Plankton sign (ultrasound)

The plankton sign refers to swirling, punctiform internal echoes within an otherwise anechoic pleural effusion which demonstrate slow, whirling dynamics, occasionally buffered by cardiac and respiratory impulses. When present, one may rule out a transudative effusion, and should be highly suspicious of a hemothorax as the likely etiology. A similar sonographic appearance is present in hemoperitoneum and hemorrhagic ascites ¹.

The slow, irregular dynamics are suggestive that no direct connection with a vascular conduit is present, and the effusion would be amenable to diagnostic thoracentesis.