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)
