How do I find the value of a field's datatype

In code below, I have not been able to interrogate the value of query1.fieldbyname(''+vfield+'').datatype.  The value type in the SQL Database is ntext.  When I retrieve the field via the sql statement and trace through the Case statement, my code drops to the end statement without stopping at any of the "ft.."  values.  

  with Query1 do
  begin
    close;
    sql.clear;
    sql.Add('Select '+vfield);
    sql.Add('from '+vTable);
    sql.Add('where code = '+inttostr(vCode));
    open;
      case query1.fieldbyname(''+vField+'').datatype of
        ftString: result := fieldbyname(''+vField+'').AsString;
        ftInteger: result := fieldbyname(''+vField+'').asInteger;
        ftFloat: result := fieldbyname(''+vField+'').asFloat;
        ftBoolean: result := fieldbyname(''+vField+'').asBoolean;
        ftMemo: result := fieldbyname(''+vfield+'').AsString;
        ftBlob: result := fieldbyname(''+vfield+'').AsString;
        ftGraphic: result := fieldbyname(''+vfield+'').AsString;
        ftUnknown: result := fieldbyname(''+vfield+'').AsString;
        ftFmtMemo: result := fieldbyname(''+vfield+'').AsString;
        ftVarBytes: result := fieldbyname(''+vfield+'').AsString;
        ftBytes: result := fieldbyname(''+vfield+'').AsString;
        ftBCD: result := fieldbyname(''+vfield+'').AsString;
        ftParadoxOle: result := fieldbyname(''+vfield+'').AsString;
        ftDBaseOle: result := fieldbyname(''+vfield+'').AsString;
        ftTypedBinary: result := fieldbyname(''+vfield+'').AsString;
        ftWord: result := fieldbyname(''+vfield+'').AsString;
        ftSmallint: result := fieldbyname(''+vfield+'').AsString;
        ftCurrency: result := fieldbyname(''+vfield+'').AsString;
        ftDate: result := fieldbyname(''+vfield+'').AsString;
   end;
end;