title: An estimation of the overlap bias in plaque assay.
abstract: A systematic underestimation of the density of PFU's of a virus suspension is caused by overlapping plaques. The degree of this bias depends on n, the number of plaques distributed over the petri dish and on d, the (mean) diameter of the plaques. The mean number of overlapping events, y, is proportional to n2 and to d2, y = cn2d2. If the experimental circumstances yield values of n or d or of both, which are too large, incorrect inferences may be drawn from the outcome of a quantitative plaque test. For example, there is danger that an existing difference in the concentration of two virus-suspensions will not be detected. It is of interest, therefore, to evaluate c, the factor of proportionality. From this an equation may be derived which relates the number of observed plaques (including the unknown number of overlapping events) directly to the expected true number of plaques on the dish. Furthermore, upper limits of the number of observed plaques, which should not be exceeded in experimentation to avoid any overlap are given at several levels of d for 5-cm dishes. The evaluation of c was performed by a set of Monte Carlo procedures.
authors.affiliation: Bundesforschungsanstalt für Viruskrankheiten der Tiere in Tübingen, Germany.
authors.forename: R J
authors.initials: RJ
authors.lastname: Lorenz
authors.forename: B
authors.initials: B
authors.lastname: Zoeth