Diagnostic tests

Question 1

Define sensitivity in relation to diagnostic test results

Answer 1

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

Question 2

What is the practical significance of high sensitivity?

Answer 2

• 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)

Question 3

Define specificity in relation to diagnostic test results

Answer 3

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

Question 4

What is the practical significance of high specificity?

Answer 4

• 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)

Question 5

what is the relationship between sensitivity and specificity?

Answer 5

As one increases, the other decreases

Question 6

Define the positive predictive value

Answer 6

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)

Question 7

What does a high positive predictive value mean?

Answer 7

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

Question 8

Define negative predictive value

Answer 8

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

Question 9

What does a high negative predictive value mean?

Answer 9

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%

Question 10

What is the prevalence of a disease?

Answer 10

The number of diseased cases within a population

Question 11

What is the effect of sample size on prevalence?

Answer 11

• 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

Question 12

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

Answer 12

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

Question 13

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

Answer 13

• 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

Question 14

Why might a provocative or dynamic hormone test be useful?

Answer 14

• 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

Question 15

What are the 2 broad categories of blood sampling tubes?

Answer 15

• Coagulation promotors
• ## Coagulation inhibitors

Question 16

Give examples of tubes that promote coagulation

Answer 16

• Clot activators
• serum separation tube with gel
• Plain

Question 17

Give examples of tubes that impede coagulation

Answer 17

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

Question 18

Compare serum and plasma

Answer 18

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

Question 19

What tube is useful for haematology and give exceptional circumstances

Answer 19

• EDTA

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

Question 20

What tubes are commonly used for clinical biochemistry?

Answer 20

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

Question 21

What tube is used for glucose assessment and why?

Answer 21

Fluoride-oxalate (or iodoacetate)

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

Question 22

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

Answer 22

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

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

Question 23

List advantages of radiographs

Answer 23

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

Question 24

List disadvantages of radiographs

Answer 24

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

Question 25

List advantages of ultrasound

Answer 25

- 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

Question 26

List disadvantages of ultrasound

Answer 26

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

Question 27

List the advantages of CT

Answer 27

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

Question 28

List the disadvantages of CT

Answer 28

- Expensive - Not often in house - Radiation - Cannot be used in large animals - GA required

Question 29

List the advantages of MRI

Answer 29

- Good for visualisation of brain - No radiation - Can see sections - Detailed images of soft tissue structures

Question 30

List the disadvantages of MRI

Answer 30

- Expensive - Not often in house - Noisy - GA required - Long time