Capillary samples aren't as accurate as a venous draw. You will almost always get hemolysis no matter how well the blood is flowing. You will get exudate fluid mixed in with the plasma or serum if it's a clot tube.
It would be easy to draw venous with a butterfly needle attached to a green top sodium heprine gel tube, yellow top gel tube, or a redtop non-additive tube and let it clot so you can pipette the serum off into a transfer tube.
Microtainer tubes for finger sticks force you to work with small volumes, the smaller the volume of plasma or serum the more things like hemolysis and exudate will throw off measurements.
The analyzers also have to be dead on with the smaller sample volume, if there's any extra protein on the analyzer sample probe it will throw off measurements.
For pediatrics you're forced to use capillary samples, but no one that works in a lab will tell you they're all that accurate, they're often crap samples that have to be handled very carefully as it's usually one and done testing, you don't get to repeat the test if it's off, you just add a note stating it's a pediatric sample, hemolyzed, micro clotted, and hope the doctor doesn't want a redraw pissing off the mom.
And the above guy brought up Theranos. I knew for years that this was a scam, microtainers simply can't carry out the sort of panel testing she was claiming, it was bullshit and I told my peers that didn't catch on, when she was finally busted I felt like Nostradamus. Her trial begins in a few months, lets see what they do on this.