Is it possible to accurately measure micronutrient levels with the blood volume from a Dermal / Capillary puncture?
Detection methods are constantly getting better, requiring lesser amounts of blood to make a determination. In newborns, a heel blood sample (of capillary blood) is used to detect up to 40 (sometimes more) disorders from maple-sugar urine disease and hypothyroidism to hemoglobinopathies and organic acid conditions. If such large molecules can be detected in capillary blood, smaller molecules should as well.

There are differences in central venous sampling and capillary blood sampling, but it could be argued that since the capillaries are the site of exchange of nutrients, this is the most accurate site to determine availability of micronutrients in blood.

One reason labs take more blood than required for the test is to run a second analysis if the first is abnormal. Although it's not done for every abnormal, clinicians often want to confirm significant abnormals before treating a possible lab error.

Newborn screening tests
Newborn Screening
Conditions Screened By State
A Layperson's Guide to Tandem Mass Spectrometry