Cut-Off Concentration
Definition
The threshold analyte concentration, set by regulatory guidance, above which a specimen is reported as a screen-positive. Choosing the cut-off balances sensitivity against the false-positive rate from cross-reactive substances. SAMHSA, EWDTS, and national guidelines each specify cut-offs by drug class and matrix.
- Screening measurement unit
- ng/mL (nanograms per milliliter)
- Hair analysis measurement unit
- pg/mg (picograms per milligram)
- Confirmation approach
- Uses a lower threshold than screening to refine positive results
- Standards body
- SOFT and SOHT publish harmonised cut-offs for major drug classes
Common questions
What is a cut-off concentration in drug testing?+
A cut-off concentration is a fixed threshold value (measured in ng/mL or pg/mg) that determines whether a test result is reported as positive or negative. Any result above the cut-off is positive. Any result below it is reported negative, even if the instrument detects the drug.
Why do screening and confirmation tests use different cut-offs?+
Screening cut-offs are set higher to catch suspected positives efficiently. Confirmation testing (using GC-MS or LC-MS-MS) applies a lower, separate cut-off to reduce false positives from the screening step. This two-stage approach balances sensitivity and specificity.
Where do hair analysis cut-offs come from?+
SOFT (Society of Forensic Toxicologists) and SOHT (Society of Hair Testing) publish harmonised cut-offs for major drug classes, measured in picograms per milligram. These standards help labs report results consistently across jurisdictions.
Related terms
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- CEDIA (Cloned Enzyme Donor Immunoassay)
- A homogeneous immunoassay using two inactive fragments of beta-galactosidase, an enzyme donor (ED) fragment conjugated to drug, and an enzyme acceptor (EA)...
- 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...
- Decontamination Wash
- A multi-solvent surface wash (typically dichloromethane, methanol and aqueous buffer) applied before digestion to strip externally deposited drug from the cuticle. The...
- Detection Window
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- 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,...
- EMIT (Enzyme-Multiplied Immunoassay Technique)
- A homogeneous competitive immunoassay in which drug in the sample competes with enzyme-labelled drug conjugate for antibody binding sites. Free enzyme-labelled conjugate...
- Lateral-Flow Immunoassay
- A point-of-care competitive immunochromatographic format on a nitrocellulose strip. In the absence of drug, colloidal gold-labelled antibody is captured at a test...
- Melanin Binding
- The non-covalent association of basic and lipophilic drugs (cocaine, opiates, amphetamines, nicotine) with melanin granules in the hair cortex. Pigmented (black, brown)...
- 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...
Explained in these topics
- Drugs of Abuse: Screening and Confirmation ProtocolsA pre-fixed analyte concentration (ng/mL) that defines positive vs negative. A separate, lower confirmation cut-off applies to the GC-MS or LC-MS-MS step.
- Hair Analysis for Drugs and Chronic PoisoningThe minimum hair concentration above which a positive result is reported. SOFT and SOHT publish harmonised cut-offs in picograms per milligram for each major d...
- Immunological Drug Testing: Screening and Forensic InterpretationThe threshold analyte concentration, set by regulatory guidance, above which a specimen is reported as a screen-positive. Choosing the cut-off balances sensiti...