A urine drug test widely used by pain management and addiction treatment doctors to screen patients for illicit drug use is wrong about half the time – frequently giving false positive or false negative results for drugs like marijuana, oxycodone and methadone.
The “point-of-care” or POC tests come with immunoassay testing strips that use antibodies to detect signs of recent drug use. Physicians like the urine tests because they can be performed in their offices, are inexpensive, and give immediate results. But experts say the tests are wrong so often that no doctor should base a treatment decision solely on the results of one test.
“Immunoassay testing has an extraordinarily high rate of false positives and false negatives as compared to laboratory testing,” said Steve Passik, PhD, Vice President of Research and Advocacy for Millennium Health, which analyzed urine samples from nearly 4,300 POC tests obtained at addiction treatment clinics.The Millennium study was published in The Journal of Opioid Management.
A false positive reading means a drug was detected that isn’t actually there, while a false negative means the POC test missed finding a drug that was present in a urine sample.
The Millennium study found plenty of both.
False positive readings for marijuana, for example, were given over 21% of the time, while false negative results for marijuana also appeared about 21% of the time.
The POC tests had an even worse track record for oxycodone, a widely prescribed opioid pain reliever. False positive results were detected over 41% of the time and false negatives over 31% of the time for oxycodone.
“We always knew it wasn’t as sensitive and we always knew that it didn’t look for specific drugs within a class. But this was revealing in regard to how much it misses, with false negative and false positives rates in 40 to 50 percent in some instances,” said Passik.
“If we were in another area of medicine, let’s say oncology, and you had a tumor marker or a test that you were going to base important treatment decisions on, and it was as inaccurate as immunoassay is, the oncologists would never stand for it.”
Passik says “the word is starting to get out” how inaccurate the immunoassay tests are. But few patients are aware of it and some doctors are still dropping patients from pain management programs after POC tests found illicit or unprescribed drugs in their urine.
Passik told Pain News Network that patients should insist on a second test if they feel the first one is wrong.
“If they think it’s a false positive, they need to ask the doctor to be re-tested. And particularly they should ask what method was used. And if they find out they were tested with immunoassay, they should say they want the same specimen either re-tested at the lab or they want to provide another specimen tested at the lab,” Passik said.
A laboratory test that uses chromatography-mass-spectrometry to break down and identify individual molecules is far more accurate than an immunoassay POC test, but it could cost thousands of dollars — something many insurers and patients are unwilling to pay for.
But addiction experts say more reliable and expensive testing is needed, simply to be fair to patients.
“Heavy reliance on immunoassays in addiction treatment can be detrimental to the patient due to their higher risk for false positives and false negatives in comparison with more reliable technology, such as chromatography-mass-spectrometry,” said Michael Barnes, executive director of the Center for Lawful Access and Abuse Deterrence (CLAAD), a non-profit that gets some of its funding from Millennium.
“A false positive can be detrimental to a patient by subjecting her to unjust suspicion or accusations, unnecessary adjustments to the treatment plan, or the deterioration of the practitioner-patient relationship. A false negative may result in delayed diagnosis or misdiagnosis, false confidence that a patient has not relapsed, and failure to catch behavior that could eventual result in a preventable overdose death. Therefore, chromatography-mass-spectrometry is often more appropriate.”
Millennium’s Passik says most doctors recognize that both tests may be needed.
“These two different methods yield very different kinds of results,” Passik said. “If I was still practicing, I wouldn’t feel that immunoassay is accurate enough to be the only test that you use.”