# TCD

karlster karlster at excite.com
Sat Sep 25 17:32:24 EST 1999

In article <Pine.GSO.3.95.990923162959.27976J-100000 at parasite>, Ruth B
Hamilton <ruthh at zoo.cam.ac.uk> wrote:

> I have been reading some fairly old virology papers which have measured viral
> titres in log(10)TCD (I have also seen TCID). So far my searches of textbooks,
> journals and websites haven't been able to define what this means.  Any
> explanation would be very helpful.
> Ruth

[posted and mailed]

Hello Ruth in Cambridge,

TCID stands for "tissue culture infectious dose"; it is/was used as an
alternative to determining virus titre by plaqueing (which gives values as
PFUs or plaque-forming units). Typically a series of dilutions of a virus
preparation is made and a given volume is inoculated in replicates
(usually between 3 and 5 tube cultures). After an appropriate time of
incubation, dependent on the type of virus, the cultures are scored for
presence or absence of demonstrable cytopathic effect or other marker of
virus infection. Based on the scoring of replicates a titre can be
calculated based on a formula for limiting dilution by Karber. The formula
is as follows:

log (base 10) TCID<50%> = - [ X<subscript p=1> + 0.5d - d(sum of p) ]

where

X<subscript p=1>  stands for the highest dilution giving 100% of tubes
positive for infection

d    stands for log(base 10) dilution interval (ie. if using 10-fold
dilutions
then d=1)

p    stands for the proportion positive for infection at any given dilution

sum of p    stands for the sum of values of p for X<subscript p=1> and all
*higher* dilutions.

To put this into context of a practical example, here's the results of a
hypothetical experiment.

A virus preparation of unknown titre is subjected to serial 10-fold
dilutions (so d = 1) and inoculated into tube cultures at five tubes per
dilution. The cytopathic effect scoring after Q number of days is:

Dilution       Proportion of tubes positive     p
10(-3)                 5/5                    1
10(-4)                 5/5                    1
10(-5)                 3/5                   0.6
10(-6)                 2/5                   0.4
10(-7)                 0/5                    0
(sum of p) = 1+0.6+0.4+0 = 2

Using the formula above the equation would yield the following

log (base 10) TCID<50%> = - [(-4) + 0.5(1) - (1)2]
log (base 10) TCID<50%> = 5.5

as such, the original virus suspension would be said to have an infectious
titre of 10^5.5 or 3.1622 X 10^5 TCID<50%>/volume of inoculum - the above
example makes this value sort of intuitive since a "perfect" dilution
which would infect 50% of the tube cultures would lie between 10(-5) (60%
positive) and 10(-6) (40% positive).

Hope this helped.
Before I forget....if you wish to report the titre per millilitre remember
to factor in the volume of inoculum each tube culture received.

Kind regards

Karlster