From what I’ve read, about 2-3ml of blood is the minimum necessary for most blood tests. Some tests require more. This is a good article about the challenges of using finger-stick blood: https://cen.acs.org/articles/95/i1/Blood-tests-fingertips.html

DNA testing can be done on very small samples because the DNA is amplified using PCR – without PCR those small amounts would be insufficient for testing. The finger clip that tests for oxygen and pulse is based on light transmission – again, totally different technologies.

One thing that always made me question Theranos is that they kept saying that their technology was super secret – for years. Generally speaking, before a company discloses an invention to anyone for any reason (counsulting, investment, manufacturing, sourcing parts ,etc) they file a patent application. Since it is impossible not to work with consultants, etc, these applications are filed early. Under most circumstances, if that application moves beyond the provisional filing (meaning anything that continues to be commercially viable), the application is published after 18 months. So most companies working at the pace that Theranos claimed to be could only keep things secret for 18 months. And even if an application is not available to the public, it can be disclosed privately under an NDA. But Theranos continued to keep this “big secret” for years and people believed them without asking the simplest questions. Walgreens invested $140 million. Considering that this was a patent driven device, why didn’t anyone talk to a patent attorney? As the saying goes, you can’t cheat an honest man. These investor groups rushed in because they thought they had the equivalent of insider information (though it wasn’t a public company). It is a Madoff-like situation.