MOI, pfu, and TCID50

  • Plaque forming units (pfu) is a measure of number of infectious virus particles. It is determined by plaque forming assay.

  • Multiplicity of infection (moi) is the average number of virus particles infecting each cell.

  • TCID50 is the tissue culture infectious dose which will infect 50% if the cell monolayers challenged with the defined inoculum. If the titer is "103 TCID50/0.2 ml, MK, 2 days," it means that when a 0.2 ml inoculum of a 1:1,000 dilution of the virus is added to each of four tubes containing monkey kidney (MK) cells, two tubes are expected to become infected.

    MOI is related to pfu by the following formula:

    Multiplicity of infection (moi) = Plaque forming units (pfu) of virus used for infection / number of cells.

    For example, if 2x106 cells is infected by 50 ml of virus with a titer of 108 pfu/ml. The moi will be 0.05*108/2*106 = 2.5.

    The fraction of cells that are not infected is

    P(0) = 1 - e-moi,

    i. e., 8% for moi = 2.5.

    To ensure 99% of cells are infected requires moi > 4.6.

    Assume the conditions used for plaque assay and TCID assay don't alter the expression of infectious virus, TCID50/ml and pfu/ml are related by

    pfu/ml = 0.7 * TCID50.

    As a working estimate, one can use

    pfu/ml = 0.5 * TCID50.