Primary keys: can they be alphanumerical?

Dear All,

I'm using postgreSQL to create a database of breast cancer patient data. I
am currently designing my tables.

I would like to know what is the general concensus on having alphanumerical
primary keys.

The reason for this is that, with a 15 character long alphanumerical key, I
could include hidden information in the key which could help users to
quickly get information without even having to search the database. For
example, the first to characters could be a two-letter code for the
refering hospital, the next four a numerical patient number and subsequent
characters refer to treating clinicians etc.

Is this a major 'faux-pas'? Am I introducing unwanted redundancy in the
key? Will it make indexing a lot slower? Should primary keys be purely
numerical?

I would be grateful for any comments/suggestions.

Best regards,

Stuart.

PS. could you cc me a copy of your posting(s).

+-------------------------+--------------------------------------+
| Stuart Rison | Ludwig Institute for Cancer Research |
| Tel. (0171) 878 4041 | Courtauld Building |
| Fax. (0171) 878 4040 | 91 Riding House Street |
+-------------------------+ London, W1P 8BT |
| stuart(at)ludwig(dot)ucl(dot)ac(dot)uk | UNITED KINGDOM. |
+-------------------------+--------------------------------------+