Viral diagnostic techniques: what the clinician should know

Question 1

Differentiate between specificity and sensitivity

Answer 1

Sensitivity: Proportion of positive samples correctly identified

Specificity: Proportion of negative samplescorrectly identified

Question 2

Outline ELISA-test principles

Answer 2

1. Add test sample to well of ELISA plate
2. Wash well to remove unbound antigens
3. Add antibody-enzyme conjugate
4. Wash well to remove unbound conjugate
5. Add chromogenic substrate for enzyme
6. Read absorbance on microplate reader

Question 3

A screening test should have high (sensitivity/specificity) to detect all positives. A test with low (sensitivity/specificity) will give false negative results.

Answer 3

Sensitivity

Question 4

A test with low (sensitivity/specificity) will give rise to false positives; if used for screening
samples, then positive results should be confirmed using another (more specific) test

Answer 4

Specificity

Question 5

How to calculate Positive predictive value

Answer 5

no. of true positive wells/ total no. of positive wells

Question 6

What does it mean when there is a test specificity of 99%

Answer 6

one false positive per 100 tests

Question 7

Calculate Positive predictive value
Test specificity of 99%
Prevalence of infection 50%

Answer 7

no. of true positive wells/ total no. of positive wells
50/51
=98%

Question 8

Diagnostic tests for FeLV infection

Answer 8

Point of care test detects FeLV antigen in a viraemic cat
ELISA for viral antigen

Confirmatory tests
Virus isolation
Real time PCR to detect provirus DNA-qPCR
Real time PCR to detect viral RNA Reverse transcriptase (RT)-qPCR

Question 9

When to test for antibody in FIV infection in kittens

Answer 9

16 weeks after maternally derived antibodies waned

Question 10

When can virus isolation of FIV be done

Answer 10

at any age