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Name: Cristen P.
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 2000-2001

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?

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.

I hope this helped.

Peter Faletra Ph.D.

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