What is the Plankton Sign?
The plankton sign is an ultrasound finding seen in pleural effusion, characterized by swirling echogenic particles within the anechoic fluid that resemble floating plankton, indicating the presence of debris, fibrin strands, or cellular material in the effusion. 1
Sonographic Appearance and Technique
- The plankton sign appears as small, mobile, echogenic particles that move freely within pleural fluid, creating a characteristic swirling pattern during real-time ultrasound examination 1
- This finding is visualized during transthoracic ultrasound (TUS) examination of the pleural space, typically using a low-frequency probe to assess for pleural effusion 2
- The sign is best appreciated with real-time imaging as the echogenic particles demonstrate movement with respiratory motion or patient repositioning 1
Clinical Significance and Diagnostic Value
The plankton sign is associated with exudative pleural effusions rather than simple transudative effusions, as it indicates the presence of cellular debris, inflammatory material, or protein-rich content within the fluid 2, 1
Differential Diagnosis Context
- Transudative effusions typically appear anechoic (completely black) without internal echoes or particulate matter 2
- Exudative effusions may demonstrate the plankton sign along with other complex features including:
- Fibrin strands
- Septations (thin linear echogenic structures dividing the fluid)
- Loculations (compartmentalized fluid collections)
- Visceral pleural thickening 2
Predictive Value for Effusion Etiology
- When septation and loculation are present, they demonstrate 98% specificity and 94% positive predictive value for exudative effusion 2
- The presence of septation alone is independently associated with complicated parapneumonic effusion or empyema (odds ratio 5.3) 2
- Loculation alone is also independently associated with complicated parapneumonic effusion or empyema (odds ratio 3.3) 2
Related Ultrasound Signs in Pleural Effusion
The plankton sign should be evaluated alongside other sonographic findings that help characterize pleural effusions 1:
- Quad sign: Four layers visible (chest wall, pleural line, effusion, lung)
- Sinusoid sign: Respiratory variation in the position of the lung within the effusion
- Thoracic spine sign: Visualization of vertebral bodies below the diaphragm due to fluid transmission
- Jellyfish sign: Floating atelectatic lung tissue resembling a jellyfish
Clinical Application
When the plankton sign is identified, clinicians should suspect an exudative effusion requiring further diagnostic workup including thoracentesis with fluid analysis to determine the underlying etiology (parapneumonic effusion, empyema, malignancy, or other inflammatory causes) 2, 1
The absence of complex features (fibrin, septation, loculation, and plankton sign) has 83% sensitivity and 78% negative predictive value for transudative effusion, helping to stratify patients who may not require invasive sampling 2