Concat the resulting value in mysql

Concat the resulting value in mysql

Can i concat result value and then send the final value as output.
WHILE(LENGTH(totalexpenseamount )>0) DO
BEGIN
SET totalshipmentexpenseamount =
CONCAT(totalshipmentexpenseamount,',',indshipmentexpenseamount);
END;
END WHILE;
but at end totalshipmentexpenseamount does not have any value in it.