Elon Musk has a long history of saying stupid things about COVID. Here's the latest, and it's a teachable moment.
He frames the tweet in a way that supports the deniers - something must be bogus with testing if he gets two positives and two negatives.
But antigen tests have low sensitivity, and will do exactly that if they're used when your viral load is borderline detectable.
Note that he has mild symptoms, and he's in a job with frequent contact with a lot of people.
Here's a model of viral load in the progression of a typical symptomatic case. If Elon's having a mild case that's almost asymptomatic, his viral load may have peaked lower and/or earlier than this model shows.
If he's at day 1, his viral load may already be down to a point where antigen tests can barely register it. He's also coming out of his highest risk time as a spreader.
Antigen tests are useful because they correlate well with when you're likely to be contagious, but they miss a lot before the viral load builds, and after it's reasonably under control. PCR will pick up most of these.
I'd give pretty good odds that he's infected and both of his PCRs will come back positive.. It's likely that he'll have a very mild case and be moving on with life by early next week.
Had he taken the antigen tests a day or two earlier, they'd likely all be positive. A day or two from now they'd all be negative. They're fast, they're cheap, and they're for screening.
This is an example of Elon's fast-and-loose history with COVID tweets. This was just after CA had started lockdowns and 3 days before NYC locked down.
A month later, he was looking at fatality graphs from the CDC, in which the most recent 2-3 weeks is always incomplete as death reports take a while to fill in. He saw the illusory declining trend on chart and declared things were winding down to zero quickly.
He also claimed that kids are "basically immune" and don't transmit the virus, which was known to be false even at the time he tweeted it.
A March 13 article on The Verge.
"not within the top 100 health risks in the US".
That escalated quickly.
tl;dr - He's almost certainly infected, with viral load on the decline in a very mild case, near the limits of what rapid tests can detect. And he's not understanding the math, which is shameful for someone with his intellect.