One problem is that blood isn't really a homogeneous substance. When dealing with small test samples, you might see different results, perhaps widely-varying ones, from one drop to the next. (Play around with a glucose meter sometime.) Things like where the blood was drawn from, what contaminants were present on the skin at the time, and the age and health of the patient can all have an effect.

Also, many tests require a centrifuge, so that's something else you have to account for if your goal is to perform the same tests with only one drop of blood. Unless your plan is to pull some Star Trek-level technology out of your hat.

TL,DR: you didn't need to consult a team of PhDs to debunk Theranos's business plan, you could have asked any experienced med tech. The investors didn't bother doing either.