[R] Is it possible to define another kind of NA

[Marc Girondot]

Dear member list,

In many experimental sciences, there is a lower detection limit (LDL) 
when a dosage of a product is done. Then some samples are evaluated to 
be below this limit.
I search for the best way to indicate in a data.frame that some values 
are such LDL. Ideally, an equivalent of NA would be the best.
Until now I manage by indicating all the column in characters.
So the question is: is it possible to define a value that could be named 
LDL and that could take place in vectors or data.frame such as:
v <- c(0.2, 0.28, LDL, 0.9) is the same way that NA can be used.
with of course a function is.ldl(v) that would return F F T F

Thanks a lot for any direction to solve this

Marc