Formalin pigment artifact in postmortem histology: Challenges and potential implications for accurate forensic differential diagnosis

Formalin pigment is the most important artifact found in postmortem histological analyses. Although it has no forensic value, formalin pigment, when abundant or not well-known to the pathologist, can morphologically resemble, mimic or misinterpret physiological or exogenous pigments of forensic interest, making their identification challenging and leading to diagnostic difficulties. To investigate these aspects, a retrospective study was conducted on 35 autopsied bodies in various states of preservation in which postmortem histological analyses had revealed the presence of brown/blackish precipitates requiring a differential diagnosis with formalin pigment artifacts. In 29 cases (83%) it could be proven that the precipitates were not due to formalin artifacts. In some cases, however, they were of minor forensic significance, so that others could be categorised as highly forensically significant. These were mainly haemosiderin, lead from gunshot residue (GSR), granular precipitates as a result of acute poisoning with various substances and malaria pigment. It should therefore be noted that precipitates that can be superficially assigned to formalin artifacts can conceal a multitude of pigments of different types, aetiologies and thus forensic significance, which also depend on the circumstantial and investigative context of the individual cases. For this very reason, it is of central importance for forensic pathologists to constantly question whether it may be useful to further investigate the diagnosis of suspected artifactual formalin pigment precipitates in autopsy specimens.