I think the exposure to all that SciFi has left people of an unrealistic expectations of detectors.
So it seems.
Why? Blood is rich in iron and that will swamp out any conductivity effects.
Iron - and for that matter lead - in blood is bound to protein. It's a bit like saying that rust
is a good conductor because it contains iron.
Assays for specific elements or compounds make use of some unique
characteristic of the material. Often nowadays in (bio)medical applications, antibodies created specifically to match the material are used, failing that chemical reagents and for elements - such as lead - mass spectrometry ()which can equally distinguish any
element would be by far the most likely technique.
Most current research focuses on (electronic) detector arrays using (many different) antibodies bonded to the sensors. Not a simple matter by any means but of course researched in the expectation of great profits.