Is it possible to have a false positive PCP result?

[UM Review RN, ASN, RN]

Had a patient a couple of weeks ago who came to us showing a positive PCP level.

Although he admitted to smoking cigarettes, he denied taking street drugs. He also had to take pain meds for a chronic pain history. Not a lot, just a couple a day.

Normally, when someone denies drug use, I have to roll my eyes and stifle a smirk. As in, you only had 2 beers and your ETOH level is 140? Yeah, right. Tell it to Sweeny.:icon_roll

But this patient seemed different--no pupil changes, no overreacting, no slurred speech or hyper behavior, everything seemed very normal to me and to others who had contact with him.

So is it possible that he was telling the truth? Or did he just fool us all? And how long does that stuff stay in the system anyway?

[sirI, MSN, APRN, NP]

hello, angie o' plasty, rn,

there have been cases where dextromethorphan, certain anti-depressants, and diazepam, can cause false postives for pcp. i'm sure there are others, but these are off the top of my head.

pcp can be detected for approx. 1 day to a month depending on how much taken and when the test was performed.

[Blackcat99]

I don't know. We had a patient who was very depressed because he was told that he was HIV positive. However, when they re- checked his blood at my hospital he was negative. Could there be a blood specimen mix-up? When I worked at the prison, the inmates always swore they had not used drugs when they tested postive. They swore it was from taking motrin.

[jmgrn65, RN]

I know that some medications and herbs can mimic other things, would need to research to know if pcp specifically can be a false positive.

[ICRN2008, BSN, RN]

It depends on what type of assay is used in your laboratory. Some methods are more prone to false positives than others, and no assay is 100% specific. I would call down to the laboratory and ask one of the techs to look up the procedure or package insert and send the list of interfering substances up to you. Then, you and the patient can get to the bottom of this. If it has not been too much time since the initial specimen collection, you could to ask the laboratory to send out a new specimen to a reference laboratory that uses a different method. Another option is to ask if they might still have some of the original sample in the laboratory. If it hasn't degraded appreciably, they may be able to send this sample out for retesting.

[Meerkat]

Interesting you should ask this! I recently had two very unlikely patients show up positive for PCP. One was a 14 year old severely autistic boy who was basically mute and developmentally delayed. The second was a 57 year old woman. I questioned both results, too but forgot to look up possible false positives. Thanks for asking this ques!

[Meerkat]

hello, angie o' plasty, rn,

there have been cases where dextromethorphan, certain anti-depressants, and diazepam, can cause false postives for pcp. i'm sure there are others, but these are off the top of my head.

pcp can be detected for approx. 1 day to a month depending on how much taken and when the test was performed.

great answer as usual from siri! and love your avatar thingy!

[rjflyn, ASN, RN]

It also depends on the test. In my exp we are only doing the rapid urine drug screen on 99.9% of the patients we test. Adding serum ASA, Tylenol, ETOH and the usual routine med level ie dilantin ect. In my many years I can only recall one or two PCP results that one would consider suspect or inaccurate. This is mostly because seeing a patient that uses this is far and few between, hence the tendancy to not trust that result. And Angio I agree it it were any other substance they would get the eye roll/ yeah right.

My favorite is the two beer answer- ok was that 2 fortys, 2 twevle paks, or two cases.

Rj