We recently reported1 that lower estimated glomerular filtration rate is associated with lower circulating concentrations of 24,25-dihydroxyvitamin D3 (24,25[OH]2D3), a metabolite that is central to the catabolism and clearance of 25-hydroxyvitamin D3 (25[OH]D3). In that article, circulating 24,25(OH)2D3 and 25(OH)D3 concentrations were quantified using high-throughput mass spectrometry assays developed and validated in our laboratory.2-4 More recently, the National Institute of Standards and Technology (NIST) developed a candidate reference method procedure and used it to quantify 24,25(OH)2D3 concentration in their Standard Reference Material (SRM) 972a.