Screening vs. Confirmatory Tests
There are two main categories of urine drug testing -- screening and confirmatory. Screening tests are initial, qualitative drug tests conducted to identify classes of drugs present in the urine and typically are done using immunoassay. They rely on a set threshold above which a positive result is produced and therefore do not detect lower concentrations of a drug. Confirmatory tests are used for further analysis of a sample -- to confirm a positive or sometimes, negative, result and typically are done using gas chromatography/mass spectometry (GC/MS) or high performance liquid chromatography (HPLC). Confirmatory testing can identify a specific drug. If the goal is to detect a synthetic or semisynthetic opioid, this test should be used as immunoassays do not typically detect these opioids. Below is a comparison of the general characteristics of these two types of testing:
|Screening (Point of care testing is usually only screening)||Confirmatory|
|Analysis Technique||Immunoassay||Gas Chromatography-Mass Spectrometry (GC-MS) or High Performance Liquid Chromatography (HPLC)|
|Sensitivity (power to detect a class of drugs)
||Low or none when testing for semi-synthetic or synthetic opioids||High|
Specificity (power to detect an individual drug)
|Varies based on assay used - can result in false positives and false negatives||High|
|Use||Qualitative analysis; detects classes of drugs (Heit and Gourley, 2004)||
Quantitative analysis; identifies a specific drug
|Cost||Inexpensive (FDA five-drug testing kit ~ $1)||More expensive, may not be paid for by insurance|
||On-site - rapid||Slow|
|Other||Intended for use in drug-free population; may not be useful in pain medicine context||Legally defensible results|
(Compton, 2007; Heit and Gourlay, 2004; Reisfeld, et al, 2007)
Due to the possibility of false positives and the qualitative nature of screening tests, confirmatory testing is recommended to affirm positive or unexpected results and to identify the presence of a specific drug (Compton, 2007; Reisfield, et al, 2007).
Point of Care UDT. Point of care UDT offers the advantage of quick results with all the limitations of screening tests described above. Providers may want to conduct screening tests at the point of care by obtaining a CLIA waiver.