Conventional diatom testing using strong acid: influence of postmortem contamination on diatoms in closed organs and lungs of highly decomposed immersed bodies.

Our previous study showed that diatoms adhering to Kjeldahl flasks can cause false-positive results in subsequent tests (28% of flasks; 1-290 diatoms/flask). Among 100 autopsy cases, ≥5 diatoms/10 g were found in the kidney of only two decomposed bodies, raising suspicion of contamination during postmortem immersion and/or tissue sampling at autopsy. In the present study, we examined 38 highly decomposed immersed bodies using only new flasks. Despite severe decomposition, no diatoms were detected in the kidney or liver in 15 cases (39%). Among 12 cases with an open thoracic and/or abdominal cavity, five (42%) had large numbers of diatoms (≥10/10 g), whereas six (50%) showed only 1-2/10 g. In three cases suspected of involving immersion in river water, with the bodies later drifting out to sea or onto the seashore, marine diatoms were scarce even in the lungs (<3%). Two non-drowning immersed bodies showed no diatoms in the lungs. In 27 cases in which algae were attached to the body surface, six (22%) revealed large numbers of diatoms (≥10/10 g) in the kidney or liver. In 20 cases with closed cavities, there was no positive correlation between diatom count in the kidney or liver and the count in the lungs, similar to our previous findings. The present findings indicate that, when new flasks are used, relatively high levels of diatom contamination are more likely to occur during tissue sampling at autopsy than during postmortem immersion or during the diatom testing procedure.