Lecture 5: Glomerular Filtration Surrogates: BUN, Creatinine, SDMA Flashcards
Endogenous
Produced by body, help to measure body function
GFR
Measure of renal function, particularly functional renal mass
GFR will change
Before routine blood chemistry changes
GFR reference range
2.5 - 5 mL/min/kg BW
Markers used for measuring GFR requirements
- Must be freely filtered
- Cannot be reabsorbed or secreted by renal tubule
- Can’t alter GFR
- Must be excreted only by kidneys, not metabolized
What is the ideal GFR marker
Inulin
Inulin type of marker
Fructose polymer
Inulin is ________ and does not bind to _________
Freely filtered; plasma proteins
Amount of inulin _____ is equal to the amount of inulin _____
Filtered; excreted
Clearance of inulin =
GFR
Filtration fraction
Expresses relationship between GFR and RPF, fraction of RPF filtered across glomerular capillaries in one pass
Filtration fraction units
No units, can be % or decimal
What is the typical filtration fraction of domestic animals?
10-40%
What happens to the RPF that is not filtered?
Goes to peritubular capillaries
Creatinine
Product of spontaneous, non-enzymatic breakdown of phosphocreatinine in muscle
What action(s) do the kidneys take on creatinine?
Filtered and secreted only
What rate is creatinine released into blood?
Relatively constant rate
What causes the release of creatinine into the blood?
Muscle
What % of functional nephrons are lost by the time creatinine increases above reference range?
75%
What hides loss of GFR with creatinine
Loss of muscle mass
In what patients will creatinine not leave reference range even when GFR severely impaired?
Patients with low muscle mass
Loss of muscle mass =
Hiding loss of kidney function
What is normal serum creatinine in dogs?
0.3 - 1.2 mg/dl
Blood serum/urea nitrogen (BUN)
Synthesized in liver using ammonia (urea is nontoxic)
Ammonia
By-product of amino acid metabolism and is toxic
Rate at which BUN is produced
Not produced at constant rate
Production of BUN can vary according
- Dietary protein uptake
- Measuring during fasting or just after eating
Normal BUN value in dog
8-25 mg/dl
BUN:creatinine ratio, what does it mean when it increases?
BUN has increased (since in numerator)
What happens to BUN:creatinine ratio in renal failure?
No change in ratio, both BUN and creatinine increase because GFR decreases so BUN + creatinine not removed from blood
What happens to BUN:creatinine ratio in dehydration?
Increased BUN:creatinine ratio (increased BUN), since urea reabsorption increases to help with water reabsorption
Urea reabsorption increases to help with
Water absorption
Hypovolemia occurs due to
Shock or hemorrhage and reduced renal perfusions
What is the kidney’s response/action to hypovolemia?
Increase renal absorption of urea and decreases urea excretion, which increases plasma urea, which leads to pre-renal azotemia
Increase in plasma urea leads to
Pre-renal azotemia
Pre-renal azotemia
Abnormally high nitrogen compounds
BUN:creatinine response to hypovolemia and pre-renal azotemia
Increased, because urea reabsorption to help with water reabsorption
SDMA can be indicative of
Renal disease
If SDMA increased and nothing else..
Have owner come back in 6 months, need more thsn one data point
SDMA shows promise as a ______ for kidney function and is specific for _________
Biomarker; kidney function
SDMA increases earlier than what as kidney function decreases
Creatinine
SDMA diagnoses occur at what % loss of kidney function
40%
How is SDMA effected by muscle mass?
Not
When diagnosing kidney disease do not
Use a single marker in isolation
Best practices for assessing kidney function in patients:
Consider history
Do thorough physical examination
Use urinary tract imaging
Do a urinalysis, especially urine specific gravity
Assess BUN, creatinine, and SDMA together
