Positivity Threshold
Definition
The minimum number of frustules per gram of tissue or per slide that a laboratory uses to declare a result positive. No universal standard exists; threshold proposals in the literature range widely, and different thresholds applied to the same case can produce opposite conclusions.
- Field
- Diatom test in drowning investigation
- Unit
- Frustules per gram of tissue or per slide
- Standard
- No universally agreed value
- Risk
- The same case can be called positive or negative depending on the threshold used
Common questions
Why has no laboratory standard been agreed on?+
Diatom counts depend on digestion method, the organ sampled, background diatom load in the water body, and the counting protocol used, so a threshold validated in one laboratory does not transfer cleanly to another. Proposed cutoffs in the literature vary by orders of magnitude for this reason.
What is the practical consequence for court testimony?+
An expert must disclose which threshold was applied and why, because a different reasonable choice could flip the interpretation from drowning-positive to inconclusive. This variability is one of the main grounds defence experts use to challenge diatom evidence.
Related terms
- Background Contamination
- Diatom frustules introduced into a sample from sources other than the victim's own body: tap water, laboratory air, unclean glassware, reagents, and...
- 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...
- Standardisation
- The production of a validated, peer-reviewed protocol that specifies every variable in the test: acid type and concentration, digestion time and temperature,...