We investigated the impact of 24,25 ( OH ) 2D3 on the performance of commercially available immunoassays for serum total 25-hydroxyvitamin D [ 25 ( OH ) D ] using ( a ) serum from a nationally representative sample of adults , ( b ) serum from a spiking experiment , and ( c ) data from the UK Vitamin D External Quality Assurance Scheme ( DEQAS ) .