Examination of autopsy tissue for diatoms is a controversial test for confirmation of death by drowning. Pollanen et al discuss the test and ways to improve the diagnostic utility. The authors are from the Coroner's Office in Ontario, Canada.
Diatoms are unicellular algae that secrete silicon skeletons called frustules, which have a size and shape that are unique for each species.
Sites where drowning may occur (Table 2, page 282):
(1) naturally occurring body of water (lake, river, stream, ditch, pond)
(2) pool or hot tub
(3) indoors - bathtub, toilet (a shitty way to die)
Proposed mechanism:
(1) A drowning person inhales water containing diatoms into the lungs.
(2) Small diatoms pass into the circulating blood.
(3) The heart is still pumping for several minutes, which takes the diatoms to sheltered locales such as the bone marrow or kidney.
Techniques (page 282):
(1) A sample of tissue (femoral bone marrow, kidney) is carefully retrieved to avoid surface contamination.
(2) The surface of the specimen is washed with 2 changes of distilled water.
(3) Approximately 50 grams of tissue are recovered with a carefully cleaned spatula and place in 50 mL of concentrated analytical grade nitric acid in a boiling flask.
(4) The flask is simmered on a hot plate for 48 hours under a chemical hood.
(5) The flask is slowly cooled and the liquid is placed in centrifuge tubes which are then spun at 250-500 g's for 20-30 minutes.
(6) The supernatant is discarded and the sediment is resuspended in distilled water and then is recentrifuged. This wash is repeated several times.
(7) The final pellet is aspirated and placed on a clean microscope slide. The material is allowed to air dry and then mounting media and a coverslip are applied before microscopic examination.
Alternative methods of tissue digestion such as lytic enzymes can be used.
A water sample collected at the time of body retrieval can be processed in a number of ways to retrieve diatoms, including millipore filtration. It is important that the collection container be carefully cleaned prior to use.
Features of cases where the diatom test has been associated with drowning:
(1) The presence of a limited number of diatom species (usually 1 or 2, up to 6).
(2) The species retrieved match species found in a sample of the water from the lake or stream.
(3) The total number of frustules typically is small (1 to 9).
(4) The size of the frustules is small (< 30 microns) enough to enter the blood stream.
(5) The method of retrieval is totally free of contamination from glassware, water or other sources.
Sources of false positive results:
(1) environmental contamination after death
(2) diatomaceous earth used in abrasive cleansers
Indications for performing the test (page 284):
(1) confirmation of drowning
(2) decomposed bodies recovered from a lake or stream
(3) bodies recovered in bathrooms
(4) selected deaths found on land where no anatomic cause of death has been established
Interpretation:
• The presence of diatoms support the diagnosis of drowning, while the absence of diatomes does not exclude it as a cause.
Performance:
• The test is insensitive (only about a third of drowning deaths) but appears to be specific provided the method used avoids causes of false positive results.
• The test is more likely to be positive at times of diatom "blooms" when the number of diatoms in the water are significantly increased.
• There are claims that diatoms can be found in people who have not drowned, but these cases may represent environmental contamination.
Specialty: Clinical Laboratory
