lecture 3A Flashcards
What does analytical test mean
Testing a particular product
What does analytical test do
– Measure quantitatively parameters
– Higher sensitivity is better
What does diagnostic tests do
–Finding the definite answer
– Relies on cut-off which defines what is
normal and what is not
– Setting cut-off will depend on our definition
of “normal” (see lecture week 1) an idea of a normal range
Are Sensitivity and specificity different
for analytical or diagnostic tests
yes
What is Sensitivity in analytical definition
Ability of the test to detect low degrees of a particular condition.
√ Detect lower amounts/sensitivity > often are earlier diagnostic
√ often defined as 2 or more standard deviations from the signal of a zero standard
The higher the sensitivity the better
What are the downside of sensitivity test
√ Sensitive tests are often less reliable (i.e. more susceptible to external factors, needs very high and consistent skill level)
√ Sensitive tests are often more expensive
√ Sensitive tests are often more invasive
What role does sensitivity have if there is a large variation?
It will be pointless, because the result will vary a lot
What is the analytical definition of specificity
√ Is a measure of the confidence one can have
that a test measures what it is supposed to measure
√ Is a measure for the lack of cross-reaction
What are the drawbacks for specific test
More specific test have a narrower detection range in that small variations in the disease can make the disease undetectable. For
example
√ Point mutation in influenza virus
√ Different strain of HIV
√ Isolate of SARs from a different geographical region
There is no point if there are no difference in the method
What does cross-reaction mean
means disease 1 and 2 are not differentiated
If test can differentiate between disease 1 and
disease 2 there is no cross-reaction.
What does cross-reactivity depend on?
-the degree of similarity between disease 1 and disease 2. The more similar, the higher the chance of cross-reaction
-the properties of the
assay
Are highly (analytically) specific tests desirable?
• Yes, to some degree
– More specificity gives a clear picture of the disease
– Different variants may have different pathogenic outcomes
• Highly infectious vs. low infectivity
• High vs low pathogenicity
• Killing disease vs benign disease
What can be a problem in highly specific tests
Too much of a good thing
– Failure to detect minor variants will results in missed cases, need a lot of different test
– If treatment and prognosis is not different do we need to know?
What are some factors for analytical test
– Quantitative (How much?) – Few samples – More expensive – Slow – Laboratory – External calibration/quality control
What are some factors for diagnostic test
– Qualitative (Yes/No) – Many samples – Cheap – Rapid – Field/farm – Within test quality control
What are the characteristics of a good test
Detect the abnormal state
• Tell us what is wrong
– How much detail do you need about what is wrong?
– Specificity
• Analytical test + Cut-off point Diagnostic test
• Can have too much sensitivity and specificity
How to differentiate “normal” from “abnormal”
– Above or below threshold
– Ability to detect different pathogens
• Are similar pathogens considered as the same?
• Need to know the difference when it impacts on treatment of animal
Waht does it mean by Minimises number if “mistakes”
Detect normal animal as sick
– Detect sick animal as normal
What is a good ability to have in diagnostic
differentiating the healthy and the sick
True positive for diagnostic
a positive result for a sample from a diseased animal
True negative for diagnostic
a negative result for a sample from a normal animal
False positive for diagnostic
a positive result for a sample from a normal animal
False negative for diagnostic
a negative result for a sample from a diseased animal
What is the formula of diagnositic sensitivity
– True positive/total number of diseased animals
– True positive/(true positive + false negatives)
Diagnostic sensitivity = TP/(TP + FN) in respect to sick animals
-Diagnostic sensitivity = TN/(TN + FP)
What is the diagnostic sensitivity for
Probability that diagnostic test is positive in the presence of disease
- to find out how accurate the tests are
How to set a cut-off point When healthy and diseased overlap
– Concessions have to be made
– Where the cut-off is set depends on the use of
the test:
• Diagnosis of individual animal
• Screening for disease presence
• Monitoring whether disease entered the country
When do you want to maximise true positive
When disease is treatable or medicine is safe, cost of false positive is high
When do you want to maximise true negative
disease is not treatable, cost of false positive is high
When should you have a medium specificity cut-off
when effect of false positive and false negatives is equally bad
