Required section · Section 3 of 6

Screening and confirmation answer different questions

A positive class immunoassay means antibody-based assay signal met the calibrator-defined decision criterion. It is a presumptive antibody-based signal, not proof of analyte identity, dose, timing, or impairment. Cross-reactivity is assay and manufacturer specific, so an unexpected positive needs review against the exact platform and reagent instructions for use.

A routine opiates immunoassay is designed around morphine-like compounds. It may not reliably detect oxycodone, hydrocodone, hydromorphone, fentanyl, methadone, or buprenorphine unless a separately targeted assay is included. The panel menu and reflex algorithm must be declared locally.

Gas chromatography-mass spectrometry and liquid chromatography-tandem mass spectrometry separate compounds chromatographically and identify them with mass spectral patterns or transitions. These methods can provide analyte-specific identification and quantitation, subject to validation, calibration, matrix effects, and reporting limits. Whether hydrolysis is used changes whether a result represents free or total analyte and is a local validated-method detail.

Federal workplace urine cutoffs are program-specific, not default clinical laboratory cutoffs. For example, the 2024 federal urine initial and confirmatory THCA cutoffs are 50 ng/mL and 15 ng/mL. Clinical laboratories must use their own validated procedure and medical director-approved reporting algorithm.

Name the exact screen, its target class, and the definitive method before explaining a discrepancy. Check the validated test menu and reagent instructions for use. Treat cross-reactivity as method-specific.

Comparison card shows an immunoassay antibody signal and a review panel distinguishing target from cross-reacting compound and requiring method-specific review. — Figure 1A class-screen signal can arise from the target or an assay-specific cross-reacting compound.

Same specimen question across two method families
Feature	Class immunoassay	GC-MS or LC-MS/MS
Measured signal	Antibody response to a class-like structure	Quantifier and qualifier transitions, accepted ion ratio, retention-time match, and stable-isotope internal standard
Usual output	Presumptive positive or negative at cutoff	Analyte-specific identification and quantitation
Principal limitation	Cross-reactivity and incomplete class coverage	Validation, calibration, matrix effects, and library or target limits
Best use	Efficient screen for a defined class	Confirmation or targeted definitive testing

Knowledge checks

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Knowledge check 1

What does a positive class immunoassay directly mean?

Choose one option.

Antibody-based assay signal met a calibrator-defined decision criterion.

Definitive identification of every drug in the class.

A concentration that establishes impairment.

Knowledge check 2

Which features belong to GC-MS or LC-MS/MS definitive testing? Select all that apply.

Choose at least 4 options.

Quantifier and qualifier transitions with an accepted ion ratio.

A retention-time match.

A stable-isotope internal standard.

Chromatographic separation before mass-based identification.

One peak at one retention time alone.

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