Background Contamination
Definition
Diatom frustules introduced into a sample from sources other than the victim's own body: tap water, laboratory air, unclean glassware, reagents, and autopsy-room surfaces. Background contamination is the most commonly identified source of false positives.
- Field
- Diatom test, drowning investigation
- Common sources
- Tap water, lab air, glassware, reagents, autopsy surfaces
- Consequence
- Leading cause of false positives
Common questions
How do examiners try to rule out background contamination in a diatom test?+
Common practice includes running reagent and glassware blanks alongside the case sample, using diatom-free distilled water, and comparing any diatoms recovered against the diatom species and concentration profile of the water source where drowning is suspected to have occurred.
Why does background contamination undermine confidence in a positive diatom result more than a negative one?+
A positive result is meant to show diatoms reached internal organs through the circulation during active drowning, but if the same diatoms could have entered the sample from the lab environment or reagents, a positive cannot reliably distinguish true drowning from a contaminated sample, which is why the test is treated as supportive rather than conclusive on its own.
Can background contamination affect samples taken from a body recovered on land?+
Yes, contamination is not limited to water-related sources; airborne diatoms in a mortuary or laboratory, or diatoms already present in commercial reagents, can be introduced into any tissue digest regardless of where the body was found, which is why blank controls are run for every batch.
Related terms
- Diatom Density
- The number of frustules per unit volume of water in the drowning environment. Low-density environments (pools, tap water, diatom-poor rivers) cannot deliver...
- False Negative
- A negative result from a preparation in which ante-mortem drowning actually did occur. Causes include low diatom density in the drowning water,...
- False Positive
- A test result that indicates the presence of a target analyte when it is absent. In forensic serology this may mean incorrectly...
- Positivity Threshold
- The minimum number of frustules per gram of tissue or per slide that a laboratory uses to declare a result positive. No...
- Standardisation
- The production of a validated, peer-reviewed protocol that specifies every variable in the test: acid type and concentration, digestion time and temperature,...