It seems that there is a consistent issue with variances of serum and functional measurements of vitamin D.
In short, serum determinations give a static quantitative measurement while functional intracellular assays reflect the long term status of Vitamin D levels.
The results of the serum and functional tests may or may not be in agreement, but this apparently conflicting information is still useful to physicians and patients. If a patient has a low serum level of vitamin D, yet the functional test is normal, these results suggest a short term deficiency of vitamin D and the patient should be repleted. If the serum test is normal, yet the functional test is deficient, the cell may need more vitamin D for optimal function and the patient should be repleted. Therefore both tests are useful in assessing the vitamin D status of patients.
The same principal described for vitamin D may occur with any analyte for which we are reporting functional test results.
What are your thoughts on the difference between functional testing and serum testing?