In addition, if you want to get rid of case statements you could use

SELECT @where_clause = @where_clause + 

ISNULL(@return+' AND Case_no# = '''+@param1+'''','')+

ISNULL(@return+' AND last_name = '''+@param2+'''','')+

ISNULL(@return+' AND [First name] = '''+@param3+'''','')+

ISNULL(@return+' AND Determination = '''+@param4+'''','')+

ISNULL(@return+' AND sender = '''+@param5+'''','')+

ISNULL(@return+' AND [Case Type] = '''+@param6+'''','')+

ISNULL(@return+' AND Date = '''+CAST(@param7 AS nvarchar(20))+'''','')+

ISNULL(@return+' AND [Qwik Due] = '''+CAST(@param8 AS nvarchar(20))+'''','')+

ISNULL(@return+' AND [Qwik sent date] = '''+CAST(@param9 AS nvarchar(20))+'''','')+

ISNULL(@return+' AND [Final due] = '''+CAST(@param10 AS nvarchar(20))+'''','')+

ISNULL(@return+' AND [Final sent date] = '''+CAST(@param11 AS nvarchar(20))+'''','')+

ISNULL(@return+' AND [on hold code] = '''+@param12+'''','')

 

doubt if better performance