Tests of renal function Flashcards
What happens if the input of a solute increases and the excretion stays constant?
The concentration of the solute will rise.
What happens if the input of a solute decreases and the excretion stays constant?
The concentration of the solute will fall.
What happen if the excretion decreases and the input stays constant?
The concentration will rise.
What happens to the concentration of a solute during water overload?
The concentration will fall. The reverse happens during water deprivation.
What are the functions of the kidney?
- excretion e.g. urea, uric acid
- regulation e.g. homeostasis, water, acid base
- endocrine e.g. renin, erythropoietin
What is the purpose of renal function tests?
- detect renal damage
- monitor functional damage
- distinguish between impairment and failure
Where are the three places problems can arise in the urinary tract?
- Pre-renal - Renal - Post-renal
What are the possible causes of renal failure?
- Pre-renal e.g. decreased ECFV or MI - Renal e.g. acute tubular necrosis - Post-renal e.g. ureteral obstruction
What are the different kinds of laboratory test of renal function?
- glomerular filtration rate - eGRF - estimate - creatinine clearance - plasma creatinine - plasma urea - urine volume - urine urea - urine sodium - urine protein - urine glucose - haematuria
What urine volume over 24h is normal in health?
750 - 2000ml/24h
What urine volume over 24h is seen in oliguria?
>80ml but <400ml
What urine volume over 24h is seen in anuria?
>100ml
What urine volume over 24h is seen in polyuria?
>3000ml/24h
What is the value of the plasma urea test?
- quick, simple measurement
- wide reference range 3 - 8 mmol/L
- sensitive but non-specific index of illness
What factors affect plasma urea concentration?
- GIT protein and tissue protein -> liver amino acids
- Distribution volume
- Kidney reabsorption excretion
- Kidney filtration
Where is urea excreted?
Filtered at the glomerulus
What percentage of filtered urea is reabsorbed by renal tubules in health?
40%
Is more or less urea reabsorbed if the rate of tubular flow is slow?
More
Why would tubular flow rate be slow?
Renal hypoperfusion
What might be the causes of increased plasma urea?
- GI bleed - trauma
- renal hypoperfusion decreased RBF decreased ECFV
- acute renal impairment
- chronic renal disease
- post-renal obstruction calculus tumour
What is the value of the urea test?
- Useful test but must be interpreted with great care
- Always consider input, output and patient’s fluid volume
What are the features the plasma creatinine test?
- 50 - 140 umol/L
- increases in concentration as GFR decreases
- analytical interferences (acetoacetate - DKA)
- NOT proportional to renal damage
- Change within an individual patient is usually more important than the absolute value
What is usually more important than the absolute value in a plasma creatinine test?
Change within an individual patient
What predicts when intervention is required in end stage renal failure?
Plot of reciprocal of plasma creatinine concentration against time.
