(Sorry, KSR23. I didn't mean for that post to come off so bluntly, it wasn't pointed at you - or anyone else.)
A TB blood test is available although not widely, check with your local health department, called QuantiFERON-TB Gold. (QTF-G)
Tests can be performed wrong, read incorrectly and other factors can influence testing like allergy, other illnesses, vaccines and time of exposure. These can lead to false-positive and false-negative skin tests.
That said, there is no such animal as "exposure TB" or "old TB". It's like being "a little pregnant"
if someone's skin test is truly
positive, it can either active or latent, but they are infected.
CDC fact sheet on skin testing, including false-positive and false-negative results [url]http://www.cdc.gov/TB/pubs/tbfactsheets/skintesting.htm[/url]