We investigated the impact of @,@ ( OH ) @D@ on the performance of commercially available immunoassays for serum total @-hydroxyvitamin D -LSB- @ ( OH ) D -RSB- 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 ) .