Access: Custom concatenation of fields

A table in Access 2000 contains the fields [Last Name] and [First Name]. On the report form design, the basic concatentation of the field is [Last Name]&”, “&[First Name]. If [Last Name] is blank, I want the report to display only [First Name] without the preceding comma. I realize this may require a VB macro or prior query. I just don’t know what I need.