[R] Creating NA equivalent

[Marc Girondot]

Dear members,

I work about dosage and some values are bellow the detection limit. I 
would like create new "numbers" like LDL (to represent lower than 
detection limit) and UDL (upper the detection limit) that behave like 
NA, with the possibility to test them using for example is.LDL() or 
is.UDL().

Note that NA is not the same than LDL or UDL: NA represent missing data. 
Here the data is available as LDL or UDL.

NA is built in R language very deep... any option to create new version  
of NA-equivalent ?

Thanks

Marc