It is self reported according to the US census recognized racial categories: white, black, asian, native American/Alaskan, native Hawaiian or Pacific islander, and other (or two+ categories). Hispanic/latino identification is a separate box you check for reasons that are hard to explain without going over decades of bureaucratic decisions.

All levels of the US education system teach (now at least) that race is a social construct. There's no concept of population-genetics taught, until much later, in hard science classes.

Yes, this is a common point of confusion when talking to Europeans about racial issues in the US (as I found out myself recently). Race in our contexts refers to your background/birthplace/heritage. On our government forms: "What race are you?" "White, black, hispanic, etc."

This is fundamentally different by intent than in Europe (using french here) where we refer to 'la race humaine' which is the _species_.

The nuance is critical during debates. While I was discussing racial differences to some Swiss folks, they thought I was talking Nazi propaganda! We are all part of the human species, the human species has many races. We are all equal!

i believe it is self-declared.

There are various risk factors, and some lab tests, that differ among racial groups.

For example, my labs include at least two that have different specified thresholds for "African-American" or "non-AA" patients.

https://en.wikipedia.org/wiki/Sickle_cell_disease#United_Sta...