PRRSV Diagnostics, specificity or PRRSV Diagnostics, specificity or sensitivity, that’s the questionsensitivity, that’s the question

Eric van EschEric van Esch

Technical director Technical director BioChek BV, Reeuwijk, Netherlands.BioChek BV, Reeuwijk, Netherlands.

PRRSV diagnostics

• Definitions

• Reasons false positives or false negatives

• Setting cut off

• Conclusions

Diagnostics, definitions

• What has to be detected?

– Antigen• Different types• Different strains

– Antibodies• Different classes

Diagnostics, definitions

• Setting up an assay– Depending on what has to be detected

• Antigen / Antibodies Which technique?• Matrix (blood, tissue, oral fluids, sperm, etc)

– Additional questions• Discrimination between field infected and vaccinated animals?• Discrimination between types / strains?• Detection limits?• Required sensitivity and specificity?• Regulations (national, OIE)?• ……

Diagnostics, definitions

• In what situation will the assay be used?

– Clinical diagnosis• Antigen present on farm, clinical cases from time to time

– Herd screening• Establishing herd status, clinical cases present or not

– Eradication• Elimination of agens from the farm

– Monitoring• Control of freedom of disease

– Vaccine control• Success of vaccination

Diagnostics, definitionsInfected

Yes No

TestResult

Positive a b a+b

Negative c d c+d

a+c b+d a+b+c+d

• Prevalence = a+c/a+b+c+d

• Sensitivity = a/a+c

• Specificity = d/b+d

• Predictive value positive = a/a+b

• Predictive value negative = d/c+d

Diagnostics, false positives and false negatives

• Selection of materials– ELISA antigen• Whole virus vs recombinants

– PCR primers• Type specific? General?

Diagnostics, false positives and false negatives

• Reasons of false negative results– Test itself– Time of sampling (too soon or too late)– (Very) low level of antibodies / antigen– Immunosuppression– Prozone, high dose Hook effect

Diagnostics, false positives and false negatives

• Reasons of false positive results– Test itself– Cross reaction with antibodies against related

antigen – Cross reactions by other factors– Bridging, non-specific adhesion (fat)

Diagnostics, setting cut off

Diagnostics, setting cut off

Diagnostics, setting cut off

Diagnostics, setting cut offClinical

diagnosisHerd

screeningEradication Monitoring

freedom of disease

Vaccine control

Prevalence 0 – 100% 0 – 100% 100 0% 0% 0 – 100%

Specificity + + + ++ +

Sensitivity ++ + ++ + +

Diagnostics, setting cut off

• Interpretation– Technical experimental samples– Diagnostic field samples

• Could be different!

• Baselines– Natural infected animals– Vaccinated animals

Conclusions, take home messages

• There is not one universal test

• Parameters Specificity and Sensitivity are important, but are competing against each other

• In different situation requirements are different, therefore parameters can change

• Evaluation of an assay should be done in each lab with respect of the purpose of the assay

Thank you for your attention!Thank you for your attention!