Answered by: Enric Mateu
In general it can be considered that the performance of diagnostic tests for PRRSV is good for detecting infected animals if a good sampling strategy is applied.
The performance of a diagnostic test is usually evaluated in terms of diagnostic sensitivity and specificity. Technically, diagnostic sensitivity is defined as the proportion of true infected (diseased) animals that are detected by the test (usually classified as positive) while diagnostic specificity is the proportion of healthy animals that are correctly classified as such (namely, negative). In general, diagnostic sensitivity depends on analytical sensitivity; namely, the lowest amount of analyte (i.e. antibodies or viral particles/genomes) that the test can detect.
Regarding serological tests for PRRSV (ELISA), most of them are able to detect infected animals between 7 to 14 days after the onset of the infection and detect antibodies for more than 4-5 months. After the second week of infection sensitivity can be considered close to 100% or 100%. Regarding specificity, most ELISAs show diagnostic specificities above 97-98%, that are quite acceptable. In our experience, the agreement of individual results between different commercial ELISAs is moderate-to-good in terms of classifying infected and non-infected animals.
However, the practical performance of a test should be evaluated not only with regards to sensitivity and specificity but also in consideration to the epidemiological context. As a concept, the use of a test that is not 100% sensitive in a population with a very low proportion of infected animals would lead to the inability to detect those animals (false negative). On the contraire, the use of a less-than-100% specific test in a healthy population, sooner or later will lead to the detection of false positive animals. Our current serological tests are very good for detecting infected animals and a little less good for classifying healthy animals.
In the case of PCR, the abovementioned concepts applied the same but some considerations are to be made. Although the analytical sensitivity is very high (it is possible to detect as little as 1 viral in the PCR tube) and the specificity is close to 100% the high genetic diversity of PRRSV affects the practical performance of the test. In other words, since the virus is so diverse it is almost impossible to design a PCR detecting all possible isolates within one genotype. In our laboratory, we did some trials to assess the potential of both commercial and in-house PCRs and the best ones detected about 95-97% of the isolates within genotype 1. This means, that occasionally, some infected animals can test negative just because the diversity of the virus. However, it is important to consider that most of the failures corresponded to isolates belonging of subtypes 2-4 of Eastern Europe.
A different subject is how reliable is a given result produced by a given laboratory. In order to assure the mentioned sensitivities and specificities is important that the laboratory have adequate protocols, including a quality control protocol, and participate regularly in ring trials or collaborative diagnostic exercises in order to test internally and externally the quality of the results.
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