…within and 15.20% between assay for 1,25-dihydroxyvitamin D. Proficiency testing was conducted with DEQAS, CAP, API, and NIST for 25-hydroxyvitamin D and with DEQAS and CAP for 1,25-dihydroxyvitamin D as described previously (Smith et al. 2005a,b, 2008, 2009a, 2012b; Zwart et al. 2011b, 2013).