Titration of virus spiked samples
To determine the virus titer, serial threefold dilution of the samples were made with cell culture medium. 0.1m aliquots of each dilution were added to 8 wells of a 96 well microtiter plate with adherent cells (in 100 µL cell culture medium per well).
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The test items (buffer or protein solution) were spiked with 5 % virus stock solution

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Note added at 8 ore (2020-05-03 07:32:51 GMT)
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En este único ejemplo en la red que he podido encontrar con la expresión original ("Titration of virus spiked samples"), se hace referencia a la titulación (viral) de muestras enriquecidas con (el) virus.

Sin embargo, la ausencia de contexto original no permite ofrecer un alto nivel de seguridad...

Saludos varios.