interpretation of serological test practicals

Question 1

what do we use diagnostic methods for

Answer 1

diagnosis
surveillance and prevention

Question 2

sensitivity

Answer 2

likelihood that an animal known to have a particular disease or infection will give a positive test result
highly sensitive = few false negative results
sensitivity is the ability to detect a disease if it is really present

Question 3

specificity

Answer 3

likelihood that an animal known to be free of the particular disease or infection will give a negative test result
highly specific = few false positive results
specificity is the ability to exclude animals which do not have the disease

Question 4

as sensitivity increases…

Answer 4

specificity decreases and vice versa

Question 5

how to choose assay

Answer 5

depends on prevalence
repercussion of a positive result
what do we want to know
which level of accuracy
in which time
how much can we spend

Question 6

PPV

Answer 6

portion of positive results that are true positive

Question 7

NPV

Answer 7

portion of negative results that are true negative

Question 8

gold standard

Answer 8

gold standard test is the most accurate diagnostic assay at this point of time under no restrictions (antemortem, results as quickly as possible, reasonable cost)

Question 9

two tier titre strategy comparison

Answer 9

titer value cannot be compared between 2 different methods

Question 10

parallel testing

Answer 10

two screening tests performed at the same time and the results are combined
higher sensitivity but lower specificity

Question 11

serial testing

Answer 11

second screening test is performed only if the result of the first screening test is positive
improves specificity but lower sensitivity

Question 12

serological methods types

Answer 12

qualitative - AGID, ELISA
quantitative - IHA, CFT, VN-test, ELISA

Question 13

paired sera

Answer 13

testing of 2 sera samples from same animal in order to confirm acute infection
fist sample ASAP, second sample 7-14 days later
paired sera are required when the only test available to detect a certain disease can only detect specific IgG or total antibodies
4-fold increase in titre is diagnostic for acute infection

Question 14

why 4-fold increase in titre

Answer 14

samples can differ - dehydration, transport
tets results can differ - variations in procedure, subjectivity go reading

Question 15

paired sera -what is a positive result

Answer 15

4-fold increase In titre is diagnostic for acute infection
statistically significant difference in titre even though sample and reading can be variable