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for calculation of virus/antibody titres (Karber,
1931)
The quantity of virus in a specified suspension volume (e.g., 0.1 ml)
that will infect 50% of a number (n) of cell culture microplate wells,
or tubes, is termed the Tissue Culture Infectious Dose 50 [TCID50]. If
the TCID50 is equivalent to a virus titre of 10m infectious doses, then
log TCID50 = m. This expression for the '50% endpoint dilution' is also
applicable to antibody titres measured in neutralisation tests.
In a test to measure the 50% endpoint dilution of, for example, a virus
suspension, individual cell monolayers are inoculated with a dilution
series of the original suspension. Each dilution is inoculated onto n
separate cultures, and observations are made of the number of ensuing
infected monolayers (as a proportion of n) for each dilution.
The calculation of TCID50 can be illustrated by reference to the set of
figures in the following table :
VIRUS DILUTION PROPORTION (p) OF
INFECTED CULTURES
(n = 5)
10 -1 5/5 = 1
10 -² a 5/5 = 1
10 -³ 3/5 = 0.6
10 -4 1/5 = 0.2
10 -5 b 0/5 = 0
10 -6 0/5 = 0
The TCID50 is given by the simple equation:
log TCID50 = m = x a - D (S p - 0.5)
where xª is the last dilution index for which all n cultures are infected
(p = 1)
D is the log of the dilution factor (log 10 = 1)
Sp is the summation of p between the last dilution for which all n cultures
are infected (p=1) and the first dilution for which all n cultures are
unaffected (p=0). [These dilutions in the above table lie between a and
b inclusively].
In this example:
log TCID50 = - 2 - (1)(1.62 - 0.5) = - 3.12
The titre of this virus suspension would thus be written as 10-3.12 TCID50
/ 0.1 ml, corresponding to a dilution of 1/1320
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