Tests of Renal Function

Question 1

For any solute (urea, Na), what does the concentration depend on?

Answer 1

Depends on the amount of the solute present and the volume of solvent (water) in which it is dispersed.

Question 2

Describe the concentration of solutes if:

a) If the input increases and the excretion stays constant?
b) If the input decreases and the excretion stays constant?
c) If the excretion decreases and the input stays constant?
d) If the volume increases (water overload)?
e) If the volume decreases (water deprivation)?

Answer 2

a) Conc will rise
b) Conc will fall
c) Conc will rise
d) Conc will fall
e) Conc will rise

Question 3

How is the kidney involved in the regulation of RBCs?

Answer 3

Produced the hormone erythropoietin –> causes bone marrow to produce more RBCs

Question 4

How is the kidney involved in the regulation of blood pressure?

Answer 4

Secretes renin –> activates RAAS

Question 5

How is the kidney involved in vitamin D metabolism?

Answer 5

The kidneys convert vitamin D from supplements or the sun to the active form of vitamin D that is needed by the body.

Question 6

It is important to establish where abouts in the pathway renal damage has occurred. What are the 3 locations?

Answer 6

1. Pre-renal
2. Renal-renal
3. Post-renal

Question 7

What is oliguria?

Answer 7

Lessened urine output: <400 mL/24 h

Question 8

What is anuria?

Answer 8

No urine output: <100 mL/24 h

Question 9

What is polyuria?

Answer 9

Too much urine output: >3000 mL/24 h

Question 10

What is the healthy urine volume range?

Answer 10

750-2000 mL/24 h

Question 11

What do serum/plasma urea concentration reflect the balance between?

Answer 11

Reflects the balance between urea production in the liver and urea elimination by the kidneys in the urine

Question 12

What is urea?

Answer 12

Urea is the principal nitrogenous waste product of metabolism and is generated from protein breakdown

Question 13

What is the purpose of the urea cycle?

Answer 13

This converts highly toxic ammonia to urea for excretion and takes place primarily in the liver and kidneys.

Question 14

Describe the urea cycle

Answer 14

1. Amino acid catabolism results in waste ammonia
2. Ammonia converted to urea by the liver
3. Urea then released into bloodstream and travels to kidneys. Here, it is filtered at glomerulus
4. Is ultimately excreted in urine

Question 15

If the rate of tubular flow is slow, how does this affect urea reabsorption?

Answer 15

More urea is reabsorbed - higher plasma levels

Question 16

What causes a slow tubular flow rate?

Answer 16

Renal hypoperfusion

Question 17

What % of filtered urea is reabsorbed by renal tubules in health?

Answer 17

40%

Question 18

What renal factors can lead to increased plasma urea concentration?

Answer 18

 Increased urea production
 Decreased urea elimination; marker of advanced renal disease (along with associated marked reduction in GFR)
 Renal hypoperfusion; decreased RBF, decreased ECFV
 Acute renal impairment
 Chronic renal disease
 Post-renal obstruction calculus tumour

Question 19

What non-renal factors can lead to increased plasma urea concentration?

Answer 19

 High protein intake increases urea production
 GI bleed (blood in the gut is effectively a high-protein meal)
 Trauma (tissue protein)

Question 20

What factors can lead to decreased plasma urea concentration?

Answer 20

o Low protein diet
o Pregnancy
o Advanced liver disease

Question 21

Urea is a sensitive but non-specific indicator of illness. Why?

Answer 21

This is because many things can affect plasma urea, not just renal conditions.

Question 22

What is creatinine?

Answer 22

Creatine is a waste product in your blood that comes from muscle activity. It’s normally removed from blood by the kidneys

Question 23

How is creatinine affected in chronic renal disease?

Answer 23

When kidney function slows down, the creatinine plasma level rises:
o Increases in concentration as GFR decreases
o NOT proportional to renal damage

Question 24

Why is change of creatinine plasma levels within an individual patient usually more important than the absolute value?

Answer 24

As GFR decreases (with renal failure), the plasma creatine increases exponentially (plasma creatine may still be in normal range long into renal failure).

Question 25

In patients who have a linear decline in renal function over time, what method is useful in monitoring renal disease progression and predicting the start time of dialysis?

Answer 25

plotting the reciprocal serum creatinine versus time

Question 26

What is eGFR? What does it require?

Answer 26

eGFR is the best test to measure your level of kidney function and determine your stage of kidney disease.

It requires a) serum creatinine b) age c) body size d) gender e) race

Question 27

Which drugs can inhibit tubular secretion?

Answer 27

Salicylate, cimetidine

Question 28

Why is GFR difficult to measure?

Answer 28

Requires 24 hour urine collection

Question 29

As chronic renal disease progresses, the creatine clearance changes.

How does it change?

Answer 29

Decreases

Question 30

How does the plasma change when the creatinine clearance is:

a) 60-120 mL/min
b) 30-60 mL/min
c) 20-30 mL/min
d) 10-20 mL/min

Answer 30

a) none
b) increased creatinine, increased urea

c) increased potassium,
decreased bicarbonate

d) increased phosphate,
increased uric acid

Question 31

What GFR would indicate the need for dialysis?

Answer 31

<15

Question 32

In the case of pre-renal oliguria, how would you expect the:

a) urine volume
b) urine conc
c) plasma values
d) urine values

to be?

Explain each answer

Answer 32

a) Urine volume expected to be low
- ADH increased
- GFR reduced

b) Urine concentration expected to be high
- ADH increased

c) Elevated plasma values of Na/urea/creatinine
- ADH increased

d) Low urea and Na urine values
- Due to Na reabsorption

Renal hypoperfusion causes renin secretion so functioning nephrons increase sodium reabsorption (aldosterone)

Question 33

What can cause pre-renal oliguria?

Answer 33

o Dehydration - sodium / water
o Haemorrhage
o Renal artery damage
o Hypotension

Question 34

Treatment for pre-renal oliguria?

Answer 34

IV fluid with saline to replenish fluids

Question 35

What renal problems could a pale colour of a patient indicate?

Answer 35

Anaemia –> intrinsic renal failure leads to lack of EPO

Question 36

What is intrinsic renal failure?

Answer 36

Intrinsic renal failure is caused by structural changes within the kidneys

Question 37

What is prerenal failure typically due to?

Answer 37

Due to decreased effective blood volume or heart failure from such conditions as dehydration or shock

Question 38

What is postrenal failure typically due to?

Answer 38

Structural abnormalities in the ureters, bladder, or urethra

Question 39

In intrinsic renal failure, how would a patient typically present?

Answer 39

Increasing lassitude
Shortness of breath
Swelling of ankles
Pale
Hypertensive

Question 40

In the case of renal-renal oliguria, how would you expect the:

a) urine volume
b) urine conc
c) plasma values
d) urine values

to be?

Explain each answer

Answer 40

a) Urine volume expected to be decreased
b) Urine concentration expected to be less due to loss of renal function
c) Plasma values: Urea and creatinine expected to be high due to reduced ability of kidney to remove them from blood
d) High urine urea and Na due to inability of kidney to reabsorb

• GFR reduced / normal
• Weak urine / low volume
• Renal renin secretion may be raised:
o	Hypertension
o	but nephrons unable to reabsorb sodium
o	Urine sodium concentration is > 40mmol/L

Question 41

What are the causes of renal-renal oliguria?

Answer 41

Intrinsic damage caused by:
 Hypertension
 Tubular necrosis
 Chronic infection
 Immunological damage - SLE
 Toxic damage - drugs, heavy metals (Hg, Ur), poisons (paraquat)
 Diabetes