Theme 4 Lecture 3: Laboratory tests of renal function Flashcards
What are the 3 main functions of the kidney?
- excretion e.g urea, uric acid
- regulation e.g homeostasis, water, acid-base
- endocrine e.g renin, erythropoietin, vitD metabolism
What are the 3 main steps that describe how a kidney functions?
- arterial input
- filter (glomerulus)
- venous output or urine output
What are the 3 types of renal impairment?
- pre-renal
- renal
- post-renal
What is the cause of pre-renal kidney failure?
decreased ECFV (extra cellular fluid volume) or MI - low BP means not enough arterial input into the kidney to drive filtrate through glomerulus
What is the cause of renal kidney failure?
acute tubular necrosis
What is the cause of post renal kidney failure?
ureteral obstruction
What are the lab tests of renal function?
- glomerular filtration rate - impractical
- eGRF
- creatinine clearance - unreliable
- plasma urea
- urine volume
- urine urea
- urine sodium
- urine protein
- urine glucose
- haematuria
What is the urine volume range typical in health?
750-2000 mL/24 hr
Define oliguria
less than 400mL/24 hr which is the minimum amount of urine produced to ensure enough urine is removed to prevent toxicity build up
Define anuria
<100 mL/24 hr
Define polyuria
> 3000 mL/hr
What is the reference range of plasma urea?
3-8 mmol/L
which factors affect plasma urea concentration?
- GIT protein
- tissue protein
- liver amino acids
- levels of reabsorption and excretion in kidney
- filtration in kidney
How is urea produced?
- 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
How is urea excreted
- 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
When is tubular flow rate slow?
renal hypoperfusion
What are the causes of increased plasma urea?
- GI bleed
- trauma
- renal hypoperfusion
- acute renal impairment
- chronic renal disease
- post-renal obstruction calculus tumour
What is the usual range of plasma creatinine?
50-140 umol/L
What is the relationship between plasma creatinine and GFR?
Plasma creatinine increases in concentration as GFR decreases
Why is GFR not used in clinical practise?
difficult to perform clinically
How is GFR measured in clinical practise?
clearance of [99Tc]-Sn-DTPA
How do we calculate creatinine clearance?
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
as creatinine clearance decreases, what plasma changes do we see?
- increased uric acid
- increased phosphate
- decreased bicarbonate
- increased potassium
- increased urea
- increased creatinine
What is the limitation of eGFR?
dosen’t take into account patients weight
What clinical decisions can be made from eGFR?
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
Explain the features of pre-renal oliguria?
- 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
What are the causes of pre-renal oliguria?
- dehydration - sodium/water
- haemorrhage
- renal artery damage
- hypotension
What are the features of renal oliguria?
- 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
What are the causes of renal oliguria?
- hypertension is main cause
- intrinsic damage
- tubular necrosis
- chronic infection
- immunological damage - sle
- toxic damage - drugs, heavy metals, poisons
