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Clinical Pathology > Theme 4 Lecture 3: Laboratory tests of renal function > Flashcards

Theme 4 Lecture 3: Laboratory tests of renal function Flashcards

1
Q

What are the 3 main functions of the kidney?

A
  1. excretion e.g urea, uric acid
  2. regulation e.g homeostasis, water, acid-base
  3. endocrine e.g renin, erythropoietin, vitD metabolism
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2
Q

What are the 3 main steps that describe how a kidney functions?

A
  1. arterial input
  2. filter (glomerulus)
  3. venous output or urine output
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3
Q

What are the 3 types of renal impairment?

A
  1. pre-renal
  2. renal
  3. post-renal
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4
Q

What is the cause of pre-renal kidney failure?

A

decreased ECFV (extra cellular fluid volume) or MI - low BP means not enough arterial input into the kidney to drive filtrate through glomerulus

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5
Q

What is the cause of renal kidney failure?

A

acute tubular necrosis

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6
Q

What is the cause of post renal kidney failure?

A

ureteral obstruction

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7
Q

What are the lab tests of renal function?

A
  • glomerular filtration rate - impractical
  • eGRF
  • creatinine clearance - unreliable
  • plasma urea
  • urine volume
  • urine urea
  • urine sodium
  • urine protein
  • urine glucose
  • haematuria
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8
Q

What is the urine volume range typical in health?

A

750-2000 mL/24 hr

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9
Q

Define oliguria

A

less than 400mL/24 hr which is the minimum amount of urine produced to ensure enough urine is removed to prevent toxicity build up

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10
Q

Define anuria

A

<100 mL/24 hr

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11
Q

Define polyuria

A

> 3000 mL/hr

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12
Q

What is the reference range of plasma urea?

A

3-8 mmol/L

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13
Q

which factors affect plasma urea concentration?

A
  • GIT protein
  • tissue protein
  • liver amino acids
  • levels of reabsorption and excretion in kidney
  • filtration in kidney
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14
Q

How is urea produced?

A
  • consumption of protein, excess amino acids are deaminated in the liver which produces urea
  • produced in the breakdown of tissue protein
  • greater protein intake = more urea
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15
Q

How is urea excreted

A
  • filtered at glomerulus
  • about 40% filtered urea is reabsorbed by renal tubules in health
  • more urea is reabsorbed if rate of tubular flow is slow
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16
Q

When is tubular flow rate slow?

A

renal hypoperfusion

17
Q

What are the causes of increased plasma urea?

A
  • GI bleed
  • trauma
  • renal hypoperfusion
  • acute renal impairment
  • chronic renal disease
  • post-renal obstruction calculus tumour
18
Q

What is the usual range of plasma creatinine?

A

50-140 umol/L

19
Q

What is the relationship between plasma creatinine and GFR?

A

Plasma creatinine increases in concentration as GFR decreases

20
Q

Why is GFR not used in clinical practise?

A

difficult to perform clinically

21
Q

How is GFR measured in clinical practise?

A

clearance of [99Tc]-Sn-DTPA

22
Q

How do we calculate creatinine clearance?

A

Ccreat = (Ucreat x V) / Pcreate

Ucreat = urine creatinine concentration (mmol/L)
V = urine volume (mL collected in 24 hours)
Pcreat = plasma creatinine concentration (umol/L)

This works out how much volume of plasma has had to go through kidney to extract all the creatinine - the rate of clearance of creatinine from volume of plasma

23
Q

as creatinine clearance decreases, what plasma changes do we see?

A
  • increased uric acid
  • increased phosphate
  • decreased bicarbonate
  • increased potassium
  • increased urea
  • increased creatinine
24
Q

What is the limitation of eGFR?

A

dosen’t take into account patients weight

25
Q

What clinical decisions can be made from eGFR?

A

Stage 1 - normal but urine or structural abnormalities
Stage 2 - mildly reduced kidney functions
Stage 3 - moderately reduced kidney function
Stage 4 - severely reduced kidney function
Stage 5 - end stage kidney failure. dialysis/ transplant

26
Q

Explain the features of pre-renal oliguria?

A
  • GFR reduced
  • ADH increased
  • concentrated urine/ low volume
  • renal hypoperfusion causes to renin secretion
  • functioning nephrons increase sodium reabsorption (aldosterone)
  • urine sodium concentration is low
27
Q

What are the causes of pre-renal oliguria?

A
  • dehydration - sodium/water
  • haemorrhage
  • renal artery damage
  • hypotension
28
Q

What are the features of renal oliguria?

A
  • GFR reduced/ normal
  • weak urine/ low volume
  • renal renin secretion may be raised
  • hypertension
  • but nephrons unable to reabsorb sodium
  • urine sodium concentration is > 40 mmol/L
29
Q

What are the causes of renal oliguria?

A
  • hypertension is main cause
  • intrinsic damage
  • tubular necrosis
  • chronic infection
  • immunological damage - sle
  • toxic damage - drugs, heavy metals, poisons

Clinical Pathology (57 decks)

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