Vitamin D Levels Determined from Dried Capillary Blood

An easy technique has been developed to determine the 25-hydroxyvitamin D level, also known as 25-OH vitamin D status, from dried blood samples on filter paper.

The method allows determination of the 25-OH vitamin D levels independently of venous blood taking, since only sampling of capillary blood is required for this original method, which could be used for screening.

Capillary blood was taken from 96 randomly selected healthy blood donors at the University Medical Center, Johannes Gutenberg-University (Mainz, Germany). Capillary blood for 25-OH vitamin D analysis was obtained with the D-Vital ID dry-blood collection system. Dried capillary blood was eluted by moistening the filter of the sampling device with activating solution and vortexing. The 25-OH vitamin D was extracted by adding precipitation solution, incubation at 37 °C for 30 minutes and followed by centrifugation. The level of 25-OH vitamin D was determined in serum and in the clear supernatant of extracted capillary blood using a commercially available immunoassay.

The mean age of the 96 blood donors was 37 years with a range of 19 to 68 years. The interassay variability was 5.18% ± 3.71%. The intra-assay variability was 3.63% ± 2.52%. The concentrations of 25-OH vitamin D in the serum samples covered the whole range from extremely low concentrations to concentrations above the recommended 25-OH vitamin D target blood concentration for healthy individuals. The 25-OH vitamin D concentration data from extracted dried capillary blood filters correlated very well with data obtained after direct measurement of venous blood samples of the same blood donor. The correlation was linear over the whole range of 25-OH vitamin D concentrations seen in this study. The D-Vital ID dry-blood collection system and the vitamin D immunoassay are products of Immundiagnostik (Bensheim, Germany).

The mean absolute differences in 25-OH vitamin D levels measured between whole venous blood and capillary blood dried on filter paper was only 6.6 nmol/L, with only three out of 96 points falling outside of the two standard deviation intervals indicating good agreement of both tests. The authors note that the sampling of capillary blood must be carried out properly. The test pipette must be filled completely so that exactly 50 μL of capillary blood are dried on the filter. If the pipette is not filled completely the vitamin D concentration will be underestimated. The study was published in the August 2012 edition of the journal Clinical Laboratory.

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Johannes Gutenberg-University
Immundiagnostik