According to the BBC:

[The paddock] contains a protein called pholasin, which gives off light when it comes into contact with “free radical” chemicals.

High concentrations of these are normally released by white blood cells as a counter-measure against infection, and as such, they can be present even if no symptoms have yet been felt.

The test uses a pinprick of blood, which is mixed with pholasin in a test tube, and the resulting light levels emitted measured.

The idea is that if athletes are feeling tired, they can check for possibility of infection. If this seems to be the case, they would know to ease up on training to recuperate, and perhaps even start an antibiotic regiment sooner.

So far, it sounds like only Britain’s sailing squad has been using the test in preparation for the Olympic Games, which kick off this Friday, August 8. Bully for them, but we’re still rooting for the US sailing team.