Clubbing is a build-up of tissue in the fingers, causing the end of the fingers to become enlarged and the nails to curve downward. This results in the fingers appearing bulb-like, or like an upside down spoon.
The most common cause of clubbing is lung cancer, but it is also seen with other diseases of the heart and lungs that result in less oxygen in the blood. Diseases beyond the heart and lungs such as cirrhosis and celiac sprue may also cause clubbing. Clubbing may occur in healthy individuals as an inherited trait.
If you find that you have clubbing of your fingers, the first thing your doctor will do is take a thorough history, including a family history. Are you aware of any other family members who have clubbing -- and if so - do they have any medical conditions which would explain the condition?
Tests your doctor may order will depend on your history and physical for the most part, but an arterial blood gas is a test that is commonly ordered.
National Institute of Health. Clubbing. Updated 05/10/13. http://www.nlm.nih.gov/medlineplus/ency/article/003282.htm
Tully, A., Trayes, K,. and J. Studdiford. Evalution of nail abnormalities. American Family Physician. 2012. 85(8):779-87.