Comprehensive Drug Screen Limits of Detection

Background

The comprehensive drug screen was designed to rapidly detect the presence of a broad array of compounds in patients presenting with a possible overdose. It consists of screening immunoassays for several drugs or drug classes, a quantitative enzymatic assay for ethanol and a gas chromatography-mass spectrometry (GC/MS) assay for a wide variety of therapeutic and abused drugs, as listed below. The assay can be run on miscellaneous pills, powders and liquids to identify a suspected ingested substance that a patient might arrive with at the hospital, and in this case, only the GC/MS assay is performed.

Because of the intended primary use of the comprehensive drug screen, the GC/MS assay was designed for rapid detection of a wide range of drugs when present at overdose concentrations, rather than for high sensitivity for any specific drug. As a result, drugs present at therapeutic concentrations may not always be detected. This limits the utility of the comprehensive drug screen for monitoring compliance or for confirming the results of screening immunoassays. Positive findings by GC/MS are highly reliable, due to the high specificity of mass spectrometry. However, negative results do not necessarily indicate the absence of the drug in question. When it is important to detect the presence of any amount of a drug, one should choose an assay designed specifically to measure the drug in question, and not a screening test.

The screening immunoassays cutoffs given are the concentrations above which the assay is reported as positive; note that a drug present below the cutoff will be reported as negative on an immunoassay screen even though it is present. The immunoassay sensitivities are listed for specific drugs, but each assay can detect related compounds with similar but variable sensitivities (i.e. the opiates assay detects morphine and codeine similarly, but oxycodone is detected poorly). Contact the laboratory with any questions about the cross-reactivity of any specific drug with this assay.

The GC/MS comprehensive screen drugs are listed with the minimum concentration of a spiked drug that was detected in a urine sample during the validation of the analytical method. Because of the complexity of this test, and because the test is not run in a quantitative mode, these sensitivities should only be used in concert with other clinical and historical data to guide clinical interpretation. In some cases, as with fentanyl, drugs are included on this list because the assay is also used to detect the drug in pharmaceutical preparations. In these cases, the claimed sensitivity is inadequate to detect the presence of the drug in urine even after an overdose.

Screening immunoassay panel (qualitative)

Enzymatic assay (quantitative)

ETHANOL <10 mg/dL ethanol reported as negative

Comprehensive screen - GC/MS assay (qualitative)