The immunoassay drug tests, which are tests designed to classify substances as either present or absent according to a predetermined cutoff threshold.
Immunoassays are chemical tests used to detect or quantify a specific substance, the presence of analytes, in a blood or body fluid sample, using an immunological reaction. Immunoassays are highly sensitive and specific. Their high specificity results from the use of antibodies and purified antigens as reagents.
An immunoassay’s ability to detect drugs will vary according to the drug’s concentration in the urine and the assay’s cut off concentration. Any response above the cutoff is deemed positive, and any response below the cutoff is negative (eg, if the cutoff is set at 50 ng/mL, 49 ng/mL would be reported as negative, while 51 ng/ml would be reported as positive, although these results are, for scientific purposes, identical).
Immunoassays are also subject to cross reactivity; ie, substances with similar, and sometimes dissimilar, chemical compositions may cause a test to appear positive for the target drug. Samples that test positive by immunoassay for classes of drug need to be tested in the laboratory by a more definitive method (LCMS/MS) if specific identification of the drug is required.