Screening Test
Definition
A fast, sensitive immunoassay (EMIT, CEDIA, FPIA, KIMS, lateral-flow) that flags a sample as presumptive positive against a class cut-off. Sensitive and cheap, but cross-reactive: positive result is not proof.
- Purpose
- Fast presumptive flag against a class cut-off concentration
- Common platforms
- EMIT, CEDIA, FPIA, KIMS, lateral-flow immunoassay
- Strength
- High sensitivity, low cost, rapid turnaround
- Weakness
- Cross-reactivity, not specific to a single compound
- Next step
- Positive result requires confirmatory testing
Common questions
Why can a screening test be positive when the specific drug alleged is not actually present?+
Immunoassay antibodies bind to a class of structurally similar molecules, so a legitimate medication, an over-the-counter product, or even certain foods can cross-react and trigger a positive flag without the target drug being present at all.
Is a screening test result ever admissible on its own to prove drug use?+
Most forensic toxicology protocols and courts treat a screening-only positive as presumptive, not confirmatory, and require a specific method such as GC-MS or LC-MS/MS before the result can support a finding of drug presence.
What does a negative screening result actually rule out?+
A negative result means the concentration was below the assay's cut-off for that drug class, not that the substance is absent entirely. A drug present below the cut-off, or one the assay's antibodies do not recognize, can still be missed.
Related terms
- Adulteration
- Deliberate tampering of a urine specimen to defeat the screen. Common adulterants include nitrite, chromate, peroxide, glutaraldehyde, niacin, vinegar and Visine. Detected...
- Confirmatory Test
- A test with sufficient specificity to establish the identity of a biological fluid for reporting purposes. Examples include Takayama and Teichmann crystal...
- Cut-Off Concentration
- The threshold analyte concentration, set by regulatory guidance, above which a specimen is reported as a screen-positive. Choosing the cut-off balances sensitivity...
- Detection Window
- The period after last use during which a drug or its metabolite is detectable in a given matrix. Varies from minutes (blood,...
- EMIT
- Enzyme-Multiplied Immunoassay Technique. Homogeneous immunoassay where drug-enzyme conjugate competes with drug in the sample for antibody binding; free conjugate retains enzyme activity,...
- MRM
- Multiple Reaction Monitoring. LC-MS-MS or GC-MS-MS mode that follows specific precursor-to-product ion transitions for each target analyte, giving very high selectivity at...
- SAMHSA-5
- The five-drug federal workplace panel of the US Substance Abuse and Mental Health Services Administration: amphetamines, cocaine metabolite, marijuana (THC-COOH), opiates and...