Elution
The neuraminidase enzyme present in the virus particle will eventually break the bond between the virus and red blood cells. This process is called elution.
When elution occurs, the red blood cells are no longer agglutinated. They roll down the side of V-bottom microwell plates to resemble the negative settling pattern, a tight button.
Some Newcastle disease virus strains elute more rapidly and the test must be read before this occurs.
Usually elution takes longer than 45 minutes. A control well with virus and red blood cells is useful to determine elution time