How to use \Q \E in GREP in a case-insensitive way?

[Stormy]

however, when I try to make grep match in a case-insensitive way, using -i flag
there are no matches, here's an example:

sh-3.2# echo "TeST" | grep -P test   ==> No match, expected
sh-3.2# echo "TeST" | grep -iP test  ==> Match, expected
TeST
sh-3.2# echo "TeST" | grep -P '\Qtest\E'   ==> No match, expected

sh-3.2# echo "TeST" | grep -P '\QTeST\E'   ==> Match, expected
TeST
sh-3.2# echo "TeST" | grep -iP '\Qtest\E'  ==> No match, UNEXPECTED

I would expect the last example to match, since the -i flag was passed.  I 
rather stay with grep than to switch to perl or parsing the large files 
manually.

Thanks.