Is postcard marketing effective?

Not really. I think if you want to keep your name in prospective client's offices you need to use some type of marketing tool that is not quickly looked at then thrown away. While you can state more about your lab and it's services on a postcard, most are lumped in with junk mail. Pens, notepads, and other "usable" office items will keep your contact information in the offices for a longer period of time.