Describe the diatom test with its principle and limitations.

Principle

Diatoms are present in almost all natural water — rivers, ponds, lakes, wells and the sea — and are of many species, each characteristic of its water → In a person who drowns, water is inhaled and diatoms reach the alveoli → They penetrate the alveolar-capillary membrane and enter the circulation → A beating heart then carries them to the distant organs — the bone marrow, brain, liver and kidney → In a body thrown into water after death, water may passively enter the lungs and stomach, but with no circulation the diatoms cannot reach the distant organs → Their presence in a closed distant site is therefore evidence that the person was alive and had a circulation when submerged

• The femur bone marrow is the specimen of choice, because it is a closed cavity which water cannot enter after death, so a positive result there cannot be explained by passive contamination. The sternum, humerus, brain, liver and kidney are alternatives
• Method — acid digestion: the tissue is digested with concentrated nitric or sulphuric acid, which destroys all organic material while the silica walls survive intact; the residue is centrifuged and examined microscopically, and the diatoms counted and identified to species
• A control sample of the water from the site of recovery must be examined alongside, and the species matched; a positive result is far stronger where the species in the marrow correspond to those in that water

Limitations

• Diatoms are ubiquitous in the environment — in drinking water, in food (especially shellfish and vegetables), in the air and in dust — so a small number may be found in people who never drowned, having entered through the gut and lungs during life. Interpretation therefore depends on the number as well as the presence
• Contamination during collection and processing is a constant danger, from tap water, glassware, reagents and instruments; the specimen must be taken with scrupulous technique, the bone surface cleaned, and the marrow taken from within after the bone is opened
• A negative result does not exclude drowning — some waters contain few or no diatoms, notably chlorinated swimming pools, some wells and tanks, and rivers in the dry season; in dry drowning little water enters the lungs at all; and death may have occurred so rapidly that few diatoms were carried
• The test is expensive, slow and requires expertise, and is not available in most Indian centres
• Its value has been debated for decades, and the responsible position is that it is useful corroborative evidence when strongly positive with matching species, and of little value when negative or weakly positive

Applied Aspects

• Take the femur, not the lung; a positive lung result is uninterpretable and a positive marrow result is the whole point of the test
• Send a control water sample from the exact place the body was recovered; without it the species cannot be matched and the result is much weaker
• Clean the bone surface before opening it, and use clean instruments and distilled water throughout; contamination produces a false positive that is worse than no test at all
• Report the number and the species, not merely "diatoms present"; a handful of mixed environmental species means little and abundant matching species mean a great deal
• State plainly that a negative result excludes nothing, and give the reasons — chlorinated or diatom-poor water, dry drowning, rapid death
• Use it as corroboration, not as proof; the diagnosis of drowning rests on the whole picture — the froth, cadaveric spasm, the circumstances and the exclusion of other causes — and the diatom test supports it
• Diatoms in the lung prove immersion only; water enters a corpse passively, and only a closed distant site carries the argument
• Acid digestion works because silica survives what destroys every other tissue, which is why the test exists at all
• Chlorinated pool water may contain none, so a drowning in a swimming pool will be diatom-negative however genuine it was
• Environmental diatoms are everywhere — in food, drinking water and dust — so a handful of mixed species means nothing
• Open the bone in a clean field with clean instruments and distilled water; a contaminated positive is worse than no test at all
• Species matching is what makes it persuasive; marrow diatoms corresponding to those in the recovery water is a far stronger finding than a bare count