The datareader.IsDBNull propertie can be used to test for NULL values against columns.
While a simple numeric index will work, it is better to test against the column names, because your code will still function correctly in the event of new fields appearing in the database changing the field numbers.
If datareader.IsDBNull(datareader.GetOrdinal("fieldname")) Then
' handle NULL value
Else
' handle field with value
fieldvalue = datareader("fieldname").ToString
End If