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

It's quite simple. The USTAF president gets on the phone to the IAAF president and tells him to make sure the B sample is negative.

Here’s your example of how it happens when you haven’t taken EPO and they botch the test.

The A sample was contaminated, mishandled or mislabeled, in either case

Or the B sample shows a false negative in the case where the athlete is using EPO.

I don't think it can show a false positive.

Not how it works. Both A +B are split from the same urine sample. The athlete certifies each sample with a marked seal and identification numbers are registered, so that the Chain of Custody documented and ensured. The only sample that counts is the B sample. The A sample serves as a notification sample -- hey theres something wrong here.

The B sample is reserved so that the athlete or his representatives have the opportunity to be present at testing.

A friend total me you can also just donate to the UCI and it makes positive tests go away

So, you are saying the A sample cannot be contaminated, mishandled or mislabeled?

Then explain how you can get an A positive and B negative as the OP asked.

GCMS testing. Same as with drug testing. A sample can be teated and a trace of the alleged substance is detected, however the amount detected is below the threshold level. Hence a negative test. Same applies with B sample testing. Masking agents are accessible unfortunately and those athletes at the absolute highest level have access to via their team ‘physician’ err pharmacist.

The two samples containers are NOT glued together.

labpedia.net

Urine changes When Urine Left at Room Temperature, and Preservatives

Dr. Riaz

Urine changes When Urine Left at Room Temperature and without preservative. There will be effect on the various chemicals and it is advised to keep urine at proper temperature and add preservatives.

Link:labpedia.net/urine-changes-when-urine-...

"And you can't see these germ-things ... they're nothing at all. Anything you can't see, ain't, that's what. Fighting things that ain't with things that ain't! They must have been all fools in them days. That's why they croaked."

-- The Scarlet Plague by Jack London

It's possible when athlete at time of sample collection was able to mix in an enzyme with the sample that with time reacts with all protein in the sample. This process takes time and the reason why A is positive and B is negative.

I would guess if you go to google scholar you could find a vey detailed answer explained by experts.

Testers should probably split the sample into three parts. One result doesn’t necessarily prove anything, two conflicting results are a conundrum, two out of three similar results are a trend. False positives will occur in testing, but not two out of three times in a certified laboratory.

I’m not sure about epo, but I had a stats professor who was mostly a cancer researcher at a big university.

He basically said, everyone has cancer. the questions are whether you have enough to matter and whether it’s growing fast enough to matter. A big tumor growing slowly could get in the way of some functions, but tumors growing quickly was the biggest concern.

In a specific sample you aren’t getting a perfect distribution of cells and the test has a margin of error. So if the sample happens to have a high percentage (1/300) and the east reads high (1/300) you could have 1/90,000 chance of a false positive. Pretty low, but if they test 90,000 times a year, someone will get a false positive. Chances of b also failing would be 1/90,000 if you weren’t positive. So it’s like 1/8 billion you’d fail both. (Testing protocol were built around high range so you would need both to fail ex if test showed .12 w/99.7 confidence interval of .06 and .18, they couldn’t really say you failed if the threshold was .10)

*99.7 percent you are w/in 3 standard deviations of the mean so ~1/300 you aren’t depending on type of distribution etc. Probably could say 1/600 because it’s only really testing at the high end but just being conservative.

Doping tests are like Covid tests we've all seen, both are completely useless. How many times have you seen someone say they tested negative for covid but was sure they had it so then they test themselves 10 more times?

The “ B” sample was taken straight from the tap.

I don't think anyone has answered the original question yet. The various posts about COVID tests and cancer tests basically say that either false negatives aren't reliable (yes lots of false negative COVID tests, but very few false positives for COVID that I am aware of) or everyone has cancer at some level so a positive doesn't really mean anything., it is the threshold that matters In this case, no person should have any exogenous EPO ESAs in their body, so there is no threshold here, and we really don't care about false negatives. How does an A sample test positive for a substance that you should have never been exposed to (exogenous EPO ESAs) and then the B sample shows negative? The only scenarios I can come up with are intentional tampering with the A sample, or maybe the time period between A and B deteriorating the B sample enough that the test is no longer effective. These seem to be unlikely. Can someone explain the likely cause of an A positive and B negative?

Except last time I was in the lab watching them run a B sample it was isoelectric focusing and not GC-MS...