I’m told that blood testing labs often use reference numbers from LiquiCheck to cross-check their values. It’s based on a supposedly-representative collection of 800 people who submitted blood samples to multiple labs. The results of identical samples submitted to different labs for Vitamin D show incredible variability:
Takeaway: it’s very hard to draw comparisons between different studies of Vitamin D. Unless you know the specific lab that processed the sample, it’s nearly impossible to tell whether the values are comparable.
Note that each of these labs is CLIA-certified, which means they have passed rigorous inspections to ensure they produce consistent results relative to that lab, but not necessarily relevant to other labs.