Diagnostic tests Flashcards

1
Q

Define sensitivity in relation to diagnostic test results

A

The proportion of true positive identified by the test as having the disease i.e. the ability of the test to identify individuals with the disease

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2
Q

What is the practical significance of high sensitivity?

A
  • Minmise false negatives, more likely to pick up false positives (but will pick up every positive)
  • Therefore make good screening tests
  • Rule out disease (SnOut)
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3
Q

Define specificity in relation to diagnostic test results

A

The proportion of true negatives identified by the test as not having the disease i.e. indicates the ability of the test to identify individuals without a disease

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4
Q

What is the practical significance of high specificity?

A
  • Will only pick up positives if are truly positive, minimises false positives
  • Will pick up every negative (and maybe some false negative results)
  • Therefore high spec is good for confirmation, rule in disease (SpIn)
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5
Q

what is the relationship between sensitivity and specificity?

A

As one increases, the other decreases

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6
Q

Define the positive predictive value

A

The number of true positives compared to the total number of positives given by a test (i.e. of 100 positive results, 88 are actually positive for the disease)

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7
Q

What does a high positive predictive value mean?

A

That the test is accurate for a positive result most of the time (of 100 positive results, 99 are trule positive so PPV = 99%)

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8
Q

Define negative predictive value

A

The number of true negatives compared to the total number of negatives

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9
Q

What does a high negative predictive value mean?

A

Can be confident in teh negative values most of the time i.e. of 100 negative results, 99 were truly negative if NPV is 99%

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10
Q

What is the prevalence of a disease?

A

The number of diseased cases within a population

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11
Q

What is the effect of sample size on prevalence?

A
  • Large sample size with little discrimination in terms of patient’s tested will have lower prevalence
  • Small sample size with strict discrimination in terms of patient’s tested for disease will have higher prevalence as the test will more often give a positive result
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12
Q

What is the effect of prevalence on positive and negative predictive value?

A

Lower prevalence means less reliable for a positive result i.e. lowered PPV (will get more negative results than positive ones)

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13
Q

Why might a single measurement of blood concentration of a hormone not reflect the true status of the gland?

A
  • May fluctuate during the day
  • May have short half life
  • May be labile and not store very well
  • Influenced by normal physiological stimuli e.g. stress, exercise
  • Influenced by feeding
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14
Q

Why might a provocative or dynamic hormone test be useful?

A
  • Removes effect or normal physiological fluctuations
  • Can assess the response to stimulation or suppression and so differentiate between normal and diseased animals
  • i.e. subnormal response in stimualtion may suggest deficiency
  • Supraphysiological response to stimulation may suggest hormone excess/lack of feedback control, as would inability to suppress response
  • may also help differentiate primary and secondary disease, localise lesion
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15
Q

What are the 2 broad categories of blood sampling tubes?

A
  • Coagulation promotors
  • ## Coagulation inhibitors
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16
Q

Give examples of tubes that promote coagulation

A
  • Clot activators
  • serum separation tube with gel
  • Plain
17
Q

Give examples of tubes that impede coagulation

A
  • Sodium citrate
  • Sodium (or lithium) heparin
  • EDTA (K2/K3)
  • Fluoride-oxalate
18
Q

Compare serum and plasma

A
  • Neither have RBCs
  • Plasma achieved by centrifuging to remove RBCs
  • Clotting factors only present in plasma
19
Q

What tube is useful for haematology and give exceptional circumstances

A
  • EDTA

- HOWEVER will bind up calcium, also has potassium in so cannot be used for Addison’s diagnosis

20
Q

What tubes are commonly used for clinical biochemistry?

A
  • Serum (plain)
  • Clot activator
  • Li-heparin
  • EDTA plasma
21
Q

What tube is used for glucose assessment and why?

A

Fluoride-oxalate (or iodoacetate)

- Fluoride inhibits glycolysis, so glucose in the sample will not be destroyed

22
Q

Why should citrate tubes not be used to measure calcium in the blood?

A
  • In order to clot the sample, citrate binds to calcium

- Thus will not give accurate result for calcium present in the blood

23
Q

List advantages of radiographs

A
  • Usually in house facilities
  • Allows comparison of structures in relation to each other
  • Good for bone and visualising altered density
24
Q

List disadvantages of radiographs

A
  • Radiation
  • Cannot view things inside skull
  • Requires anaesthesia
  • Obscuring of lower structure
  • Cannot distinguish what kind of pathology, only enlargement/atrophy etc
25
Q

List advantages of ultrasound

A
  • No need for sedation
  • Recumbent or standing
  • ivsualisation of internal structure of an organ
  • Can see functionality of the organ
  • Can assess thickness of walls etc
26
Q

List disadvantages of ultrasound

A
  • sahdowing
  • Some images/views difficult to achieve
  • Need to clip area
  • Only see small field
  • Quality/age of machine may affect ability to visualise
27
Q

List the advantages of CT

A
  • Can move through sections, multiple views of one organ
  • Internal structure of organs visible
  • Removes overlapping
  • More precise location identification
28
Q

List the disadvantages of CT

A
  • Expensive
  • Not often in house
  • Radiation
  • Cannot be used in large animals
  • GA required
29
Q

List the advantages of MRI

A
  • Good for visualisation of brain
  • No radiation
  • Can see sections
  • Detailed images of soft tissue structures
30
Q

List the disadvantages of MRI

A
  • Expensive
  • Not often in house
  • Noisy
  • GA required
  • Long time