Cross reactivity happens when an unrelated but structurally similar substance triggers a false positive on an onsite rapid UA (Urinary Analysis) device. This is something that is unavoidable with any onsite UA devices within the market because they only provide qualitative results. Substances on a reacted immunoassay fall into two categories; the targeted drug class which are expected to be detected as a positive reading and structurally similar but unrelated substances that can bring a false positive. When the results are disputed and more information about the results are needed then confirmation testing is done. Confirmation testing is generally done by a registered lab with GCMS testing equipment. GCMS testing allows for a greater level of sensitivity and specificity. This testing allows the true positives to be separated from the false positives.
There are thousands of both OTC (Over the Counter) and POM’s (Prescription Only Medications) available that have the potential for causing cross reactivity. First it is best to request details of a donor’s medication if they are currently using one, then send the non-negative UA sample for confirmatory testing at a registered lab such as Labcorp or Alere.
Some drugs tested for do have a higher chance of resulting in cross reactivity such as amphetamines and methamphetamines. There are compounds with structural similarities to AMP and MAMP contained in some over the counter cough syrups or over the counter cold and nasal decongestant medications.
There are almost endless possibilities of substances that can cause cross reactivity. So we recommend understanding cross-reactivity as best you can so you can comfortably handle situations where it may have occurred.