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module 104 - theme 3 > measuring renal function > Flashcards

measuring renal function Flashcards

1
Q

what are the 2 reasons for measuring renal function?

A

1) identification of renal impairment in your patient

2) modification of dosages of drugs which are cleared by the kidneys

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2
Q

what are the 7 different types of patients at risk of renal failure?

A

1) extremes of age
2) polypharmacy
3) specific disease states
4) patients receiving long-term analgesia
5) transplant patients
6) drug therapy
7) patients undergoing imagine procedures

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3
Q

what are the 3 main ways to monitor a patients renal function?

A

1) patients clinical condition
2) modern imaging techniques
3) biochemical data

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4
Q

what is used for patient clinical condition?

A
  • weight charts
  • fluid balance charts
  • degree of oedema
  • results of urine dipstick testing
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5
Q

how are modern imaging techniques used?

A
  • include macroscopic views of renal blood flow, filtration and excretory function
  • some of these are used clinically but some ca only be currently used experimentally in the lab
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6
Q

what 3 types of scan fit into renography?

A
  • gamma camera planar scintigraphy
  • positron emission tomography (PET)
  • single photon emission computerised tomography (SPECT)
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7
Q

what is biochemical data useful for?

A

identifying renal impairment

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8
Q

what types of blood markers are used for renal function?

A
  • plasma or serum creatine (SCr)

- plasma or serum urea or blood urea nitrogen (BUN)

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9
Q

what are the key characteristics of creatine?

A
  • breakdown of product of creatine phosphate in muscle
  • generally produced at constant rate
  • filtered at the glomerulus with some secretion into proximal tubule
  • normal range in plasma: 40-120 micro mol/L
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10
Q

what is plasma creatine increased by?

A
  • large muscle mass, dietary intake
  • drugs which interfere with analysis
  • drugs which inhibit tubular secretion
  • ketoacidosis
  • ethnicity
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11
Q

what is plasma creatine decreased by?

A
  • reduced muscle mass
  • cachexia/starvation
  • immobility
  • pregnancy
  • severe liver disease
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12
Q

what are the key characteristics of urea?

A
  • liver produces urea in the urea cycle as a waste product of protein digestion
  • filtered at the glomerulus, secreted and reabsorbed in the tubule
  • plasma urea described as BUN
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13
Q

what does BUN stand for?

A

blood urea nitrogen

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14
Q

what is the normal range of BUN?

A

2.5-7.5mmol/L

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15
Q

what level of BUN indicates moderate to severe renal failure?

A

> 20mmol/L

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16
Q

what is BUN increased by?

A
  • high protein diet
  • hyper catabolic conditions
  • GI bleeding
  • muscle injury
  • drugs
  • hypovolaemia
17
Q

what is BUN decreased by?

A
  • malnutrition
  • liver disease
  • sickle cell anaemia
  • SIADH
18
Q

what are the key features of an ideal marker of kidney function?

A
  • naturally occurring molecule
  • not metabolised
  • only excreted by the kidney
  • filtered but not secreted or reabsorbed by the kidney
19
Q

what is an example of a substance that is freely filtered but not reabsorbed or secreted by the kidney?

A

inulin

20
Q

what is an example of a substance that is freely filtered and partly or mostly absorbed by the kidney?

A

electrolytes

21
Q

what is an example of a substance that is freely filtered but fully reabsorbed by the kidney?

A

glucose and amino acids

22
Q

what is an example of a substance that is freely filtered, not reabsorbed but fully secreted by the kidney?

A

PAH

23
Q

what is clearance?

A

the volume of plasma completely cleared of a given substance in unit time

24
Q

what 3 basic functions does renal clearance provide information on?

A
  • glomerular filtration (F)
  • tubular reabsorption (R)
  • tubular secretion (S)
25
Q

what is the rate at which kidneys excrete solute into urine equal to?

A

the rate at which solute disappears from blood plasma

26
Q

what is the equation for renal clearance?

A

Cx = Ux x V / P

27
Q

what are some drawbacks of renal clearance?

A
  • measuring clearance means measurement of overall nephron function
  • this gives the sum of all transport processes occurring along nephrons
  • no information is given about precise tubular sites or mechanisms of transport
28
Q

what should the glomerular filtration rate be?

A

125mL/min

29
Q

how is glomerular filtration rate measured?

A
  • GFR can be measured clinically and used as an indication of renal function
  • GFR can be estimated by measurement of clearance of creatine
  • creatine is filtered and secreted into tubule
30
Q

what is a more accurate estimation of renal function?

A

inulin clearance

31
Q

how is inulin clearance used for measurement of GFR?

A
  • it is freely filtered but it is not secreted and it is not absorbed
  • therefore rate of excretion in urine equals the rate of filtration by the kidneys
  • inulin clearance = GFR
  • if a substance has clearance greater that inulin then it must also be being secreted; less means that it must be being reabsorbed or not filtered freely at the glomerulus
32
Q

what is inulin?

A

inulin is a plant polysaccharide Mw 5200Da

33
Q

what are the drawbacks of inulin clearance to measure GFR?

A
  • inulin must be administered by IV to get relatively constant plasma or serum levels
  • chemical analysis of inulin in technically demanding
  • could use radio labelled compound instead
  • however, these may also bind to proteins and distort results
  • problems of IV infusion of GFR marker are avoided by using an endogenous substance with inulin-like properties
34
Q

how does creatinine clearance work?

A
  • creatinine is filtered at the glomerulus but some is secreted into the proximal tubule
  • therefore using the equation above would actually over-estimate GFR by about 20% in humans
  • however the colorimetry methods used to measure creatinine, under-estimate creatinine concentrations by about 20%
  • luckily these 2 errors cancel each other out, and calculated creatinine clearance is approximately inulin clearance
35
Q

what are the advantages to creatinine clearance?

A
  • cheap, easy, reliable, used clinically

- avoids IV infusion, just requires venous blood and urine samples

36
Q

how is creatinine usually produced?

A

by creatinine phosphate metabolism in muscle

37
Q

how do you use PAH clearance to measure renal blood flow?

A
  • if a substance is completely cleared from the plasma, its clearance rate will be equal to the renal plasma flow
  • clearance of PAH - para-aminohippuric acid can be used to estimate this
  • PAH another normally present in the blood
  • when given, almost all cleared from kidney in one passage, some filtered in glomerulus and remainder secreted by proximal tubules
  • around 10% by-passes tubule, travels from efferent arteries into peritubular capillaries an then venous renal blood, and is not secreted
  • uncorrected value for PAH clearance often used: known as effective renal plasma flow
38
Q

what are the biomarkers of renal disease?

A
  • indicators of renal function such as plasma creatinine or BUN increase only after there is a significant loss of renal function: typically a 60% loss in renal function before plasma or creatinine or BUN increases
  • urinary albumin/protein excretion can also be used as an indicator of chronic kidney disease
  • currently a lot of research interest in identifying blood and urinary markers which increase in the early stages of renal failure and can be measured
39
Q

what are the main proteins released into the plasma and/or urine to show renal disease?

A
  • kidney injury molecule
  • IL-18
  • fatty acid binding proteins
  • neutrophil gelatinise associated lipocalin
  • cystatin C

module 104 - theme 3 (13 decks)

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