Antigen and antibody testing

Question 1

What are the 3 broad methods of diagnosing an infection?

Answer 1

• Diagnose based on clinical signs (may be pathognomonic)
• Detect antigen (from pathogen)
• Detect host response to the pathogen e.g. antibodies

Question 2

What should you be mindful of when trying to detect an antigen in the body?

Answer 2

Some antigens are only very briefly found in the body

Question 3

What should you be mindful of when trying to detect the host response to the pathogen (e.g. antibodies)?

Answer 3

This response will be delayed compared to detecting the antigen/pathogen itself

Question 4

Tests to detect the antigen (pathogen)

Answer 4

• ELISA
• PCR
• Culture (slow but allows us to further characterise the virus e.g. has it undergone antigenic drift?)
• Latex agglutination test
• Lateral flow devices

Question 5

What is a latex agglutination test? Give an example of a pathogen it is used to detect?

Answer 5

Latex agglutination test: latex beads are coated with antibodies to the pathogen’s antigen.

Used to detect Taylorella equigenitalis

Question 6

Which antibody isotype is produced rapidly after infection and is typically seen in the primary immune response?

Answer 6

IgM

It typically indicates an active infection and doesn’t remain for long after the infection has subsided.

Question 7

Which antibody isotype is likely to be present for some time after the infection has subsided?

Answer 7

IgG

Question 8

A sandwich ELISA easily detects the presence of which antibody isotype?

Answer 8

IgM

e.g. as used when screening for toxoplasmosis in cats (as you only treat active infections)

Question 9

Describe the agglutination inhibition test

Answer 9

• If the virus is present, antibodies cannot bind to the RBCs
• therefore no agglutination = virus present

Question 10

Describe the single radial haemolysis test

Answer 10

• If the sample added to agarose contains antibodies against the virus, interaction between complement and antibodies lyses the RBCs

Question 11

Describe the complement fixation test

Answer 11

• Relies on the ability of antibodies to lyse sensitised RBCs
• If there are antibodies present, complement will bind to them and there will be no free complement
• If there are no antibodies present, the complement will be free and cause lysis of cells

Question 12

Methods of testing for presence of antibodies

Answer 12

• Indirect ELISA
• Agglutination inhibition
• Single radial haemolysis
• Complement fixation test

Question 13

What does a sensitivity of 90% mean for a herd of 10 cows?

Answer 13

SnOUT = if a test is highly sensitive and you can a negative result, you can rule out the disease.

• If we have 10 infected cattle, with this test, 1 positive animal will be missed.

Question 14

What does a specificity of 90% mean for a herd of 10 cows?

Answer 14

SpIN = if you have a high specificity test, it will have a high proportion of genuine positives, so good to rule in the disease.

In this herd, there will be 1 false positive in 10 uninfected cattle

Question 15

If you were testing an individual that you wanted to treat, you would favour a test with:

a) high sensitivity
b) high specificity

Answer 15

a) high sensitivity

b) high specificity

Question 16

If a positive result of the test would result in dire consequences, you would favour a test with:

a) high sensitivity
b) high specificity

Answer 16

a) high sensitivity

b) high specificity

Question 17

True/false, if you want to perform a PCR, use a lithium-heparin tube to collect the blood sample.

Answer 17

False

Do not use lithium-heparin tube if you want to perform PCR as it will cause interference

Question 18

How long after infection might IgG be present?

a) days
b) weeks
c) months
d) only seen in active infection

Answer 18

a) days
b) weeks

c) months

d) only seen in active infection