Experts, please explain how an A sample can test positive for EPO and a B sample does not

Exactly. Thank you. I posted the same thing and my post was removed.

There are two parts to this answer. One is statistics, the other one is chemistry/medicine.

Statistics: a test for a substance uses a certain sequence of processes, materials, human capital etc. in this process, things can become contaminated, not only by manipulation as suggested by others here, but also just as part of the process. Think about wearing a white shirt, but maybe during the day somehow you got a stain on it and when they hold that small piece of your shirt under the microscope, they will say "hey, this is brown, not white" (positive A sample) and then someone checks a different part of your shirt without the stain and they say "nvm, it's white" (negative B sample). There are a million ways the stain could have gotten there. So the test relies on probability - it says "ok, given what we know about white shirts, our process is so good that in 99% of cases we correctly identify a white shirt". And to a scientist, that's fine. To the very very rare athlete with the small stain, it's tough. And this goes both ways, there can be false positives or false negatives. The more sensitive you make the test to detect stains (i.e. get a crazy good microscope) the more likely you will find even the tiniest stains (traces of doping). But the more sensitive it is, the more likely it becomes that you zero in on a tiny tiny stain that really does not say much about the shirt as a whole. You can look this up in scientific terms: specificity and sensitivity.

The second is medical science and depends on the substance: often, the test does not detect the actual substance, it detects certain proteins or other biochemical structures that are associated with that substance and concludes that it must be the substance. EPO tests have been criticized for this by cyclists (which is a dirty sport, so take with a grain of salt, but there is at least some scientific evidence from the lab to back it up). And there can be other substances that shouldn't be in a human body but that have the same or very similar proteins as the doping substance. We don't eat clean, our food/water is treated with chemicals etc., so the test might not be able to distinguish between different sources of that particular thing it's looking for (not "monospecific" is the term for it). And then sometimes they use more/different testing methods on the B-sample to rule out that this happened and find that with the other method for the same substance it's negative. So yeah, it can be legit.

Thanks for this answer. This was my impression. From what the Bol lawyer’s comment was, he thinks that there is a potential for the scenario at the end of your first paragraph. That they narrowed in on one band with a potentially oversized conclusion from the finding. Granted, he’s the lawyer and it’s his job. They’re asking for further testing on the A and B samples and it feels like we will get a satisfactory answer here if solely because there is so much scrutiny and there aren’t potential red herrings (eg meat contamination) involved.

Translation: I got $300 per hour to pick apart a test that works completely fine. I equate suboptimal to unreliable and engage in other forms of sophistry that are not befitting a science so skilled in the art….

Trust the science

... and offers $ incentive. Or the head of (major shoe company) gets on the phone with USATF and tells them to make sure the B sample is negative.

Pro track is rife with corruption on all levels -- athlete, coach, sponsor, national organization, international organization.

Who has deemed your skills worth paying you to oversee such a test? Classic social media...

Perhaps you were wearing diapers and don't remember but USADA was thoroughly embarrassed by the Lagat case, which proved that their test did NOT work fine, beyond just the transport-induced degradation. And I do consider electing not to use techniques to ensure sample integrity which are available to them for no clear reason other than convenience or laziness to be suboptimal, particularly when appropriate controls are not used to demonstrate that sample stability is not being impacted.

Rather than addressing the issues USADA limited observer mobility within the lab, put a gag on the observer under penalty of being thrown out, refused to allow them to ask questions until the end, and at that point conveniently made the lab head unavailable to answer questions at the end as promised...or at any other time point after.

But yeah...totally a process on the up-and-up.

Is this an actual situation or are you asking about a hypothetical?

Thankfully we have heaps of EPO experts on this site to help us all understand.

Is it possible to cheat your way through college?

big thanks to "been there done that" and "basicresearch"

i suspect most people have never even seen what the slides stains etc. look like in real life. they probably think the test is run and then the computer spits out a binary answer to signal positive or negative. truth is the interpretation of the result is an art form, non-binary if you will. but the non-binary result must be transposed into binary form.

so it's open to error in that aspect as well.

I don't know why the brojos removed that (noticed a long time ago now), but here's a working link to the MS word file:

Clearly what is needed is a C sample for a tie-breaker. Two out of three.

Absolute clowns (WADA and the test labs, not the Brojos). WADA transported Lagat's urine during the summer time in a hot car with temperatures of 38-40 degrees for many hours and when Lagat's team questioned why it wasn't kept cool to prevent deterioration, the WADA people complained it's too much hassle and costs too much for a bunch of ice and a styrofoam cooler? These are the types of clowns we are all paying to supposedly protect us from the evils of doping?

So, comparing it to the white shirt with a brown stain:

It's like saying this spot in the sample is dirty but this other spot is clean?

And you need a completely dirty shirt to call the shirt dirty?

If you unironically call yourself a scientist skilled in the art you're a clown. I get that this is a term you heard lawyers use, but you should've immediately realizes how dumb it sounds, if you weren't a moron.

Your argument is not compelling. Read the report (from the non-broken link).

I have

Now respond to my comments.