You still need a negative PCR Test within 3 days of your return to the US. I just did an IST run. I stayed 4 days so had to get another COVID test arranged through the hotel to get back into the US. With your quick turn, if you get a PCR test before departure, that original test might still qualify for the return, but only if that test was taken within 3 days of your entry back into the US. Hadn't thought of that possibility but I don't see why it wouldn't work if the timing was right. The issue is how fast you can get the PCR test results before departure and still stay within the 3 day time limit required by the US.