Question:
I am teaching an intro bio course lab and we did an
experiment to determine the LD50 of NaCl on brine shrimp. There is a bit
of debate among the TA's over the correct definition of LD50. I say that
it is determined in a time point. Therefore, if our experiment was one
hour it would be the concentration of NaCl that killed 50% of organisms
in the 1 hour time point. Am I right?
Replies:
I cannot say you are wrong but I do not know the other side of the
"argument" so I will not say whether the others are wrong. The term LD-50
is a general term (no hard definition) to mean the point in an experiment
where 50% of the experimental population dies. The parameters of the
experiment might have a discrete time point involved or might have an
extended time...over a period of minutes, days ... weeks. The death most
often (in my experience) involves a dose-response which will then play into
what is the defining limit of the LD-50, which is the case in your
experiment. As a point of logic, a time period or limit would usually be
assigned to the experimental group that describes the terms under which the
LD-50 will be measured along with how death determined. It might be
possible to have an LD-50 experiment measuring death where death is an all
or none response independent of time...but I think this would be quite
unusual.
In your case you have two limits on the LD-50...time and salt concentration.
If you measure death at a certain point in time against increasing salt
concentrations that is one approach....which is also the far more typical
approach. If you keep the salt concentration steady and measure when 50% of
the population has died over successive times it is unusual but still a
valid experiment, as long as your control is not dying at the same time...of
old age.
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