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How good are the diagnostic tests for infectious salmon anaemia virus?
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Virus-induced anaemia is a big problem in salmon farms. Currently there are three diagnostic tests available on the market and there seems to be no `gold standard`. This very interesting article compares specificity and sensitivity of these three procedures. Which method is the best? The disappointing result: None of them - all need to be improved...
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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 92100%), IFAT is the least sensitive method (range 6576%) while the three tests have similar high specificities (range 96100%).
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.
Source: Nérette, P, Dohoo, I & Hammell, L (2005)
Estimation of specificity and sensitivity of three diagnostic tests for infectious salmon anaemia virus in the absence of a gold standard. In: Journal of Fish Diseases 28 (2), 89-99.
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