Testing the specificity of the diatom test: search for false-positives

The diatom test is widely used by forensic pathologists as proof of drowning, notwithstanding some criticisms mainly concerning the occurrence of false-positive results (presence of diatoms in the tissues of subjects who died from causes other than drowning). The aim of the present study was to verify the claimed inaccuracy of the method caused by an excessive rate of false-positives related to inadvertent exposure to diatoms of the general population. The study was carried out to investigate the presence of diatoms in the tissues (lungs and sternum) of subjects who died from causes other than drowning. Two groups of cadavers that underwent an autopsy at the Institute of Forensic Medicine of the University of Verona were included in the study. Group A comprised 45 individuals who died from causes other than drowning, whereas Group B comprised 20 bodies which had been recovered from water. The extraction of the diatoms was performed by incubation of samples in nitric acid for 48 hours at 60°C. The analysis of the samples from Group A showed the absence of diatoms in both lung and sternum samples. In Group B all lung samples showed the presence of diatoms, whereas only six sternum samples were shown to contain diatoms. The difference between Groups A and B was statistically highly significant. The absence of diatoms in the samples collected from Group A falsified the hypothesis that false-positive results from the diatom test may occur due to diatoms entering living bodies through the respiratory and/or digestive tracts via air, water or food, supporting the validity of the diatom test as proof of drowning.