Drug screens use cutoff concentrations to distinguish between negative and positive samples. For a qualitative test like a urine drug screen it is important to consider that some arbitrary threshold has to be met for the assay to be positive. The cutoff points for drugs of abuse on screening panels are usually determined by the immunoassay manufacturer. However, they can be adjusted by the laboratory if the laboratory prefers a higher or lower cutoff.
Clinicians can over-interpret cutoffs. They should be reminded that a negative result on a screening test does not necessarily mean that the drug is not present in the sample, only that it is less than the cutoff concentration established by the manufacturer or laboratory for that drug. For example, if a sample screens negative for oxycodone, there may be oxycodone present in the sample, but the concentration could be less than the laboratory's cutoff, of 100 ng/mL.
Cutoff concentrations should be posted with all laboratory screening results.
Below are some typical cutoff concentrations for DOA screens:
Drug
| Typical Cutoff Concentration
|
Amphetamine
| 500 ng/mL
|
| Barbiturates | 200 ng/mL
|
THC
| 50 ng/mL
|
Cocaine/BE
| 300 ng/mL
|
Oxycodone/Oxymorphone
| 100 ng/mL
|
Opiates
| 300 ng/mL
|
Methadone
| 300 ng/mL
|
Benzodiazepines
| 200 ng/mL
|
