The bold claim is based on a recent study from the University of Nottingham, where researchers used antibodies (immune cells) to detect the presence of antigens (markers for a foreign presence within the body) for breast tumour tissue. The results were positive, showing that detection using this method was definitely possible, although with only a 37% success rate at pointing out positive cases: it definitely leaves a lot to be desired before this test can be useful in the clinical setting.
“The results of our study show that breast cancer does induce auto-antibodies against specific tumour-associated antigens,” said Daniyah Alfattani, one of the researchers who worked on the study. However, he expresses that more work is needed before it can have a clinical use: "Once we have improved the accuracy of the test, then it opens the possibility of using a simple blood test to improve early detection of the disease."
This is promising research, which could lead to the development of a simple blood test to detect an individual's future risk of breast cancer. However, many news websites could be over-exaggerating just how promising the study is. There isn't a blood test that could detect the onset of breast cancer; there could, theoretically, be a blood test that detects that, in the near future. That is all the research has shown. Some news sites didn't even bother with the "could".
As researchers continue to work on the accuracy of these tests (hoping that, someday, they can become a reality), thousands across the world will already be believing that this test already exists and is running smoothly. It underlines an important issue within science communication: journalists not trained in science reading through these press releases, without really understanding the actual implications of them - then misinforming their readers about the current state of modern medicine. Or worse, purposeful misinterpretation of these press releases in order to create a "clickbait headline" for their website.