An international comparison of serum 25-hydroxyvitamin D measurements

Vitamin D status is usually assessed by measuring the serum 25-hydroxyvitamin D (25(OH)D) concentration. This mainly depends on sunshine exposure, nutrition and age. Interlaboratory variation may hamper comparison between results from different populations. This study reports cross-calibration of the 25(OH)D assays of five laboratories. In study 1, serum 25(OH)D was measured with three different assays in 104 serum samples from a large vitamin D supplementation study. The mean serum 25(OH)D level was 80% higher when measured by competitive protein binding (CPB) assay than by high-performance liquid chromatography (HPLC), while radioimmunoassay (RIA) gave intermediate values. The highest correlation was observed between RIA and HPLC (r = 0.84, p < 0.01). Of the serum 25(OH)D values in the lowest quartile by HPLC, 25% were not recognized by CPB and 21% were not recognized by RIA as belonging to the lowest quartile. In study 2, the five laboratories analyzed serum 25(OH)D in eight serum samples covering the concentration range very low to high, with live different assays. The differences between the mean values for serum 25(OH)D between the laboratories with the highest and lowest values was 38%. The ranking order of individual samples according to the serum 25(OH)D value was very similar in all laboratories. The results show that 25(OH)D values from different laboratories can not be assumed to be comparable unless a careful cross-calibration has been performed

Influence of food and antacid administration on fluoride bioavailability from enteric-coated sodium fluoride tablets.

The relative bioavailability of enteric-coated sodium fluoride (NaF) tablets (10 mg F-) has been assessed following administration with a standard calcium-rich breakfast or calcium-poor lunch, and 2 h before or simultaneously with antacid administration (2.4 g aluminum-magnesium hydroxide), versus intake on an empty stomach. Twelve volunteers were studied 3 times according to an open, three-way crossover design over a 24 h period at weekly intervals. Meals were found to decrease the peak serum concentration of NaF from 122 micrograms/L during fasting (after baseline subtraction) to 71 and 88 micrograms/L with breakfast and lunch respectively, and to slow its absorption rate with Tmax increasing from 3.3 to 7.3 and 11.2 hours, without altering its bioavailability. Antacid impaired the bioavailability of NaF by 80% when administered simultaneously, with AUC decreasing from 987 to 155 micrograms.h/L, but had no significant effect when taken 2 h before NaF. In conclusion, the enteric-coated NaF tablets used in this study can be administered with food or after a 2-hour delay following antacid administration, but should not be taken simultaneously with antacid