Diagnostics and Lab Testing

Question 1

What is the definition of sensitivity?

Answer 1

The ability of a test to correctly identify those with the disease (true positive rate)

Question 2

What is the equation for sensitivity ?

Answer 2

True positives / (true positives +false negatives)

Question 3

What is the definition of specificity?

Answer 3

The ability of a test to identify those without the disease (true negative rate)

Question 4

What is the equation for specificity?

Answer 4

True negatives / (false positives + true negatives)

Question 5

What is the positive predictive value?

Answer 5

The disease percentage in those testing positive

Question 6

What is the equation for positive predictive value?

Answer 6

True positives/ (true positives + false positives)

Question 7

What is the negative predictive value?

Answer 7

The percent testing negative in those without disease

Question 8

What is the equation for negative predictive value?

Answer 8

True negatives/ (true negatives + false negatives)

Question 9

What is the likelihood ratio?

Answer 9

The likelihood of a result meaning disease versus not meaning disease

They are basically used to assess the value of performing a certain test, and predicting whether it will significantly change the pre test probability.

These can be useful to figure out whether to perform a certain test, when put in combo with pre test probability

Question 10

What is the equation for positive likelihood ratio?

Answer 10

Sensitivity (1-specificity)

Question 11

What is the equation for negative likelihood ratio?

Answer 11

(1-sensitivity)/specificity

Question 12

What do you use a receiver operating curve for?

Answer 12

To see the ‘trade off’ between sensitivity and specificity, to allow optimisation of a cut off point for a test

Question 13

What is the change in probability if the likelihood ratio is:

a) 2
b) 5
c) 10
d) 0.1
e) 0.2
f) 0.5

Answer 13

a) 15
b) 30
c) 45
d) -45
e) -30
f) -15

ie. the higher positive value, the more likely the post test probability to change = DO THE TEST
ie. the higher the decimal point (closer to 1), the less likely the post test probability to change = DONT DO THE TEST

Question 14

What is analyte specificity?

Answer 14

The ability of a test to detect the analyte alone. This can be interfered with through:

• Related molecules
• Matrix effects
• Antibodies etc

Question 15

What is the limit of detection?

Answer 15

The smallest conc of an analyte that can be distinguished from zero

Question 16

What is the limit of quantification?

Answer 16

The smallest conc of analyse that can be measured with ACCEPTABLE PRECISION (SD>20%)

Question 17

What is precision?

Answer 17

The repeatability of a measurement (measure of random error)

Question 18

What is accuracy?

Answer 18

How close the result is to the actual amount present (measure of systematic error)

Question 19

What is spectrophotometry?

Answer 19

A lab technique in which light is passed through a sample, and the intensity correlated with absorption over certain wavelengths

Question 20

What are antibody based methods?

Answer 20

Immunoassays containing antibodies that bind to a specific part of a foreign antigen, called the epitope. They can be POLYCLONAL or MONOCLONAL.
Usually they are attached to an enzyme that produces a colour or a flurophore

Question 21

What is the difference between polyclonal and monoclonal antibodies?

Answer 21

Polyclonal - high affinity, recognises multiple epitopes

Monoclonal - high specificity, very expensive

Question 22

What are the two types of immunoassay?

Answer 22

Sandwich - signal produced is proportional to the amount of analyte present

Competitive - signal produced is inversely proportional to amount of analyte present (as it accounts for whatever isn’t captured by the antibody)

Question 23

What are heterophilic antibodies?

Answer 23

Abs produced against poorly defined antigens, which are generally weak with multi specific activities. They can sometimes interfere with immunoassay tests

Question 24

What are immunoassays used for?

Answer 24

Primarily diagnosis of primary malignant tumours, such as HER2 staining in breast cancer

Question 25

What are the limitations of immunoassays?

Answer 25

• Cross-linking
• Background staining
• Difficult to standardise

Question 26

What is 2d electrophoresis?

Answer 26

A lab technique which separates out analytes based on structure and weight

Question 27

What is mass spectrometry?

Answer 27

A lab technique used to target detection of specific molecular fragmentation. It sorts ions based on their mass to charge ratio.

Question 28

What is Bayes Theorem?

Answer 28

It describes the probability of an event, based on prior knowledge of conditions that might be related to the event.

ie. it considers the test characteristics (likelihood ratio), as well as the pre-test probability

Question 29

What should be the outcome if you have

a) very low pre test probability
b) intermediate pre test probability
c) very high pre test probability

Answer 29

a) consider if you really need to test and treat?
b) TEST and treat on the basis of the results
c) consider if you really need to test, or should just get on with treatment?

Question 30

Why is it useful to do a test if the pre test probability of having a disease is intermediate?

Answer 30

It influences interpretation of a borderline result