Interval Appearance of Hematoidin Crystals in Empyema.

A 63-year-old man receiving apixaban for pulmonary embolism developed a loculated empyema requiring repeated pleural drainage. Blood-tinged purulent pleural fluid obtained during repeat drainage showed no hematoidin crystals on microscopic examination. Twenty-one days later, recurrent fluid accumulation in the same pleural loculation required further drainage. Microscopy revealed golden-brown to golden-yellow crystals with needle-shaped, radiating, and star-like configurations, consistent with hematoidin crystals. Hematoidin is an iron-free pigment produced during hemoglobin degradation following erythrocyte extravasation, particularly in closed spaces with low oxygen tension. The interval appearance of these crystals was consistent with antecedent bleeding into the pleural space and illustrates how prior bleeding may leave a recognizable microscopic footprint during the evolution of an infected pleural collection.