  The infectivity of a virus sample is measured by the infections it causes,
via a plaque or focus forming assay (PFU or FFU) or an endpoint dilution (ED)
assay (TCID$_{50}$, CCID$_{50}$, EID$_{50}$, etc., hereafter collectively
ID$_{50}$). The counting of plaques or foci at a given dilution intuitively and
directly provides the concentration of infectious doses in the undiluted
sample. However, it has many technical and experimental limitations. For
example, it relies on one's judgement in distinguishing between two merged
plaques and a larger one, or between small plaques and staining artifacts. In
this regard, ED assays are more robust because one need only determine whether
infection occurred. The output of the ED assay, the 50% infectious dose
(ID$_{50}$), is calculated using either the Spearman-Karber (SK, 1908,1931) or
Reed-Muench (RM, 1938) mathematical approximations. However, these are often
miscalculated and their ID$_{50}$ approximation is biased. We propose that the
PFU and FFU assays be abandoned, and that the measured output of the ED assay,
the ID$_{50}$, be replaced by a more useful measure we coined Specific
INfections (SIN). We introduce a free, open-source web-application, midSIN,
that computes the SIN concentration in a virus sample from a standard ED assay,
requiring no changes to current experimental protocols. We demonstrate that the
SIN/mL of a sample reliably corresponds to the number of infections the sample
will cause per unit volume, and directly relates to the multiplicity of
infection. midSIN estimates are shown to be more accurate and robust than those
using the RM and SK approximations. The impact of ED plate design choices
(dilution factor, replicates per dilution) on measurement accuracy is also
explored. The simplicity of SIN as a measure and the greater accuracy of midSIN
make them an easy, superior replacement for the PFU, FFU, and ID$_{50}$
measures.
