We DO know that Alan has at least one autoimmune condition--though, admittedly, psoriasis is not particularly associated with positive ANA titers--still, while there can be ANA "false positives", it is worth checking out some of the more specific tests to see what kind of ANA is showing up (speckled, bouble-banded, etc.) as these have greater/lesser associations with various types of autoimmune disorders.

Take a look at:

http://en.wikipedia.org/wiki/Anti-nuclear_antibody

http://www.rheumatology.org/practice...itions/ana.asp