False Positive
Definition
A test result that indicates the presence of a target analyte when it is absent. In forensic serology this may mean incorrectly identifying a fluid, a blood group, or a species. False positives arise from cross-reactivity, non-specific binding, contamination, or assay cut-offs set too low.
- Also called
- A presumptive positive (in blood and semen screening, all initial positives are presumptive, not confirmatory)
- Why it matters
- Every forensic test has a false-positive rate. Relying on a single test to prove something can lead to wrong conclusions. Confirmation tests and independent methods are the standard safeguard
Common questions
What makes a test result a false positive in forensic analysis?+
A false positive is a positive test result that doesn't mean what it appears to mean. The sample shows the expected chemical, biological, or physical signature, but the source is something other than the forensic target. For example, the Kastle-Meyer blood test can be triggered by plant peroxidases, not blood. This is why presumptive tests require confirmation with a second method.
Can geophysical equipment for grave detection give false positives?+
Yes. Ground-penetrating radar and similar tools flag anomalies that match the signature of a buried body, but they can't distinguish between a forensic target and buried infrastructure, tree roots, animal burrows, or decomposing garden waste. Each anomaly needs physical investigation to confirm.
Why is the diatom test for drowning prone to false positives?+
Diatoms appear in organs when someone inhales water before death, but they can also arrive by contamination during handling, post-mortem diffusion through tissue, or prior environmental exposure. The test shows diatoms, but not necessarily drowning. This vulnerability is why the diatom test alone cannot confirm cause of death.
Related terms
- 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,...
- Acid Phosphatase (AP)
- An enzyme present at high concentrations in seminal plasma. The Brentamine Fast Violet B (or SERI AP) spray test detects acid phosphatase...
- Alert Fatigue
- The condition in which analysts receive more alerts than they can meaningfully review, leading to delayed responses, dismissed true positives, and reduced...
- Asset Criticality
- A pre-assigned score or label that records how important a system, service, or data set is to the organisation. Used during triage...
- 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...
- Brentamine Fast Violet B Test
- A two-component spot test in which sodium alpha-naphthyl phosphate (substrate) is hydrolysed by AP to alpha-naphthol, which couples with fast violet B...
- Confidence Ranking
- A classification scheme that assigns each detected anomaly a confidence level based on how many independent methods detected it and how well...
- Confirmatory Hierarchy
- The structured sequence of tests applied to biological evidence, in which a presumptive (screening) test with high sensitivity is followed by a...
- Confirmatory Test
- A test with sufficient specificity to establish the identity of a biological fluid for reporting purposes. Examples include Takayama and Teichmann crystal...
- Cross-Reactivity
- The capacity of an antibody raised against one analyte to bind structurally related compounds. In RIA, cross-reactivity is the main driver of...
- Decision Framework
- A structured protocol for selecting geophysical methods based on documented site characteristics (soil type, target depth, cultural noise) before fieldwork begins, with...
- 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...
Explained in these topics
- Acid Phosphatase Screening for SemenA positive AP test result on a non-semen stain, caused by vaginal AP, faecal AP, plant extracts, or bacterial AP. Not confirmatory of semen; all positive scree...
- Integrating Multiple Geophysical MethodsAn anomaly that matches the expected signature of a forensic target but originates from a non-forensic source: buried infrastructure, geological inhomogeneity,...
- Kastle-Meyer TestA positive colour response from a substance other than blood. In the Kastle-Meyer test, plant peroxidases and some oxidising agents can produce false positives...
- Limitations and Controversies of the Diatom TestA test result indicating diatoms consistent with ante-mortem aspiration in an organ sample when the diatoms actually arrived by contamination, post-mortem diff...
- Sources of Error and False Results in Serological TestingA test result that indicates the presence of a target analyte when it is absent. In forensic serology this may mean incorrectly identifying a fluid, a blood gr...
- Triage and Incident PrioritisationAn alert that fires on a benign event and does not represent a real security incident. High false positive rates are a primary cause of alert fatigue. Document...