Tests of Renal Function Flashcards
For any solute (urea, Na), what does the concentration depend on?
Depends on the amount of the solute present and the volume of solvent (water) in which it is dispersed.
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)?
a) Conc will rise
b) Conc will fall
c) Conc will rise
d) Conc will fall
e) Conc will rise
How is the kidney involved in the regulation of RBCs?
Produced the hormone erythropoietin –> causes bone marrow to produce more RBCs
How is the kidney involved in the regulation of blood pressure?
Secretes renin –> activates RAAS
How is the kidney involved in vitamin D metabolism?
The kidneys convert vitamin D from supplements or the sun to the active form of vitamin D that is needed by the body.
It is important to establish where abouts in the pathway renal damage has occurred. What are the 3 locations?
- Pre-renal
- Renal-renal
- Post-renal
What is oliguria?
Lessened urine output: <400 mL/24 h
What is anuria?
No urine output: <100 mL/24 h
What is polyuria?
Too much urine output: >3000 mL/24 h
What is the healthy urine volume range?
750-2000 mL/24 h
What do serum/plasma urea concentration reflect the balance between?
Reflects the balance between urea production in the liver and urea elimination by the kidneys in the urine
What is urea?
Urea is the principal nitrogenous waste product of metabolism and is generated from protein breakdown
What is the purpose of the urea cycle?
This converts highly toxic ammonia to urea for excretion and takes place primarily in the liver and kidneys.
Describe the urea cycle
- Amino acid catabolism results in waste ammonia
- Ammonia converted to urea by the liver
- Urea then released into bloodstream and travels to kidneys. Here, it is filtered at glomerulus
- Is ultimately excreted in urine
If the rate of tubular flow is slow, how does this affect urea reabsorption?
More urea is reabsorbed - higher plasma levels
What causes a slow tubular flow rate?
Renal hypoperfusion
What % of filtered urea is reabsorbed by renal tubules in health?
40%
What renal factors can lead to increased plasma urea concentration?
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
What non-renal factors can lead to increased plasma urea concentration?
High protein intake increases urea production
GI bleed (blood in the gut is effectively a high-protein meal)
Trauma (tissue protein)
What factors can lead to decreased plasma urea concentration?
o Low protein diet
o Pregnancy
o Advanced liver disease
Urea is a sensitive but non-specific indicator of illness. Why?
This is because many things can affect plasma urea, not just renal conditions.
What is creatinine?
Creatine is a waste product in your blood that comes from muscle activity. It’s normally removed from blood by the kidneys
How is creatinine affected in chronic renal disease?
When kidney function slows down, the creatinine plasma level rises:
o Increases in concentration as GFR decreases
o NOT proportional to renal damage
Why is change of creatinine plasma levels within an individual patient usually more important than the absolute value?
As GFR decreases (with renal failure), the plasma creatine increases exponentially (plasma creatine may still be in normal range long into renal failure).
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?
plotting the reciprocal serum creatinine versus time
What is eGFR? What does it require?
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
Which drugs can inhibit tubular secretion?
Salicylate, cimetidine
Why is GFR difficult to measure?
Requires 24 hour urine collection
As chronic renal disease progresses, the creatine clearance changes.
How does it change?
Decreases
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
a) none
b) increased creatinine, increased urea
c) increased potassium,
decreased bicarbonate
d) increased phosphate,
increased uric acid
What GFR would indicate the need for dialysis?
<15
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
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)
What can cause pre-renal oliguria?
o Dehydration - sodium / water
o Haemorrhage
o Renal artery damage
o Hypotension
Treatment for pre-renal oliguria?
IV fluid with saline to replenish fluids
What renal problems could a pale colour of a patient indicate?
Anaemia –> intrinsic renal failure leads to lack of EPO
What is intrinsic renal failure?
Intrinsic renal failure is caused by structural changes within the kidneys
What is prerenal failure typically due to?
Due to decreased effective blood volume or heart failure from such conditions as dehydration or shock
What is postrenal failure typically due to?
Structural abnormalities in the ureters, bladder, or urethra
In intrinsic renal failure, how would a patient typically present?
Increasing lassitude Shortness of breath Swelling of ankles Pale Hypertensive
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
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
What are the causes of renal-renal oliguria?
Intrinsic damage caused by: Hypertension Tubular necrosis Chronic infection Immunological damage - SLE Toxic damage - drugs, heavy metals (Hg, Ur), poisons (paraquat) Diabetes
