
Purists often distinguish between "true" clubbing and "pseudo"
clubbing, the difference being that with "true" clubbing the
angle of the nail when viewed from the side is constantly
negative when proceeding distally (towards the fingertip).
With "pseudo" clubbing, the angle is initially positive, then
negative, which is the normal situation. "Real" internists
can talk for hours about clubbing. I'm limited to a couple
of minutes.

Whether this distinction has anything to do with reality is
entirely unclear, but it is one of those things that internists
love to paw over during rounds. Supposedly, only "true" clubbing
is associated with disease. The problem is that the list of
diseases associated with clubbing is quite long, and includes
both congenital conditions and acquired disease. Since many of
these diseases are associated with cardiopulmonary problems
leading to right to left shunts and chronic hypoxemia, it is
very reasonable to get a chest xray. However, many of the 
congenital abnormalities would only be diagnosed with a cardiac
catheterization. 

The cause of clubbing is unclear, but presumably relates to
some factor causing blood vessels in the distal fingertip to
dilate abnormally. 

Clubbing is one of those things from an examination which is
a tipoff to do more extensive examination. Often, however,
the cause of the clubbing is quite apparent.