The pathophysiology of clubbing is not well understood; however, it has been associated with a variety of disease states, including infectious, neoplastic, inflammatory, and vascular. Clubbing can also be a benign hereditary condition. In children, clubbing usually occurs with cystic fibrosis or uncorrected cyanotic congenital heart disease. Although usually symmetric, clubbing can be bilateral, unilateral, or even unidigital. Patients with clubbing may also have hypertrophic osteoarthropathy, a disorder most commonly associated with bronchogenic carcinoma, but also associated with extrapulmonary malignancies and non-malignant pulmonary conditions. Clubbing is almost always painless, unless it is associated with hypertrophic osteoarthropathy; most patients are even unaware of any abnormality in their fingers. Conditions associated with clubbing are listed in (see accompanying table).

There is no “gold standard” for the evaluation of clubbing and no imaging tests that can confirm its presence. Physicians must, therefore, rely on their clinical examination skills to verify a diagnosis of clubbing. Visual inspection and palpation are usually sufficient to detect advanced clubbing, but early stages of clubbing can be more difficult to detect. Myers and Farquhar review the available literature about clubbing to determine the precision and accuracy of clinical examination in diagnosing the condition.

Nail curvature by itself may occur with increasing age and does not necessarily indicate clubbing. The nail-fold angle is one method used to define clubbing. In normal fingers, the nail extends from the bed (the profile angle) at about 160 degrees, but in clubbed fingers this angle approaches 180 degrees. The phalangeal depth ratio is also used to distinguish normal nails from clubbed nails. In normal fingers, the distal phalangeal depth should be smaller than the interphalangeal depth; this relationship is reversed in the finger with clubbing. Since calipers may not be readily available, physicians may estimate the phalangeal depth ratio.

The Schamroth sign describes the “window” that is created when the dorsal surfaces of the terminal phalanges on opposing fingers are placed together. A normal finger shows a diamond-shaped window, while the clubbed finger will have no space visible between the opposing nails. This sign has not been formally tested but is easy to perform at the bedside. When palpating the fingernail, the nail may seem to be loose within the soft tissue, and the base (proximal edge) of the nail may be felt through the skin in advanced cases. Clinical examinations were found to be moderately precise in describing clubbing.

Because of methodologic factors, none of the studies reviewed led to better than grade C recommendations. The authors suggest using the following values in clinical examinations: the phalangeal depth ratio (PDR) should not exceed 1.0, and the profile angle should not exceed 180 degrees. While a normal PDR does not eliminate the possibility of lung cancer, an abnormal PDR is associated with an increased probability of underlying lung cancer. In studies, some patients with chronic obstructive pulmonary disorder were found to have a ratio of more than 1.05, but none had a ratio greater than 1.1. Because non-hereditary clubbing is a marker of important and potentially serious conditions, the presence of clubbing should prompt a search for an underlying disease.