Neuron-specific enolase in medullary thyroid carcinoma.

CLINICALCHEMISTRY, Vol. 34, No. 11, 1988 2375 trile, and applied the extract to a SepPak C18 cartridge (Waters Associates, Milford, MA) that had been washed sequentially with acetomtrile and water. To remove 24,25dihydroxy vitamin D, we washed the cartridge with water, then with methanol:water (7:3 by vol). 250HD was eluted with acetonitrile; the eluate was evaporated under nitrogen and the dried extract was stored under nitrogen at -20 #{176}C until assay. (Analytical recovery for each specimen ranged from 60% to 80%.) We reconstituted the extract with 500 L _____________________________________________ of ethanol and assayed by competitive protein binding assay, using plasma from a vitamin D-deficient pig, diluted 4000-fold. All values were corrected for losses through the whole procedure. The precision (CV) of the assay for 250111) at 28 g/L was 6.9%. This in-house assay (y) correlated well with a commercial assay (x) (Amersham International, Amersham, U.K.), yielding the regression equation: y = 1.25x + 0.34 ,ug/L, r = 0.991, n = 20, P <0.001). Blood samples were collected from 47 young healthy subjects in September and again in January. The mean (±SD) plasma 250HD concentration in September (26.8 ± 5.7 ug/L) was significantly higher than that in January (23.4 ± 6.3 tg/L, P <0.001 by Student’s paired t-test). In a cross-sectional study of 18-month-old infants, the plasma 250HD in June (30.0 ± 7.1 gfL, n = 9) was significantly greater (P <0.005) than in January (22.0 ± 6.3 j.tg/L, n = 20) (Leung SSF, Lui S, Swaminathan R, unpublished observations). This study illustrates the importance of seasonal variation in plasma 250HD, even in Hong Kong, where the winter is not severe or prolonged.