PRRSV oral fluids ELISA testing at the AHL

Davor Ojkic, Jim Fairles

Animal Health Laboratory, University of Guelph, Guelph, ON

AHL Newsletter 2021;25(1):12.

Oral fluid samples are non-invasive to pigs, simple and cost-effective to collect.  These characteristics have made them attractive as samples for detecting antibodies against porcine reproductive and respiratory syndrome virus (PRRSV).  According to manufacturer’s data, the IDEXX PRRSV oral fluid ELISA has a diagnostic sensitivity of 100% and a diagnostic specificity of 98.7% when the manufacturer's cut-off of sample to positive (S/P) ratio ≥ 0.4 is used.  Because the test specificity is less than 100%, it will occasionally generate false-positive results.  It has also been reported that non-specific positive oral fluid ELISA results can be generated in PRRSV-negative pigs that were fed with a feed containing spray-dried porcine plasma (1).

Regardless of the cause, unexpected positive results have limited the application of oral fluid ELISA for PRRSV surveillance.  A recent study suggested that increasing the manufacturer’s ELISA cut-off to S/P ratio ≥ 0.8 improved the specificity of the test and decreased the chance of false-positive reactivity in presumed-negative herds (2).  In another study, raising the cut-off to S/P ratio of 1.0 increased the oral fluid ELISA specificity to 99.2%, but still did not completely remove false-positive reactivity.  However, as a consequence of raising the cut-off, the test sensitivity decreased to 96.5% (3).

Therefore, results of IDEXX PRRSV oral fluid ELISA tests performed at AHL will continue to be reported using the manufacturer's cut-off of S/P ratio ≥ 0.4 which is appropriate for diagnostic applications and ensures the optimal test sensitivity.  While we have considered a different cut-off for surveillance applications, our accredited status does not permit us to alter the current kit recommendations.  We have reached out to the manufacturer who is aware of the situation and is investigating possible options to increase the utility of the test for both diagnostic and surveillance purposes.

When performing the oral fluids PRRSV ELISA in presumed-negative herds, practitioners may use their discretion to consider a cut-off of S/P ratio ≥ 1.0 to increase the test specificity for surveillance purposes.  To support this usage, the following comment has been added to all AHL PRRSV oral fluid ELISA test reports:

"Test results are interpreted using the manufacturer's cut-off which is appropriate for diagnostic applications:

Negative: S/P ratio less than 0.400

POSITIVE: S/P ratio equal to, or greater than 0.400

However, S/P ratio cut-off equal to, or greater than 1.000 may be suitable to increase the test specificity for surveillance in presumed-negative herds (Henao-Diaz A, et al. Prev Vet Med. 2021. Adapting a PRRSV oral fluid antibody ELISA to routine surveillance).  

Unexpected positive results should be further investigated by additional testing with PRRSV serum ELISA and/or PRRSV PCR.”   AHL

1. Johnson et al. Exogenous source of PRRSV antibody in positive oral-fluid ELISA results. J Swine Heal Prod 2012:20:215.

2. Crofet et al. Field application of a commercial PRRSV oral fluid antibody ELISA. Can Vet J 2020:61L420-423

3. Henao-Diaz A et al. Adapting a PRRSV oral fluid antibody ELISA to routine surveillance. Prev Vet Med 2021:188:1-6