Red lunula: A reflection of systemic disease.

The lunula, the half-moon shaped visible portion of the distal matrix most apparent in the thumbs, is typically white in appearance and contributes to the shape of the nail plate. Abnormal coloration of the lunula may signify an underlying disease. Red lunula, a rare but clinically salient finding, can implicate systemic disease or a local disorder isolated to the fingernail. First described in 1954, numerous other cases have since been delineated and an association with cardiovascular, hepatic, hematologic, dermatologic, and rheumatologic disorders has been apparent. Though the precise etiology remains unclear, red lunulae may reflect both systemic disease activity or local pathology such as subungual tumors. Given the rarity of the occurrence, its potential diagnostic relevance is often overlooked. When identifying red lunulae, clinicians should consider connections to underlying systemic or dermatologic disease. This subtle clinical sign may facilitate early detection and identification of salient systemic disease.