LT50 is lethal time. So, at a specified dose (or dosage), LT50 is the time point at which mortality of inoculated hosts is 50%.  Example: At a dosage of 1000 infective units, 50% of hosts have died at 10 days post inoculation.  LT50 for 1000 units is, therefore 10 days. If the dosage is 10,000 and 50% have died at 5 days, then LT50 for 10,000 units is 5 days. 

LD50 is lethal dose (dosage. So at a specified time post inoculations, it is the dose (dosage) at which 50% of hosts die. It is calculated by inoculating hosts with a series of dosages and calculating mortality using regression analysis. (Time must be specified such that the LD50 is not being calculated based on normal lifespan, so could be 1-2 days post inoculation for acute toxins or pathogens, e.g. Bacillus thuringiensis Cry proteins, usually longer for most living pathogens. Time, however, is not the focus for LD50.)

Yes.  In general, we use the Probit Analysis for both.

However, if the mortality increase after LT50, it means the LT50 is inappropriate.  For example, if the LT50 is 4 days at 500ppm, but after 5 days the mortality at dose 500ppm increases to 60 or 70%.  Then, the LT50 = 4days is meaningless.

LT50 is lethal time. So, at a specified dose (or dosage), LT50 is the time point at which mortality of inoculated hosts is 50%.  Example: At a dosage of 1000 infective units, 50% of hosts have died at 10 days post inoculation.  LT50 for 1000 units is, therefore 10 days. If the dosage is 10,000 and 50% have died at 5 days, then LT50 for 10,000 units is 5 days. 

LD50 is lethal dose (dosage. So at a specified time post inoculations, it is the dose (dosage) at which 50% of hosts die. It is calculated by inoculating hosts with a series of dosages and calculating mortality using regression analysis. (Time must be specified such that the LD50 is not being calculated based on normal lifespan, so could be 1-2 days post inoculation for acute toxins or pathogens, e.g. Bacillus thuringiensis Cry proteins, usually longer for most living pathogens. Time, however, is not the focus for LD50.)

Dear Raad

We use essentially the same methodology for both in our bioassay testing and we also use log-probit analysis for both.  For LT50  the important factor is to settle on a discriminating dose of the toxin, i.e. one that will, at least in theory, kill most or all of the susceptible organisms but which potentially will not eliminate all the tolerant or resistant ones, and to use that discriminating dose consistently.  

This approach is the basis of the WHO insecticide testing system for susceptibility/resistance in which standard test papers are used that are prepared in a single laboratory and distributed to interested researchers.  That process is not without its flaws but for most purposes serves quite well.  In our lab we generally use test papers made up to our own recipe because we find that the solvents used by WHO can have either an inhibitory effect on bioavailability or in some cases can artificially enhance activity.

However, I must say I disagree with the point made by Hsin Chi.  If the LT50 of a toxin is, for the example given, 4 days at 500ppm and then on the 5th, 6th, or a later day increases to 60%, 70% or some other figure it does not invalidate the LT50 result.  

In an ideal world the aim is to identify both LT50 and LT95 as the benchmark activity point, in much the same was as for dosing we look for LD50 and LD95.  So in the test, assuming there is no mortality due to ageing, starvation, dehydration, etc., it should run until there is no increase in mortality as a result of intoxication.

hope this helps, Ian

If the insects are treated with LD50, we will expect 50% mortality.  The mortality will not increase with time. We can expect the 50% survived will live normally. 

If you use 1000ppm to treat your insects and record the mortality at different time, then you can calculate the LT50 by using Probit analysis.  If you get LT50 = 2 days.   It means 50% insects will die after 2 days.  The problem is:  If 1000 ppm will cause 70% mortality after 3 days and 90% mortality after 5 days.  Then we have to be very careful about the LT50 = 2 days, because it is biologically meaningless.  It is why LT50 is usually used for bacteria or virus pesticide.  LT50 based on LD50 will be more rational.

Since LT50 is normally an estimate based on a series of bioassay tests using a fixed dose of toxin, all of the test replicates must at some point pass 50% of the organisms killed.  You do not stand and watch the test chambers until you see 50% of the organisms fall over and then proclaim that as your LT50 for the simple reason that if you run the same experiment on a different day you are likely to obtain a different result due to intra-population variation, even if you are capable of duplicating all the other parameters exactly.  This is why you should perform several replications of the experiment - minimum five but preferably at least twice this number and then perform probit analysis on the data.

I find so many of my colleagues who are familiar with LD50, which is often quite repeatable, falling into the trap of assuming that LT50 is conceptually identical.  It is not, and can be quite variable as a result of unpredictable intrinsic and extrinsic factors that we cannot control precisely.  So LT50 is only a guide to activity not an absolute measure.  However, it is a useful guide because in practice we need to know how rapidly a toxin works in order to design our regimen of post treatment follow ups and monitoring.

I agree with Ian Burgess.

We should have care about LD50. This indicates a dose that kills 50% of a popululation, at a specific time. In ticks we evaluate mortality 24 hours posexposition to an insecticide. After 48 or 72 hours we will observe more mortality. Then LT50 and LD50 should be complementary to measure insecticide activity, using log-probit analysis in both.

Dear Ruben , You are right sir ,using linear ,exponential or log-probit analysis;Thanks

Yes it is same

LD50 and LT50 both have importance. LD50 gives the idea about dose and LT50 decides the duration of suffering before death. Both factors must be considered   critically.  Log-probit analysis should be useful.

Sir,

As per my opinion you should go for SPSS probit analysis.

Calculation remain same for LT50 also as you have done for LD50.

LD depicts mortality verses concentration. LT mortality verses time. Do as in case of probit or through regression analysis