Chvostek sign is a clinical indication of hypocalcemia. It manifests when the physician strikes the facial nerve at the jaw. The sign manifests as a twitch of the nose or lips on the same side. This is due to the nerves in the area being hyperexcited due to the lack of calcium.
The presence of the sign can be helpful in a differential diagnosis as it only appears in a few conditions such as hypoparathyroidism and Vitamin D overdose.
House tests a man for this symptom during Last Resort.