L3: Diagnosis of viral infections

Question 1

What is the difference between sensitivity and specificity?

Answer 1

Sensitivity = (True positive : True positive + False negative)

Specificity = (True negative/true negative + false positive)

Question 2

What are the requirements of diagnostic procedures? (8)

Answer 2

• tests are standardised
• miniaturised assays (for reduced cost and time)
• cheap
• accurate
• simple
• automated (high throughput)
• objectivity in recording/reporting

Question 3

If something is 98% sensitive, what does it mean?

Answer 3

2% of patients with disease will receive normal results

Question 4

If a test is 95% specific, what does that mean?

Answer 4

5% of positive results are incorrect diagnoses

Question 5

Name 3 methods of viral diagnosis.

Answer 5

• Direct = cell culture, EM, viral Ag detection, ELISA, hemagglutination assay
• From IR = serological testing

Question 6

What is LD50?

Answer 6

Lethal dose 50% = dilution of the virus needed to kill 50% of inoculated animals (changes with different means of administration)

Question 7

What is TCID50?

Answer 7

Tissue culture infectious dose 50% = the amount of virus needed to observe cytopathic effects in 5-% of dish(es). Note this measures lysis as well as other CPEs (eg syncitia)

Question 8

What results might you expect from a hemagglutination assay?

Answer 8

Uninfected cells = pellet forms

Infected cells = lattice forms and cells stay in suspension

Question 9

What are hemagglutination assays based on?

Answer 9

Some (hemadsorbing) viruses contain proteins that bind RBCs together.