laboratory tests of renal function Flashcards
for solutes what happens if the input increases and the excretion stays constant?`
the concentration will rise and vice versa
what happens if the input of a solute stays the same but the output decreases?
the concentration will rise and vice versa
what is incorporated in a healthy state?
not just what is being excreted and ingested but also what is being produced in the body
what can be used as a marker for compromised excretion rate in the kidney?
rise in urea - if urea is constant but the excretion decreases then it rises
for solvents what will happen if the volume increases or decreases?
the concentration will rise when there is a decreased volume of solvent and vice versa
e.g. higher concentration in water deprivation and lower in water overload
what feeds into body volume?
intake and metabolic production and excretion feeds out
what does concentration of a solute depend on?
amount of the solute present and the volume of water in which it is dispersed
what are the three main functions of the kidney and give examples?
excretion - urea and uric acid
regulation - homeostasis, water, acid and base
endocrine - renin and erythropoietin
what is erythropoietin for?
for maintaining Hb levels
25 hydroxycholecalciferol to 1,25 dihydroxycholecalciferol which is vit D
what is involved in regulation?
helps with compensatory mechanisms
what are the processors and the filters of the kidney?
the processors are the tubules and the filter is the glomerulus
where does the input arterial and the output venous go from and to?
input arterial into the glomerulus
output venous out of the tubules
what are renal function tests for?
detecting renal damage, monitor functional damage and distinguish between impairment and failure
what happens when there is renal damage?
function is impaired
how can we monitor functional damage?
physical and functional are connected so can do through blood in urine or proteins
what is the therapy with failure?
dialysis and eventually transplant
what are the classifications of renal damage?
pre-renal
renal
post-renal
the distinctions will determine the treatment
how can we assess kidney damage?
clinical symptoms
tests
urine output - often overlooked
what is normal for urine output?
40ml/hour
what are causes of pre renal damage?
decreased ECFV or MI
MI results in low BP meaning that there is not enough blood flow to kidney and therefore little filtration as is not driven through the glomerulus
what is a cause of renal damage?
acute tubular necrosis - major injury will results in a BP drop so there is ischaemia in the tubules - this will regenerate if it is not severe
what are causes of post renal damage?
ureteral obstruction
can be from seriously enlarged prostate bladder, bladder cancer, peritoneal fibrosis (sticks to back wall) or cancer of the ureter or wrong tubes being tied
what lab tests are there to assess renal damage?
GFR, eGFR, creatinine clearance, plasma creatinine, plasma urea, urine excretion, volume, urea, sodium, protein, glocuse and haematuria
what is GFR used?
is is seldom used in clinical practice unless a very accurate measure is needed such as in donors or dose of drugs
it is the clearance of [99Tc]-Sn-DTPA
what are the characteristics of creatinine clearance?
in health this will be 10-30% higher than GFR
there are problems with incomplete collection as when out of house is difficult
unreliable as use wrong flow time or part of flow
there is a 1000 fold difference in units
is done ideally over 24 hours
what increases and inhibits tubular secretion of creatinine?
it is increased in chronic renal disease
drugs such as salicylates such as cimetidine and aspirin inhibit the secretion
how can you calculate creatinine clearance from a set period of time?
measure the amount of creatinine excreted in fixed time period and calculate using the plasma creatinine conc and therefore can calculate how much plasma has passed through kidney to produce this
clearance = urine creatinine con (mmol/L) x volume of creatinine (ml over 24 hours) /plasma creatinine conc (micromol/L)
what is the reference range for creatinine clearance?
100-130ml/min
how would you calculate the creatinine clearance from results?
scatter plot with line of best fit
y axis of 1/[pCreat]
x axis of time
what are the characteristics of plasma creatinine?
the reference ranges of plasma creatinine are 50-140 micromol/L
increase in concentration as GFR decreases
not proportional to renal damage
analytical interferences
is the measure of acetoacetate - DKA
chemical reaction is method - cheap but not accurate
non linear change
why can GFR not be measured against the change is plasma creatinine?
the change is not proportional
in chronic renal disease PC may increase to 1000 micromol/L
the creatinine may also still be in the reference range but the GFR may have plummeted as well as function
what needs to be considered when using plasma creatinine to measure function?
in terms of the reference range and individual in hand
what can be used to measure GFR and PC?
a reciprocal graph
predicts when intervention is required in end stage renal failure - dialysis and transplant
what interferes with the chemical reaction?
other chemicals can affect it and therefore are aiming for enzymatic method
what is important to consider with patients graphs?
change within an individual patient is usually more important than the absolute value
what are the characteristics of plasma urea?
it is a quick simple measurement with a wide reference range of 3-8mmol/L
it is a sensitive but non specific index of illness
as renal reabsorption decreases then urea is amplified - can be useful or over exaggerate
what else can affect plasma urea?
protein (tissue or GIT) - breaking down tissue protein such as in surgery or damage to muscles making urea
AA - when they are deaminated and the constituents are urea - used for nitrogen metabolism - protein intake will increase
distribution volume - anything that changes the water volume and then changes urea and function filtration rate
kidney filtration, reabsorption and excretion
what is the urine volume reference range per 24 hours?
750-2500mL/24 hours
in health
what is oliguria?
<400mL
this is the minimum volume that is needed to produce in order to remove toxic metabolites
what is anuria ?
<100 - failure to produce urine - depends on the time of day and activities
what is polyuria?
it is >3000
in syphilis and diabetes where there is little ADH
what are the characteristics of urea excretion?
urea is filtered at the glomerulus and around 40% is reabsorbed at the renal tubules in health
what happens when there is renal hypoperfusion?
the tubular flow rate is slower meaning that more urea is reabsorbed
what is an early phase of pre renal failure?
there is early high urea
in chronic kidney disease what is the change to plasma as the creatinine clearance changes:
a) 60-120
b) 30-60
c) 20-30
d) 10-20?
a) no change in plasma
b) increased creatinine and urea
c) increased potassium and decreased bicarbonate
d) increased phosphate and uric acid
when could plasma urea increase?
chronic renal disease, acute renal impairment, renal hypoperfusion, trauma, GI bleed, post-renal obstruction
what is an example of post - renal obstruction?
calculus, tumour
slows filtration as there is back pressure
why does renal hypoperfusion increase urea?
there is decreased ECFV and RBF
what is an example of a GI bleed that will increase urea?
gastric ulcers - feed into GIT and push the area up
what is needed to be considered for urea?
the input, output and the patient’s fluid volume
always interpret with care
in current practice how is eGFR calculated?
it is calculated in the lab using an international formula by taking creatinine and estimating
what were the initial problems of this method of eGFR?
there is variance between labs in chemical methods
ethnicity impacts
different formulas are used
need to compensate for the patients weight
creatinine will test different in different places
how were the initial issues identified with eGFR overcome?
it is now classified into bands - more standardised and reliable
what equation does the UK use for eGFR?
it uses the abbreviated MDRD equation
186(creatinine /88.4) - 1.154(age) - 0.203(if female x0.742 and if black then 1.210)
what does the grade of eGFR determine?
the methodology of referral from GP and the treatment
what is stage 1 and 2 in terms of eGFR findings?
1 - eGFR of 90+
stage 1 means that the kidney function is normal but urine, structural abnormalities or genetic traits point to kidney disease and therefore is managed with observation and control of BP
2 - eGFR of 60-89 meaning that the function is mildly impaired and other findings point to kidney disease - the treatment is observation, management of risk factors and blood pressure
describe stage 3 of eGFR and its divisions?
there are stages 3a and 3b
3a will have an eGFR of 45-59 and 3b of 30-44. The treatment is observation and control of BP and RFs and there is moderate reduced kidney function
what is stage 4?
it is an eGFR of 15-29. The kidney function is severely reduced and there is planning for end stage kidney failure
what is stage 5?
the eGFR of less than 15 or on dialysis. It is very severe - endstage/established renal failure. The treatment choices are dialysis or transplant
what is the result of pre renal oliguria?
the GFR is reduced
when might ADH be increased?
when we are dehydrated - the urine concentration is high with low volume
what is needed for pre renal oliguria?
IV fluids and saline - need water and sodium
what are causes of low renal perfusion?
dehydration strictures post operative lack of fluid haemorrhage renal artery damage hypotension
what is a result of renal hypoperfusion?
renin secretion
what is the result of renin secretion from hypoperfusion?
functioning nephrons will increase the sodium absorption through the release of aldosterone and urine concentration of sodium is therefore low
what is the main cause of pre renal oliguria and what is the difference between renal and pre renal?
the main cause of pre renal is hypoperfusion
renal is a reduced urine volume that implies AKI
what are the characteristics of renal oliguria?
GFR is reduced or normal and there is concentrated urine with low volume
cannot reabsorb sodium
intrinsic damage
why is the renal renin secretion raised in renal oliguria?
unable to reabsorb Na but need to
this results in hypertension from angiotensin and the urine sodium concentration remaining high above 40mmol/L
what can cause renal oliguria?
intrinsic damage
tubular necrosis which is reversible if not too sever
chronic infections
immunological damage such as in SLE
toxic damage such as drugs, heavy metals and poisons
hypertension is the main cause
what is the sodium concentration of urine in PR?
<20mmol/L
what is the P/U urea ratio?
> 5 fold in PRU
<2 fold in RRF
what are other clues for renal renal failure?
from urine - anaemia, haematuria, proteinuria, urine casts
what are other problems of renal renal failure?
the calcium and phosphate resulting in metabolism disorders and bone disease
how can we figure out which stage of the kidney process is affected?
through urine sodium and urine tests
what is renal failure?
when there is fluid in but nowhere to go resulting in oedematous and cardiac overload and failure - need dialysis to overcome
what is the treatment consideration for dialysis?
balancing the input and overload
which lab test of renal function is often forgotten?
urine volume
what are the plasma lab tests for renal function?
plasma creatinine, urea and sodium
what are the urine tests for renal function?
urea, volume and sodium and dipsticks
what else can be done with creatinine to identify renal problems?
creatinine clearance
what are the issues of the lab tests?
creatinine clearance is unreliable
GFR is impractical
plasma urea is subject to problems
plasma creatinine is specific but it is insensitive
