TABLE 7.

Lessons learned from the folate and vitamin D NHANES crossover studies¹

Topic	Lessons learned
Folate	Use of HPLC-tandem MS method and comparison to international reference materials revealed systematic bias for BioRad radioassay
	BioRad radioassay was stable over time and precise (<5% CV) and it showed a high correlation to the microbiological assay; this led to robust regression equation used to adjust data
	Several regression models were evaluated to find the best fit for the data, paying particular attention at the tails of the distribution where prevalence for low or high folate status is derived
	Relation between BioRad radioassay and microbiological assay varied by matrix
	Trending data over time needs to be based on adjusted assay data
Vitamin D	DiaSorin radioassay shifted over time, masking the ability to monitor “true” population trends over time
	Separate regression equations linking the DiaSorin radioassay and HPLC-tandem MS method had to be derived for different time periods
	Several regression models were evaluated to find the best fit for the data, paying particular attention at the tails of the distribution where prevalence for low or high vitamin D status is derived
	Use of in-house QC materials in each analytical run provided information on method shifts; this was used initially to QC-adjust the data and thus “smooth out” method shifts
	Trending data over time needs to be based on adjusted assay data

¹QC, quality control.