[page143]

Is it possible to test positive for Cocaine but negative for Benzoylecgonine on the same Hair Test?

On the first test, Cocaine was positive, but Benzoylecgonine was negative.  It did not indicate the quantitation levels, but the cutoff for the the initial test level and the MS confirm test level was 300 pg/mg.

On the second test, the Screen cutoff was 500 pg/mg.  Confirm cutoff was 50 pg/mg for Benzoylecgonine and 100 pg/mg for Cocaine.  Quantitation for Benzoylecgonine was 99 pg/mg and showed positive.  Quantitation for Cocaine was 821 pg/mg and showed positive.

If the Benzoylecgonine level is below the cutoff, how is the Cocaine level higher?

[Babcock_Hall]

Did you try to search on "benzoylecgonine" here?  I did a search and a number of threads popped up, although I did not see an answer to your specific question.

[page143]

Yes, I have been searching constantly.

I can find everything to support a positive Benzoylecgonine test as the confirming factor even if the Cocaine is negative because Cocaine leaves the system sooner than the Benzoylecgonine.

I can't find anything that would explain a positive Cocaine level, and a negative Benzoylecgonine level.

Something just doesn't make sense. The person did have surgery during the testing window and thought maybe liquid cocaine was used during his anesthesia, but we can't find anything to support that either.  I don't know if that would indicate presence of cocaine, and also explain that it was not metabolized.

[Furanone]

Maybe he fell asleep in a bed that had cocaine on his pillow and he rolled over on it, or maybe he was packaging cocaine and as some dropped into bag it powderized into the air like when you squeeze a bottle of baby powder and this got on his hair -- thus no metabolite! (OK overactive imagination, I know)

[page143]

Can anyone point me in the right direction here or have any insight as to what would cause a positive test for cocaine but negative test for benzoylecgonine. 

I have researched this site and find information about what might cause a false positive test for benzoylecgonine other than cocaine.  In these cases, it seems that positive results were seen for both cocaine and benzoylecgonine, or just positive for benzoylecgonine which is normally what the test is searching.

I am looking for a reason why cocaine would be positive and benzoylecgonine is negative.  Are there any medical reasons that would cause a false positive for cocaine, but not show the benzoylecgonine?  Are there external factors that could cause this, meaning he didn't ingest cocaine for it to be metabolized through the liver, but somehow it is getting picked up on a hair test?

[Borek]

I am afraid you may not get any answers - not because nobody is willing to help, but because we don't know. These questions surface now and then, but I have yet to see a discussion that I would classify as satisfying.

[Arkcon]

Also try: http://www.chemicalforums.com/index.php?topic=2456.0

Like Borek: said, this sort of question is problematic.  The information on the shortcomings of analytical methods for drug detection simply aren't common knowledge.  The vendors of instruments and kits acknowledge there are some shortcomings, but won't give away the details.

[Archer]

Hi,

Cocaine is always in higher concentration in hair than benzoylecgonine, this observation has been documented:

http://jat.oxfordjournals.org/content/24/7/489.full.pdf

Drug elimination is a very complex process the following processes may contribute to the observed results:

Cocaine is extensively metabolised by the liver, however it is not common practice to consume cocaine orally in most countries where coca leaves are subject to control. Insufflation, injection and inhalation of vapours put cocaine into the bloodstream but bi-passes the liver thus avoiding the "first pass" effect.

The higher the polarity of a drug the less likely it is to be deposited in the medulla of the hair, therefore cocaine has a higher affinity for deposition (benzoylecgonine is zwitterionic).

A proportion of ingested cocaine enters the kidneys before the liver and is eliminated as unchanged drug. Benzoyleconine is also passed into the urine but due to it's zwitterionic properties the pH of urine has to be very precise (I don't know what) in order for passive diffusion back into the blood. However if urine pH is slightly elevated then cocaine can passively diffuse back into the bloodstream.

Directly after ingestion it can take up to one hour before plasma benzoylecgonine levels exceed plasma cocaine levels.

Co-administration of cocaine and ethanol forms cocaethylene via a competing transesterification reaction with ethanol during metabolism with cocaine hydrolase in the liver.

All of these factors could contribute to the observed results, however, the zwitterionic nature of benzoylecgonine will probably have the highest contribution to what you have observed.

I hope this helps.

Archer

[Archer]

Directly after ingestion it can take up to one hour before plasma benzoylecgonine levels exceed plasma cocaine levels.

Apologies, I should have backed this statement up with evidence but I could not find the article immediately last night and I figured the information was more important than the reference in the short term.

The data for cocaine pharmacokinetics can be found here:

http://jat.oxfordjournals.org/content/19/6/459.full.pdf

Archer