Enzymes usually have cofactors. What if cofactors have cofactors that affect the activity of the cofactor and thus the activity of the enzyme?
For example, a number of enzymes use thiamine pyrophosphate (TPP) as a cofactor and TPP uses magnesium. In this case both cofactors are essential. I am wondering if most essential nutrients bound to enzymes have one or more non-essential nutrients as cofactors bound to them, out to several coordination spheres, to at least where the milieu provides the last coordination spheres.
In which case, nothing beats a biological activity measurement because any cofactors of the cofactors are already integrated into the output. One may not know why a measurement is low, but at least the measurement is more valid than a mere measurement of concentration.
Most medical measurements are simple measures of concentration and they will totally miss this effect.