Distinguishing true antemortem thrombus (AMT) from artifactual postmortem clot (PMC) can occasionally be challenging at autopsy. Lines of Zahn are cited as pathognomonic of AMT, but review of literature reveals heterogeneous definitions of the term. Neutrophil karyorrhexis and CD61 immunohistochemistry can also be used to define AMT, but there has been no systematic study determining the specificity of these features.


To identify features that distinguish AMT from PMC, and to clarify the definition of lines of Zahn.


PMC from the heart was collected in 50 hospital autopsies. Fifty arterial and 50 venous surgical thrombectomy specimens were reviewed for comparison. The microscopic features with hematoxylin-eosin staining, phosphotungstic acid–hematoxylin (PTAH) staining, and CD61 immunohistochemistry were documented.


Thin curvilinear strands of fibrin and clumps of fibrin were frequently observed in both AMT and PMC. Thick bands of nested platelets wrapped in fibrin were nearly exclusive to AMT. Neutrophil karyorrhexis was readily apparent on low power in AMT but was seen in 40 of 50 PMCs (80%) only sparsely on high-power examination. Bone marrow elements were identified in 38 of 50 PMCs (76%). CD61 staining showed a geographic pattern in AMT and a speckled pattern in PMC. PTAH staining confirmed features seen with hematoxylin-eosin.


Thin curvilinear strands of fibrin are found in both AMT and PMC and can be misinterpreted as lines of Zahn. We define lines of Zahn as thick bands formed by nested platelets wrapped in fibrin. Diffuse neutrophil karyorrhexis is common in AMT; in contrast, bone marrow elements are often seen in PMC.

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Author notes

Preliminary results of this research were presented virtually at the 2021 United States and Canadian Academy of Pathology Meeting; March 15, 2021.

Competing Interests

The authors have no relevant financial interest in the products or companies described in this article