In this episode, I’ll discuss potential reasons for an unexpected positive urine barbiturate screen.
When a urine drug screen comes up positive for barbiturates unexpectedly, healthcare practitioners often suspect the result is a false positive. Both ibuprofen and naproxen have been reported to cause a false positive urine barbiturate result in a study of healthy volunteers. This study found that 2 of 102 patients taking NSAIDs had a false-positive for barbiturates by the fluorescence polarization immunoassay (FPIA). One patient was taking ibuprofen and one was taking naproxen. The good news is this method of barbituate assay is less frequently used in hospital laboratories today as newer immunoassays predominate. Theoretically, phenytoin could cause a false-positive on an immunoassay, although I can’t recall ever encountering this in practice.
In my practice, I have found that it is far more common for a patient with an unexpected positive urine barbiturate screen to actually be taking a barbiturate. The barbiturate that often flies under the radar of clinicians is butalbital. This short-acting barbiturate is a component of Fioricet or Esgic, a commonly prescribed analgesic. However because butalbital is not used for any other indication or available separately, many clinicians do not realize that it is in fact a barbiturate. Another sneaky one is primidone. Clinicians might see “primidone” on the home medication list and forget it is a barbiturate precursor to phenobarbital.
Whenever I am asked to investigate an unexpected urine barbiturate screen, the first thing I do is look for butalbital or primidone in the patient’s home medication list or medication administration record. More often than not, one of these two barbituates explains the positive screen. Also, if I recommend a urine drug screen for a patient I first check to see if they have been taking butalbital or primidone. If so, I make sure to let the patient’s nurse and physician know to expect the barbiturate screen to be positive.
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