Lecture 10 9/24/24 Flashcards
What are the factors influencing renal excretory function?
-glomerular filtration rate
-tubular secretion
-tubular resorption
Why are glomerular filtration, tubular secretion, and tubular resorption important?
they govern what is removed from vs. retained in the plasma and what ends up being excreted in the urine
What is azotemia?
increased plasma conc. of non-protein nitrogenous waste products
What are the characteristics of urea/BUN and creatinine?
-part of the routine biochemical profile
-an increase in both is classified as azotemia
-should be interpreted along with USG for max. diagnostic value
-insensitive biomarkers of decreased GFR
What is urea?
by-product made from ammonium and bicarbonate in the liver
What is creatinine?
by-product made from creatinine phosphate in skeletal muscle
What are the characteristics of SDMA?
-biomarker of decreased GFR
-interpretation analogous to urea and creatinine; increased SDMA = decreased GFR
-may be more sensitive for detecting GFR compared to creatinine
-less impacted by extra-renal factors
What are potential causes of increased urea and creatinine?
-hypovolemia
-shock
-renal disease/injury
-urinary tract obstruction/rupture
What are potential causes of only increased urea?
-high-protein meal or diet
-GI hemorrhage
What are potential causes of only increased creatinine?
-interferents
-large muscle mass
What are potential causes of decreased urea and creatinine?
-over-drinking
-aggressive fluid therapy
What are potential causes of only decreased urea?
-increased renal excretion
-low-protein diet
-decreased functional hepatic mass
-GI utilization of urea
What are potential causes of only decreased creatinine?
-muscle wasting/cachexia
What are the characteristics of USG?
-measured using refractometer
-reflects soluble substances
-no reference interval; interpretation depends on hydration status
-should be interpreted before giving fluids
-can be influenced by extra-renal factors impacting renal tubular function
Why is USG a good measure of urine osmolality?
the relationship between the two is mostly linear
Which conditions can mildly and falsely increase USG?
-proteinuria
-glucosuria
What are the USG values for fully concentrated urine?
dogs: > 1.030
cats: > 1.040
large animal: > 1.025
What are the USG values for hyposthenuric urine?
< 1.007
What are the USG values for isosthenuric urine?
1.008-1.012
What does fully concentrated urine indicate about the glomerular filtrate?
it was concentrated due to the body conserving water
What does hyposthenuric urine indicate about the glomerular filtrate?
it was diluted due to the body getting rid of water
What does isosthenuric urine indicate about the glomerular filtrate?
it was not altered from the osmolality of plasma
What are the expected urine outputs associated with each type of urine concentration?
-fully concentrated: minimal output that is physiologically appropriate
-hyposthenuric: lots of output that can be physiologic or pathologic
-isosthenuric: minimal to excessive output depending on functional nephron mass
What are the characteristics of azotemia classification?
-pre-renal, renal, and post-renal categories
-has prognostic implications
-pre-renal azotemia is relatively benign
-renal and post-renal azotemia are treated very differently
-different types are NOT mutually exclusive; can have 2 or all 3
What are the lab findings associated with pre-renal azotemia?
azotemia and fully concentrated urine
What are the lab findings associated with renal azotemia?
azotemia and isosthenuric urine
What are the lab findings associated with post-renal azotemia?
azotemia and variable USG
What are the mechanisms of primary polyuria?
-renal tubular dysfunction
-decreased renal medullary tonicity
-osmotic diuresis
-ADH issue
Why is it difficult to distinguish pre-renal azotemia complicated by an extra-renal cause of impaired concentrating ability?
it can mimic renal azotemia and renal disease/injury
What should the approach be with patients with a partly concentrated USG?
review history and medications; always look for extra-renal factors first
What are the limitations of using urea, creatinine, and USG to diagnose renal disease/injury?
-polyuria does not occur until 67% of nephrons are lost, making low USG insensitive
-low USG is non-specific
-azotemia does not occur until 75% of nephrons are lost, making it insensitive
-polyuria typically happens before azotemia
What other information is needed to definitively dx AKI or CKD?
-history
-physical exam
-urinary tract imaging
-other lab data
What are the characteristics of FGF23 testing in cats?
-secreted by osteocytes and osteoblasts
-major regulator of phosphate metabolism
-early biomarker of mineral derangements in CKD
-helps guide need for dietary phosphate restriction
What are the external causes of impaired urine concentrating ability and polyuria?
-diuretic use
-hyponatremia; Addison’s or other causes
-low urea: liver insufficiency or lack of protein
-uncontrolled/poorly controlled diabetes mellitus
-sugar administration
-nephrogenic diabetes insipidus (dec. ADH response)
-central diabetes insipidus (dec. ADH production)
What are the potential causes of acquired nephrogenic diabetes insipidus?
-endotoxemia
-hypercalcemia
-hypokalemia
-possible corticosteroid use
-alcohol/caffeine consumption (humans)
What are the potential causes of acquired central diabetes insipidus?
-brain disease
-certain anticonvulsant drugs
-corticosteroid use
