Estimation of specificity and sensitivity of three diagnostic tests for infectious salmon anaemia virus in the absence of a gold standard

Abstract

Reverse transcriptase-polymerase chain reaction (RT-PCR), virus isolation (VI) and indirect fluorescent antibody test (IFAT) are three tests currently used by the salmon industry to identify fish infected with the infectious salmon anaemia virus (ISAV). However, very limited information is available on the sensitivity and specificity of these methods. In order to evaluate these tests in fish representing a range of farmed Atlantic salmon populations, five laboratories participated in a blind study of 400 kidney samples from four groups of fish with different prevalences of ISAV. Each laboratory used its own testing protocols. Estimates of the specificity of each test were determined directly from a population assumed to be free of infection. Indirect estimates of the sensitivity and specificity of each test were obtained using maximum likelihood estimation of a latent class model (i.e. no gold standard test result available). There was a substantial difference in sensitivity and specificity of RT-PCR among the three laboratories using this test. If only the best results for the RT-PCR tests are taken into account, the maximum likelihood estimates obtained from this study suggest RT-PCR and VI are of similar high sensitivity (range 92-100%), IFAT is the least sensitive method (range 65-76%) while the three tests have similar high specificities (range 96-100%). The results of the study suggest: (1) RT-PCR tests should be standardized before they are used as a diagnostic test for prevention and control of ISAV, (2) the sensitivity of VI was higher than expected and (3) IFAT has a low sensitivity but might be a good screening test because of its low cost.

Diagnostic validation of three test methods for detection of cyprinid herpesvirus 3 (CyHV-3)

“…Uneven distribution of CyHV-3 MB2010 virions in the kidney may also have affected test precision estimates since tissues were not homogenized prior to sample distribution into tubes for our study. It was expected that this impact would be more evident with same-fish samples with a low virus load (Nérette et al 2005, Caraguel et al 2009, 2012. In our study, the concordance correlation plots for duplicate sample sets tested by 2 laboratories revealed greater variation in results reported for samples with low Cq values, suggesting that the precision of the test was reduced for samples with a high virus load (data not shown).…”

“…Low pathogen loads may also confound electrophoretic gel reading for the cTK test. Gel reading subjectivity has been presented as a possible explanation for the low precision reported for cPCR tests in other validation studies (Nérette et al 2005, Caraguel et al 2009). Uneven distribution of CyHV-3 MB2010 virions in the kidney may also have affected test precision estimates since tissues were not homogenized prior to sample distribution into tubes for our study.…”

Detection of infectious haematopoietic necrosis virus and infectious salmon anaemia virus by molecular padlock amplification

“…The current guidelines determining the officially recognized criteria for confirmation of IHNV and ISAV infection in fish, as established by the OIE, require monitoring clinical, pathological, histopathological and haematological changes, as well as detection of viral particles via virus isolation (VI), indirect immunofluorescent antibody testing (IFAT) and RT‐PCR (Office Internationale des Epizooties 2003). Five laboratories participating in a blind study of 400 ISAV‐infected kidney samples representing four populations of farmed Atlantic salmon to compare the precision (Nerette, Dohoo, Hammell, Gagne, Barbash, MacLean & Yason 2005b) and accuracy (Nerette, Dohoo & Hammell 2005a) of these methods concluded that none can be used as a ‘gold standard’ for viral detection. Found to be highly accurate and highly precise, VI studies are typically expensive to conduct and, although possible to accomplish in as little as 1 week, may take 30–45 days to complete.…”

The application of epidemiology in aquatic animal health -opportunities and challenges

“…However, the test characteristics of relatively few diagnostic tests have been validated under field conditions and in the absence of a gold standard, the exception being tests for infectious salmon anaemia [99,100]. The techniques and expertise exists; once again the main constraint is the lack of resources to test a sufficient number of populations (with different levels of disease).…”