creatinine
- 24h urine collection (challenge in vet med)
- Slight underestimate of GFR
Endogenous - Creatinine Clearance Method
- Creatinine given SQ or IV in a high dose and 24h urine collection
- Very good estimate of GFR
Exogenous - Creatinine Clearance Method
- Iodinated water-soluble contrast agent
- Non-toxic, EXTRAcellular only, little protein binding and removed only by the kidneys
- Newer and simpler way to estimate GFR in clinical setting
- Give a specific dose IV
Iohexol Clearance
What is a problem with Inulin, Creatinine Clearance Method, and Iohexol Clearance?
They measure GLOBAl GFR, Not individual kidney GFR
- Imaging method to determine individual kidney GFR, effective
renal plasma flow, and filtration fraction - Give the isotope 99mTc-DTPA and image kidneys with a Gammacamera
Radioisotopes
What do we use in ‘real life’ (regular practice) to estimate the GFR?
BUN and Creatinine
Product of protein and aa catabolism
BUN
Made in the LIVER with ammonia from aa’s
Blood Urea Nitrogen (BUN)
Can be made from endogenous or exogenous proteins
Blood Urea Nitrogen (BUN)
Freely permeable across cell membranes, distributed in Total body water
BUN
- Renal excretion most important
- Filtered at glomerulus
- Passively reabsorbed in tubules (most of it)
* Less absorbed with rapid flow
* More absorbed with slower flow rates
BUN
BUN is NOT secreted by ______________
renal tubules
- Varies with protein intake and catabolism
- Blood level of urea nitrogen reflect renal excretion of urea and protein catabolism
- Requires no metabolic work by kidneys
- Some degraded by bacteria
BUN
- High protein meal
- GI bleeding
- Increased catabolism
- Drugs
Are all examples of what?
Non-Renal Factors that INCREASE BUN
- Pre-renal – slow tubular flow rates = more reabsorption
- Renal
- Post-renal
Renal Factors that INCREASE BUN
- May reflect high tubular flow rates (lots of urine made) with decreased urea resorption
- Low protein diet
- Liver dysfunction or portosystemic shunt
*Urea not made - Anabolic steroids - slow protein catabolism
Are examples of what?
Decreased BUN
- Product of muscle breakdown (phosphocreatine)
- Produced at a relatively constant rate for an individual
Creatinine
Creatinine
- Rate of production determined by age, sex, and muscle mass
- Young animals have a ___________ creatinine
lower
Creatinine
- Rate of production determined by age, sex, and muscle mass
- Males have a slightly ___________ creatinine
higher
Creatinine
- Rate of production determined by age, sex, and muscle mass
- Well-muscled animals have a __________ creatinine than poorly
muscled ones (e.g. Greyhounds)
higher
Creatinine
- Rate of production determined by age, sex, and muscle mass
- ________ (is/is not) influenced by diet
is not
Creatinine
- Rate of production determined by age, sex, and muscle mass
- _________ (is/is not) metabolized in the body
is not
Non-Renal: uncommon
* Massive muscle necrosis
* Prolonged strenuous exercise
* Greyhound Norma
Increased Creatinine
Renal: Important
* Pre-renal – decreased renal perfusion to kidneys (dehydration, shock)
* Primary renal – can’t filter (not enough functional nephrons to do the job)
* Post-renal – can’t eliminate (obstruction of ureters, urethra, or ruptured urinary tract)
Increased Creatinine
- Muscle loss – only one that is really significant (think ancient skinny cats)
- Small size
- Young age
Decreased Creatinine
After eliminating pre-renal and post-renal causes…
Increased BUN and Creatinine indicate that at least 75% of
____________ are NOT functioning
nephrons
Pre-renal azotemia is usually ________
Mild
Post-renal azotemia can be __________
severe
What is it that you CAN’T tell from BUN and Creatinine?
- Cause of azotemia
- Reversibility of azotemia
- Origin of azotemia (pre-renal, renal, post-renal)
- Acute vs. chronic disease
- Progressive vs. non-progressive disease
- Made by all cells and excreted almost exclusively by the KIDNEYS
- Increases in the serum earlier than creatinine
*EARLIER detection of kidney disease - Not affected by muscle mass
SDMA (IDEXX)
Symmetric dimethylarginine
When do you get a Urine Specific Gravity?
BEFORE fluids or diuretics are given
Localization of Azotemia:
*If the animal can concentrate its urine, its NOT ____________ in origin
renal
Localization of Azotemia:
*If it concentrates but not as much as it should, it is partially ____________ in
origin
renal
Localization of Azotemia:
*If the animal ________ (can/cannot) concentrate its urine, some or all of it is renal in origin (unless drug interference)
cannot
The BUN is increased for which types of Azotemia?
- Prerenal
- Renal
- Postrenal
The Creatinine is increased for which types of Azotemia?
- Prerenal
- Renal
- Postrenal
What type of Azotemia is the following?
- Increased BUN & Creatinine
- USG > 1.030 (concentrated)
- BUN, Creatinine is better after IV fluids
- Dehydrated
- able to urinate
- no abdominal fluid
- no large firm bladder
Pre-Renal
What type of Azotemia is the following?
- Increased BUN & Creatinine
- USG < 1.030 (not very concentrated)
- BUN & Creatinine DOES NOT get better after fluids
- Hydrated/Dehydrated (variable)
- Able to urinate unless anuric
- no abdominal fluid
- no large firm bladder
Renal
What type of Azotemia is the following?
- Increased BUN & Creatinine
- USG is Variable
- BUN & Creatinine DOES NOT get better after fluids
- Hydrated/Dehydrated (variable)
- NOT able to urinate
- Maybe abdominal fluid
- Maybe large firm bladder
Post-Renal
normal level of creatinine in cats is:
0.6 and 2.4 mg/dL
What are some causes for PU/PD?
- Kidney Disease
- Hypercalcemia
- Hyperadrenocorticism
What MUST be done before moving to a water deprivation test in evaluating PU/PD?
A first morning urine sample specific gravity
