Lecture 10 9/24/24

Question 1

What are the factors influencing renal excretory function?

Answer 1

-glomerular filtration rate
-tubular secretion
-tubular resorption

Question 2

Why are glomerular filtration, tubular secretion, and tubular resorption important?

Answer 2

they govern what is removed from vs. retained in the plasma and what ends up being excreted in the urine

Question 3

What is azotemia?

Answer 3

increased plasma conc. of non-protein nitrogenous waste products

Question 4

What are the characteristics of urea/BUN and creatinine?

Answer 4

-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

Question 5

What is urea?

Answer 5

by-product made from ammonium and bicarbonate in the liver

Question 6

What is creatinine?

Answer 6

by-product made from creatinine phosphate in skeletal muscle

Question 7

What are the characteristics of SDMA?

Answer 7

-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

Question 8

What are potential causes of increased urea and creatinine?

Answer 8

-hypovolemia
-shock
-renal disease/injury
-urinary tract obstruction/rupture

Question 9

What are potential causes of only increased urea?

Answer 9

-high-protein meal or diet
-GI hemorrhage

Question 10

What are potential causes of only increased creatinine?

Answer 10

-interferents
-large muscle mass

Question 11

What are potential causes of decreased urea and creatinine?

Answer 11

-over-drinking
-aggressive fluid therapy

Question 12

What are potential causes of only decreased urea?

Answer 12

-increased renal excretion
-low-protein diet
-decreased functional hepatic mass
-GI utilization of urea

Question 13

What are potential causes of only decreased creatinine?

Answer 13

-muscle wasting/cachexia

Question 14

What are the characteristics of USG?

Answer 14

-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

Question 15

Why is USG a good measure of urine osmolality?

Answer 15

the relationship between the two is mostly linear

Question 16

Which conditions can mildly and falsely increase USG?

Answer 16

-proteinuria
-glucosuria

Question 17

What are the USG values for fully concentrated urine?

Answer 17

dogs: > 1.030
cats: > 1.040
large animal: > 1.025

Question 18

What are the USG values for hyposthenuric urine?

Answer 18

< 1.007

Question 19

What are the USG values for isosthenuric urine?

Answer 19

1.008-1.012

Question 20

What does fully concentrated urine indicate about the glomerular filtrate?

Answer 20

it was concentrated due to the body conserving water

Question 21

What does hyposthenuric urine indicate about the glomerular filtrate?

Answer 21

it was diluted due to the body getting rid of water

Question 22

What does isosthenuric urine indicate about the glomerular filtrate?

Answer 22

it was not altered from the osmolality of plasma

Question 23

What are the expected urine outputs associated with each type of urine concentration?

Answer 23

-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

Question 24

What are the characteristics of azotemia classification?

Answer 24

-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

Question 25

What are the lab findings associated with pre-renal azotemia?

Answer 25

azotemia and fully concentrated urine

Question 26

What are the lab findings associated with renal azotemia?

Answer 26

azotemia and isosthenuric urine

Question 27

What are the lab findings associated with post-renal azotemia?

Answer 27

azotemia and variable USG

Question 28

What are the mechanisms of primary polyuria?

Answer 28

-renal tubular dysfunction
-decreased renal medullary tonicity
-osmotic diuresis
-ADH issue

Question 29

Why is it difficult to distinguish pre-renal azotemia complicated by an extra-renal cause of impaired concentrating ability?

Answer 29

it can mimic renal azotemia and renal disease/injury

Question 30

What should the approach be with patients with a partly concentrated USG?

Answer 30

review history and medications; always look for extra-renal factors first

Question 31

What are the limitations of using urea, creatinine, and USG to diagnose renal disease/injury?

Answer 31

-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

Question 32

What other information is needed to definitively dx AKI or CKD?

Answer 32

-history
-physical exam
-urinary tract imaging
-other lab data

Question 33

What are the characteristics of FGF23 testing in cats?

Answer 33

-secreted by osteocytes and osteoblasts
-major regulator of phosphate metabolism
-early biomarker of mineral derangements in CKD
-helps guide need for dietary phosphate restriction

Question 34

What are the external causes of impaired urine concentrating ability and polyuria?

Answer 34

-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)

Question 35

What are the potential causes of acquired nephrogenic diabetes insipidus?

Answer 35

-endotoxemia
-hypercalcemia
-hypokalemia
-possible corticosteroid use
-alcohol/caffeine consumption (humans)

Question 36

What are the potential causes of acquired central diabetes insipidus?

Answer 36

-brain disease
-certain anticonvulsant drugs
-corticosteroid use