Some tests we have are not good predictors for some nutrients.

Magnesium for example only shows very low or very high with good predictability. The "normal" range is very large and some deficiency may exist within it.

It is thought that measuring serum is less accurate than measuring what is within the cells themselves.
This lab does intracellular measurement:
http://www.spectracell.com/

The "functional medicine" chiropractors do a more comprehensive testing panel, but that is often not covered by insurance.

Some common tests in US have antiquated ranges for normal for B12 and Vit D. The low end of this scale is considered "deficient" by many well informed doctors today. But people are given "normal" verbal results when in the very low end of "normal". So knowing your levels is important here.

IMO people respond to nutrients often irrespective of their tests therefore. Allopathic interpretation of nutrients blood serum testing therefore is not always accurate, or predictive of the patient's response to many nutrients.