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Y2 Clinical Pathology > Laboratory Tests of Renal Function > Flashcards

Laboratory Tests of Renal Function Flashcards

1
Q

What contributes towards total body volume?

A

In health body volume is in a steady state

(this means what goes in should come out)

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2
Q

What does the concentration of any solute in the body depend on?

A

For any solute, the concentration depends on the amount of the solute present and the volume of solvent (water) in which it is dispersed

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3
Q

What happens if the input of a solute increases and the excreteion stays constant?

A

The concentration will rise

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4
Q

If the input of a solute decreases and the excretion stays constant, what will happen?

A

The concentration of the solute will fall

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5
Q

What will happen if the excretion of a solute decreases and the input stays constant?

A

The concentration of the solute will rise

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6
Q

What will happen to the concentration of solute if the body volume increases and decreases?

A

If the volume increases due to water overload, the concentration will fall

if the volume decreases due to water deprivation, the concentration will rise

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7
Q

What are the 3 main functions of the kidney?

A

Excretion:

  • the kidneys excrete the end products of metabolism
  • e.g. urea from amino acid breakdown
  • uric acid from purine (nucleic acid) metabolism

regulation:

  • e.g. homeostasis, water, acid base balance

endocrine:

  • e.g. renin, erythropoietin
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8
Q

What are some of the homeostatic functions of the kidney?

A
  • Maintenance of water balance by regulating urine volume
  • acid base balance by altering hydrogen ion excretion
  • sodium balance by altering the rate of sodium reabsorption
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9
Q

What are the endocrine functions of the kidney?

A
  • Secretion of renin from the JGA which influences aldosterone
  • erythropoietin effects the rate of red cell production
  • 1,25 - dihydroxycholecalciferol is the active form of vitamin D, which affects calcium homeostasis
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10
Q

What types of defects will be seen in patients with chronic renal disease and impaired renal functions?

A

They show defects in endocrine and excretory functions before the loss of homeostatic control

when homeostatic functions cease, the patient is in renal failure and would die if there were no interventions

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11
Q

What are the purposes of renal function tests?

A
  1. They detect renal damage
  2. They monitor functional damage
  3. They distinguish between impairment and failure
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12
Q

What is an early test to detect renal damage?

What is a test used to monitor a patient with renal disease?

A

An early test to detect renal damage is a simple strip test for haematuria - important in screening for heavy metal poisoning

there is a clinical need to monitor a patient with renal disease - this is acheived by serial plasma measurements

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13
Q

About how many nephrons would have to lose their function before tests could detect this?

A

In renal disease, about half the nephrons have to lose their functioning before the abnormality can be detected by conventional laboratory tests

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14
Q

What are the pre-renal, renal and post-renal causes of kidney failure?

A

Pre-renal:

  • e.g. decreased ECFV or MI

renal:

  • e.g. acute tubular necrosis

post-renal:

  • ureteral obstruction
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15
Q

What are all the laboratory tests of renal function?

A

The detection of substances such as red cells or glucose could be an early indicator of renal damage

  • glomerular filtration rate
  • eGFR - estimate
  • creatinine clearance
  • plasma creatinine
  • plasma urea
  • urine volume
  • urine urea
  • urine sodium
  • urine protein
  • urine glucose
  • haematuria
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16
Q

What is the normal urine volume in health and what does this depend on?

A

Urine volume depends on how much you drink and sweat

In health it is closely matched to water balance by the hormone ADH, or vasopresssin (AVP)

750 - 2000 mL / 24 hours is typical in health

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17
Q

What is oliguria?

A

Abnormally low urine volume is a 24 hour volume less than 400 mL

this is oliguria

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18
Q

What is the definition of polyuria?

A

There is no absolute definition of polyuria as some people can drink an full lot and match it with a high urine output

if the urine volume is greater than 3 litres per day, and the patient is not drinking, this is polyuria

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19
Q

What is the output for anuria?

A

Anuria is less than 100 mL / 24 hours

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20
Q

What is the reference range for plasma urea?

How sensitive / specific is it?

A

It provides a quick, simple measurement

it has a wide reference range of 3 - 8 mmol / L

it gives a sensitive but non-specific index of illness

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21
Q

Why is the plasma urea measurement sensitive but non-specific?

A

The concentration of urea is increased in many different conditions, this makes it sensitive to the presence of disease but a non-specific test

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22
Q

What are the factors influencing plasma urea concentration?

A
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23
Q

Where is urea filtered?

What % of filtered urea is reabsorbed in health?

A

Urea is filtered at the glomerulus

about 40% of filtered urea is reabsorbed by renal tubules in health

60% of filtered urea is excreted

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24
Q

How does the amount of urea reabsorbed change with the rate of tubular flow?

A

The rate of urea reabsorption is variable and depends on the rate of tubular flow

more urea is reabsorbed if the flow rate is slow as there is more time for urea to diffuse into the peritubular capillaries

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25
Q

When does tubular flow rate tend to be slow?

A

When there is renal hypoperfusion

this may be following myocardial infarction

more urea is reabsorbed and plasma urea increases

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26
Q

What types of conditions result in renal hypoperfusion?

A

Fluid loss, circulatory insufficiency, renal artery stenosis

27
Q

What are the causes of increased plasma urea?

How are they divided?

A

Acute renal impairment:

  • gastrointestinal bleed

trauma

chronic renal disease:

  • renal hypoperfusion
  • decreased RBF
  • decreased ECFV

post-renal obstruction:

  • Calculus tumour
28
Q

Why are the risks of performing a urea test?

A

It is a useful test but needs to be interpreted with great care

you need to consider input, output and a patient’s fluid volume

29
Q

What is normal plasma creatinine?

What does this measurement show?

A

50 - 140 umol / L

it has a wide reference range

a body builder may have a plasma creatinine at the top end, and an old lady a value at the low end, reflecting muscle mass

measurement of plasma creatinine can provide a specific test of glomerular function

30
Q

How long after a meal should plasma creatinine be measured?

A

Plasma creatinine should not be measured until 8 hours after a meal

there is some evidence that the concentration increases after meat ingestion

31
Q

How is plasma creatinine concentration linked to GFR?

A

Plasma creatinine concentration increases when GFR falls

GFR has to fall quite considerably before the plasma creatinine reliably increases

32
Q

What important analytical inferences need to be checked with a lab when interpreting plasma creatinine?

A

A patient with ketoacidosis, jaundice or infection might have agents in the plasma which could invalidate the measurement of creatinine

33
Q

How is plasma creatinine linked to renal damage?

A

Plasma creatinine is not proportional to renal damage

34
Q

How is the plasma creatinine change within an individual patient related to the absolute value?

A

The change within an individual patient is usually more important than the absolute value

35
Q

How does plasma creatinine change in patients with chronic renal disease?

A

Patients with chronic renal disease have elevated plasma creatinine concentrations

this can be up to 1000 umol / L

plasma creatinine increases in an exponential fashion

36
Q

How is a plot of reciprocal of plasma creatinine concentration used in clinic?

A

Renal physicians need to predict when a patient will require dialysis or transplantation

the plot of reciprocal of plasma creatinine concentration against time is used to predict when intervention is required

37
Q

When is glomerular filtration rate measured?

A

It is rarely measured in clinical practice and is required when a patient comes to hospital

people who are considering donating a kidney whilst they are alive have their GFR measured

38
Q

how is GFR measured?

A

It used to be measured by calculating the clearance of inulin

now radioactive substances are used

this is either technetium labelled diethylenediaminetetra acetic acid DTPA or 51-chromium labelled EDTA ethylenediaminetetra acetic acid

39
Q

What is used as a marker of GFR?

A

Creatinine clearance

40
Q

What are typical values for creatinine clearance in health?

A

100 - 130 mL / min

41
Q

What samples must a patient give in order to calculate creatinine clearance?

A

The subject collects all the urine produced over 24 hours into a container

a portion of this urine is used to measure creatinine concentration - Ucreat

a blood sample during the 24 hours is needed to measure plasma creatinine concentration - Pcreat

42
Q

What is the calculation for creatinine clearance?

What measurements are required ?

A

Ccreat = [Ucreat x V] / Pcreat

Ucreat = urine creatinine concentration (mmol/L)

V = urine volume (mL collected in 24 hours)

Pcreat = plasma creatinine concentration (umol / L)

43
Q

In health, how much higher should creatinine clearance be compared to GFR?

Why is it higher?

A

Creatinine clearance should be 10 - 30% higher than GFR

it is higher as creatinine is secreted by the renal tubules

as plasma creatinine increases, the rate of creatinine secretion increases, so more creatinine in urine gets there by tubular secretion rather than by glomerular filtration

44
Q

What does the fact that creatinine clearance is higher than GFR mean clinically?

A

Creatinine clearance is not a reliable marker of GFR in chronic renal disease

secretion is also inhibited by common drugs such as aspirin and cimetidine

45
Q

Why is creatinine clearance an unreliable test?

A
  • Tubular secretion is increased in chronic renal disease
  • tubular secretion is inhibited by drugs such as salicylate and cimetidine
  • problems with incomplete collection of urine
46
Q

What is normal creatinine clearance, as a marker of GFR, in health?

How much must renal function be reduced until creatinine clearance is affected?

A

120 mL / min in health

there is no increase in plasma creatinine concentration until renal function is halved

with creatinine clearance at 20 - 60 mL / min, increased creatinine and urea can be measured in plasma

47
Q

What are the plasma changes associated with the creatinine clearance decrease in chronic renal disease?

A
48
Q

When creatinine clearance falls below 30 mL/min , what kind of changes are observed?

A

Hyperkalaemia and acidosis

when creatinine clearance becomes very low at 10 - 20 mL / min, then there is an increase in phosphate and uric acid

49
Q

Currently, how is eGFR calculated in practice?

A

It is calculated in the lab using an international formula

In the UK, the abbreviated MDRD equation is used

50
Q

What would the treatment decisions be if a patient had these eGFR readings?

A
51
Q

In pre-renal oliguria, how would ADH levels change and why?

A

ADH increased

If a patient has lost water from the ECFV, then ADH is secreted

this produces a low volume of concentrated urine

urine osmolality is greater than 500 mmol / kg H2O

52
Q

Why is urine urea concentration high in pre-renal oliguria?

A

There is increased urea reabsorption

plasma urea concentration will also be elevated

53
Q

What does renal hypoperfusion (low RBF) lead to in pre-renal oliguria?

A

Renal hypoperfusion causes renin secretion

This leads to hyperaldosteronism - increase in aldosterone levels leads to functioning nephrons increasing sodium reabsorption

this means that urine sodium concentration is very low - less than 20 mmol/L

54
Q

What can low renal perfusion in pre-renal oliguria/failure lead to?

A
  • Dehydration - sodium / water
  • haemorrhage
  • renal artery damage
  • hypotension
55
Q

What would you expect the GFR, urine properties and renal renin secretion to be like in renal oliguria?

A
  • GFR normal or reduced
  • weak urine and a low volume produced
  • renal renin secretion may be raised
56
Q

Why may renal renin secretion be raised in renal oliguria?

What are the consequences of this?

A

Hypertension is present but the nephrons are unable to reabsorb sodium

this leads to urine sodium concentration being high and above 40 mmol/L

57
Q

What types of things may cause renal oliguria / failure?

A

intrinsic damage:

  • tubular necrosis
  • chronic infection
  • immunological damage - SLE
  • toxic damage - drugs, heavy metals, poisons
58
Q

What other clues and problems may be present that suggest renal failure?

A
  • Anaemia
  • haematuria
  • proteinuria
  • urine cases
  • problems with calcium / phosphate or bone disease
59
Q

What do PRU and ARF stand for in the differentiation of oliguria?

A

PRU - prerenal uraemia:

  • refers to conditions that produce renal hypoperfusion
  • e.g. Fluid loss and circulatory problems

ARF - acute renal failure:

  • involves tubular necrosis
  • there is damage to the nephrons
60
Q

Why should a patient with ARF not be infused with a fluid?

A

Patients with ARF have nephrons which cannot respond to aldosterone or frusemide

urine sodium concentration is usually greater than 40 mmol/L

if they are infused with a fluid, this may lead to fluid overload with pulmonary oedema

61
Q

Complete the table for the differentiation of oliguria

A
62
Q

What are the laboratory tests of renal function?

A
  • Plasma creatinine
  • plasma urea
  • plasma sodium
  • urine volume
  • urine sodium
  • urine urea
  • creatinine clearance
  • urine dipsticks
63
Q

Which tests can be used to assess both hydration status and renal function?

A

Plasma urea and urine volume

Y2 Clinical Pathology (76 decks)

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