A twenty-year survey of drowning cases in Verona territory: strengths and limits of diatom test

BACKGROUND: The medico-legal classification of cases of corpses found in water is often challenging due to the need not only to distinguish between submersion of a corpse and death in water, but also, with respect to the latter hypothesis, to understand in which cases the cause of death is actually attributable to drowning. The complexity of the diagnosis explains the constant interest of the scientific community in the subject with the development in the last hundred years of different lines of research for the identification and validation in forensic practice of diagnostic procedures, which can substantiate or exclude, on an objective basis, the hypothesis of drowning, which remains the prevailing one in these cases. This is the context in which diatom test takes place. It was first proposed at the beginning of the 1900’s, and it is based on the theoretical assumption that diatoms are present in fresh and seawater and not in the human body and then that their identification in biological fluids and tissues is an index of inhalation of water and therefore drowning. From the first reports, various studies have followed with consequent increase in knowledge on the strengths and limitations of the diatom test, even if the aspects concerning diagnostic specificity and sensitivity of this test are still debated. The purpose of this work was to critically analyze the results of diatom test in cases of bodies retrieved in water in different areas of North-Eastern Italy to evaluate the frequency of this mode of death in our area, as well as to formulate considerations on the sensitivity and specificity of the diatom test on the basis of the real forensic practice. The study period ranged from 2000, when the diatom test had been introduced in the routine practice of the Institute of Forensic Medicine of the University of Verona, to 2022.METHODS: The search of diatoms was performed on lung and sternal bone marrow through an extraction procedure based on the incubation of samples in nitric acid for 48 hours at 60 °C. The extracts were examined by light microscopy at 200X, 400X and 630X magnification. The cut-off used to consider positive the test was 10 diatoms/5 g of lung tissue and 5 diatoms/10 g of sternal bone marrow. A positive pulmonary diatom test was considered indicative of drowning in any case, even if the sternal diatom test was negative.RESULTS: Overall, 186 cases underwent diatom test. Of them, 172 were freshwater cases, five seawater cases and nine treated water cases (swimming pool, bathtubs). In 153 cases, a quantitative diatom test was performed. Of them, 106 cases (69%) yielded a positive pulmonary diatom test result, confirming the diagnosis of drowning which had been originally formulated on the basis of circumstantial data. Of the remaining 47 cases, three showed a number of diatoms below the cut-off and then were classified as doubtful cases, while 17 cases showed a cause of death different of drowning and then were classified as true negatives. In the remaining 27 cases, it was not possible to formulate a conclusive explanation of the negativity test results, as it could be attributed both to a rapid death in the water from natural causes or to complications linked to immersion and to paucity of diatoms in the aqueous medium. These cases were then classified as false negatives.CONCLUSIONS: The data of the present study confirmed the usefulness of diatom test to objectively support a diagnosis of drowning. Indeed, the diatom test shows a good diagnostic specificity, provided that it is performed taking all the suitable precautions to limit or eliminate post-mortem contamination, and that it is interpreted on a quantitative basis. In addition, compared to other tests, this diagnostic approach is significantly cheaper and can be performed at any forensic pathology facility.