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J C Netelenbos

Publications and source records attributed to J C Netelenbos.

103 records · Page 6Linked to original sources
PubMed1985

Vitamin D status in urinary calcium stone formation.

Serum vitamin D metabolites were measured in 160 normocalcemic urinary calcium stone formers and in 217 control subjects. No difference in concentrations of 25-hydroxy-vitamin D (25[OH]D), 24,25-dihydroxyvitamin D (24,25[OH]2D), and 1,25-dihydroxyvitamin D (1,25[OH]2D) was found between stone formers and control subjects. Values for 25(OH)D and 24,25(OH)2D were higher in hypercalciuric stone formers than in normocalciuric stone formers independent of seasonal fluctuation. No difference in concentration of serum 1,25(OH)2D was found between hypercalciuric and normocalciuric stone formers. No correlations were present between the serum concentrations of the measured vitamin D metabolites and of measures of calcium and phosphate metabolism. These findings suggest no major pathophysiologic role of the main vitamin D metabolites in urinary calcium stone formation.

24,25-Dihydroxyvitamin D 3↗
PubMed1984

Measurement of vitamin D metabolites in anephric subjects.

Circulating concentrations of 1,25-dihydroxyvitamin D, 24,25-dihydroxyvitamin D and 25-hydroxyvitamin D were measured in 21 anephric subjects. 13 subjects had no therapy with vitamin D, dihydrotachysterol or 1 alpha-hydroxyvitamin D3. In 7 subjects of this group 1,25-dihydroxyvitamin D was undetectable (less than 5 pmol/l). In the other 6 patients concentrations ranged from 10 to 43 pmol/l (reference value 111 +/- 33 pmol/l). All subjects taking high doses of vitamin D showed detectable 1,25-dihydroxyvitamin D concentrations in the same range. Dihydrotachysterol therapy caused spuriously high '1,25-dihydroxyvitamin D' values, probably by interference of a metabolite of dihydrotachysterol in our assay. In subjects on vitamin D or dihydrotachysterol therapy 25-hydroxyvitamin D concentrations were significantly elevated (314 +/- 146 nmol/l and 98 +/- 19 nmol/l, respectively; reference value 52 +/- 22 nmol/l). Concentrations of 24,25-dihydroxyvitamin D were only measured in subjects without vitamin D2 intake. In general very low but detectable concentrations were found. One subject on a high dose of vitamin D3 showed a 24,25-dihydroxyvitamin D3 concentration of 10.2 nmol/l (reference value 4.4 +/- 2.9 nmol/l). Our results therefore confirm earlier reports on extrarenal synthesis of 24,25-dihydroxyvitamin D and suggest that there may be extrarenal production of 1,25-dihydroxyvitamin D as well.

24,25-Dihydroxyvitamin D 3↗
PubMed1984

An international comparison of vitamin D metabolite measurements.

An international 19-laboratory survey was organized to compare assays for 25-hydroxyvitamin D, 24,25-dihydroxyvitamin D, and 1,25-dihydroxyvitamin D in plasma. Each participant received two ethanolic standard solutions of each metabolite and eight plasma samples. Each laboratory used its usual procedures. Mean interlaboratory coefficients of variation (CVs) for the eight plasma samples were 35%, 43%, and 52% for 25-hydroxyvitamin D, 24,25-dihydroxyvitamin D, and 1,25-dihydroxyvitamin D, respectively. Average CVs for the standard solutions were 27%, 23%, and 25%, respectively. Of the eight plasma samples, five had the same concentration for one of the metabolites. One sample was diluted to 0.6 times its original concentration and three samples were fortified with one or more of the metabolites under investigation. Fourteen of 18 laboratories (78%) could distinguish between the five unchanged samples and the modified ones with their 25-hydroxyvitamin D assay. Nine of 12 (75%) could distinguish the modified samples from the other samples with the 24,25-dihydroxyvitamin D assay. Only eight of 15 (53%) could do this their 1,25-dihydroxyvitamin D assay. Values from different laboratories evidently cannot be intercompared without making an actual comparison of the assay procedures. Furthermore, in case of clinical applications of these assays, each laboratory should establish its own reference values and should continually use an internal reference sample to assess the precision of the procedures.

24,25-Dihydroxyvitamin D 3↗
PubMed1984

Radiologic morphometry in patients with femoral neck fractures and elderly control subjects. Comparison with histomorphometric parameters.

Radiologic morphometry was performed in 125 patients with femoral neck fractures and 74 elderly control subjects to evaluate simple parameters for screening of the population at risk. Reproducibility of metacarpal cortical thickness was high when measured in six metacarpals. Values of this parameter were similar in patients and control subjects. Reproducibility of the Singh index was poor. The patients with femoral neck fractures had a higher vertebral index, more vertebral crush fractures, and a lower Singh index than the control subjects. There was no significant difference in these radiologic indices between cervical and intertrochanteric fractures. The radiologic indices correlated poorly with each other and with histomorphometric data from transilial bone biopsies of the patients. Stepwise discriminant analysis allowed correct classification as patient or control subject in only 69.8% of the cases, with the Singh index having the highest predictive value. The reported observations indicate a need for a more accurate technique for local assessment of the bone mass in the femoral neck.

Aged↗
PubMed1982

Interlaboratory variation of vitamin D1) metabolite measurements.

Interlaboratory variation of the measurement of 25-hydroxy vitamin D, 24,25-dihydroxy vitamin D and 1,25-dihydroxy vitamin D by six laboratories in the Netherlands was studied. Three different serum samples and two different standard solutions of each metabolite were assayed. Substantial interlaboratory variation was found for the measurement of serum samples. The mean interlaboratory CV's for the 25-hydroxy vitamin D, 24,25-dihydroxy vitamin D and 1,25-dihydroxy vitamin D assays in the three sera were 48%, 38% and 20% respectively. The measurement of standard solutions of all metabolites showed relative little variation (mean CV 8%). The small number of samples allowed no evaluation of intralaboratory variation. The much higher CV's of the measurement of serum samples, when compared to standard solutions, may be attributed to differences in extraction and purification procedures which are probably responsible for the presence of varying amounts of interfering substances during the final quantification of metabolites.

24,25-Dihydroxyvitamin D 3↗
PubMed1981

Analysis for 1,25-dihydroxyvitamin D in human plasma, after a liquid-chromatographic purification procedure, with a modified competitive protein-binding assay.

1,25-Dihydroxyvitamin D in plasma is measured by competitive protein-binding assay after isolation from plasma. The present method is improved, as compared with those hitherto described, with regard to receptor preparation, isolation of vitamin D metabolites from plasma, and procedure for the competitive protein binding. Receptor preparation from healthy rather than rachitic chicks, together with a fast isolation procedure, results in a high yield of active receptor protein: 12 animals provide receptor for about 4000 incubations. No loss of binding activity was observed during one year. 1,25-Dihydroxyvitamin D is isolated from plasma by a single extraction and a one-step chromatographic purification. Analytical recovery for the entire procedure averaged 78.1% (SD 4.7%). Other vitamin D metabolites (25-hydroxyvitamin D and 24,25-dihydroxyvitamin D) can also be separated with this procedure. The main features of the modified binding assay are the use of a stabilized cytosol receptor and dextran-coated charcoal instead of polyethylene glycol. The smallest detectable amount in the binding assay is 1-2 pg (2.4-4.8 fmol). Intra- and interassay CVs are 7.0% and 4.8%, respectively. 1,25-Dihydroxyvitamin D concentrations in plasma of 20 healthy subjects averaged 51.7 (SD 10.8) ng/L [124 (SD 26) pmol/L]. Three anephric patients showed values of 3, 6, and 7 ng/L (7, 14, and 17 pmol/L).

Animals↗
PubMed1981

Simultaneous determination of 25-hydroxyvitamin D, 24,25-dihydroxyvitamin D, and 1,25-dihydroxyvitamin D in plasma or serum.

We describe a simultaneous assay for the principal vitamin D metabolites: 25-hydroxyvitamin D, 24-25-dihydroxyvitamin D, and 1,25-dihydroxyvitamin D. Special attention has been paid to simplification of the extensive extraction and purification procedures used in previously described simultaneous assays. All three metabolites were isolated with a single extraction step, followed by only one gradient liquid-chromatographic procedure. For final quantitation we used competitive protein binding assays, involving readily available binding proteins and commercially purchased tritiated vitamin D metabolites. Concentrations in the plasma of healthy subjects (mean age, 27 years), sampled during December were 51 (SD 17) nmol/L, 4.1 (SD 1.3) nmol/L, and 124 (SD 26) pmol/L for 25-hydroxyvitamin D, 24,25-dihydroxyvitamin D and 1,25-dihydroxyvitamin D, respectively. Intra- and interassay CVs for the three metabolites were 4.4 and 3.9%, 6.7 and 8.0%, and 7.0 and 4.8%, respectively.

24,25-Dihydroxyvitamin D 3↗
PubMedDate not supplied

Determining reference ('normal') limits in medicine: an application.

We provide an account of a study to assess reference limits for eight routine laboratory determinations at the Academic Hospital of the Free University, Amsterdam and emphasize methodological issues rather than results. We argue that reference limits have use mainly in the first phase of the diagnostic process. Reference and target populations should be grossly comparable, and therefore patients (after slight selection) could well serve as references. However, we found major differences between in- and out-patients, so we suggest that this factor, together with age and sex, be taken into account. To arrive at reliable limits, the size of the reference sample should be at least 100. Laboratory reports should provide percentiles, which enable a more flexible decision than do fixed limits.

Adult↗