| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Letters to the Editor |
Unit of Hormonal Biology, Hôpital Saint-Louis, Assistance Publique, Hôpitaux de Paris, Paris, France
aAddress correspondence to this author at: Unit of Hormonal Biology, Saint-Louis University Hospital (AP-HP) and INSERM U 671, 1 avenue Claude Vellefaux, 75475 Paris cedex 10, France. Fax 33-1-42-49-42-80; e-mail philippe.boudou{at}sls.aphp.fr.
To the Editor:
The major problem in measuring 25-hydroxyvitamin D (25-OHD) is attributable to the molecule itself (1). Thus, an analytical method must be selected that will accurately estimate total circulating 25-OHD independent of the circulating concentrations of 25OHD2 and 25OHD3. Although HPLC remains the method of choice, many convergent data indicate that the 25-OHD RIA (DiaSorin Inc) should be incorporated as a secondary reference method (2)(3). With the RIA method actually used in our laboratory, we evaluated the 25-OHD LIAISON® chemiluminescent immunoassay (CLIA) proposed by the same manufacturer as an alternative assay. We also investigated whether a 25-OHD value <50 nmol/L, a well-known arbitrary RIA cutoff value to define vitamin D insufficiency, is applicable to CLIA.
We studied 199 people [79 men/120 women; mean (SD) age 48.1 (19.8) years] who were residents of Paris and its suburbs and who suffered primary hyperparathyroidism (HPT), secondary HPT with hemodialysis or obesity, mild asthenia, depression, and/or osteopenia/osteoporosis. This study was approved by the ethics committee and study participants gave written informed consents. Blood samples were collected and centrifuged at 4 °C for 10 min with a force of 950g, and serum aliquots were frozen at –80 °C. All samples were assayed during the same day using assay reagents from the same lot. None of the samples was frozen and thawed repeatedly. Samples were assayed either in duplicate for RIA (3), or singly for CLIA(4), according to manufacturers’ instructions. With the RIA method, the intra- and interassay CVs were, respectively, <15.0% and <17.0% at a mean concentration of 9 nmol/L, <10.0% and <11.0% at a mean concentration of 30 nmol/L, and <7.5% and <11.0% at a mean concentration of 150 nmol/L. With the CLIA method, the intra- and interassay CVs were, respectively, 15.5% and <18.0% at a mean concentration of 19 nmol/L, 12.6% and 14.6% at a mean concentration of 33 nmol/L, and 4.0% and 4.5% at a mean concentration of 117 nmol/L. The relation between both methods was calculated by the equation of the regression analysis (CLIA = 0.99 RIA + 5.77 nmol/L). A clear dispersion of 25-OHD values started at 27.5 nmol/L, as randomly highlighted by the insert presented in Fig. 1A
. The Bland-Altman plot (Fig. 1B
) confirmed this random tendency, which was independent of concentration and of sex, and also demonstrated an amplified dispersion at 25-OHD concentrations >50 nmol/L.
|
Our data (Fig. 1B
) suggested that CLIA results tended to be higher than RIA at low and high concentrations, conversely to data previously published (4). With a threshold of 25-OHD 
30 nmol/L but <50 nmol/L to define vitamin D insufficiency, in 33% of the 54 individuals with 25-OHD concentrations classified as insufficient by RIA, 25-OHD concentrations were normalized by CLIA (range: 53.0–109.5 nmol/L). In contrast, 21% of the 62 study participants with 25-OHD concentrations 50 nmol/L by RIA had insufficient 25-OHD concentrations by CLIA (range: 17.5–48.0 nmol/L). For optimal serum 25-OHD concentration defined as >75 nmol/L in osteoporotic patients, 35% of the 23 patents with 25-OHD concentrations above this threshold by RIA had concentrations below it by CLIA. The RIA method used a primary antibody to 25-OHD in a homogenous phase with a 2nd antibody used as precipitating agent, whereas CLIA used the same primary antibody immobilized onto coated magnetic particles. This antibody interacts differently with the first calibrator [i.e., 17.5 nmol/L (CLIA); 12.5 nmol/L (RIA) with an optional calibrator of 6.25 nmol/L (B/B0: 91%) created by diluting 12.5 at 1:2 as suggested by the manufacturer], indicating different affinity profiles that are probably responsible for these random results. These divergent results may also be attributable to different calibrators, constituted in either human- (RIA) or horse-based serum (CLIA), different incubation times (90 min by RIA vs 30 min by CLIA), or an insufficient quantity of reagents used to dissociate 25-OHD from its binding protein.
Overall, these 2 methods did not similarly classify individuals with reference to well-known arbitrary cutoff values. The random tendency observed whatever the concentrations measured did not permit the definition of a clear strategy concerning patient follow-up, particularly for those needing treatment with respect to their vitamin D status. The discrepancy between these 2 methods is consistent with either important negative (4)(5) or positive (our data) intercepts traducing differences in the assay response to the calibrant matrix.
Finally, our results are consistent with the poor correlation previously reported between RIA and CLIA (5) and demonstrate that in disagreement with recently published data (4), a 25-OHD value <50 nmol/L used to define vitamin D insufficiency with the DiaSorin RIA is not suitable for use with the LIAISON® assay (CLIA).
References
The following articles in journals at HighWire Press have cited this article:
|
|
 |
|
  E. Hypponen, S. Turner, P. Cumberland, C. Power, and I. Gibb Serum 25-Hydroxyvitamin D Measurement in a Large Population Survey with Statistical Harmonization of Assay Variation to an International Standard J. Clin. Endocrinol. Metab., December 1, 2007; 92(12): 4615 - 4622. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
) and women (
)]. (A), a plotting as well as an insert highlighting patients having 25-OHD concentrations <50 nmol/L by RIA. (B), Bland-Altman plot.