Hi, pathology resident here. Even with large volume venous blood draws, you can change the values quite substantially. one of my professors likes to tell the story how just by pumping his fist as is usually directed by The phlebotomist, he was able to change his potassium level from 4 to 5 to 6 to 6.5. This is the difference between normal and starting to worry about heart problems. so it is far from shocking at least to the experienced, that this is a problem for microfluidics.<p>This essentially equivalent to a sampling error problem. A large venous sample (10 mL) is enough to get a pretty good average. Microliters of blood from any given location in the body are likely to be different from microliters somewhere else in the body. I seriously doubt anyone in a clinical lab would be surprised by these results.