Fingernail clubbing is a sign of several diseases, most notably of the heart and lungs. It is also known as "Hippocratic fingers" and is best assessed by looking for Schamroth's sign, also discussed below.
Profile angle: tangential angle made by nail as it exits from terminal phalanx (cuticle)
Angle is abnormal if it approaches 180°
Phalangeal depth ratio
Ratio of diameter of the finger at cuticle to the diameter of the finger at DIP joint
Ratios of > 1.0 indicate clubbing
Digital index
Obtained by summing the phalangeal depth ratio for all 10 fingers
An index of > 10.2 indicates clubbing
Schamroth's sign
Combines qualities of nail-fold angle and phalangeal depth ratio
Have patient oppose index fingers so that they're nail-to-nail, pointing downwards
Negative/normal: diamond-shaped window (arrowhead) is created
Positive/abnormal: obliteration of diamond shape
Palpation
Grip sides of subject’s distal end of finger between thumb and middle finger with one hand and exert downward pressure at base of nail bed with index finger of other hand
Clubbing
Increased sponginess
Perception that nail is “floating” within soft tissue
Palpable proximal edge of nail
References
Bickley LS. The thorax and lungs. In: Bickley LS, Szilagyi PG. Bates' Guide to Physical Examination and History Taking. 10th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2009:283-321.
Lampe RM, Kagan A. Detection of clubbing—Schamroth’s sign. Closing the window and opening the angle. Clinical Pediatrics. 1983;22(2):125.
Myers KA, Farquhar DRE. The rational clinical examination: Does this patient have clubbing? JAMA. 2001;286(3):341-347.