Prevalence of vitamin D deficiency and consequences for PTH reference values
Introduction
Standardization of PTH assays is lacking. Method comparison studies of PTH immunoassays from various suppliers show 2–3 fold differences in PTH levels [1], [2], [3], [4]. The lack of standardization has major clinical implications as PTH is used not only to exclude hyper- or hypoparathyroidism but also to monitor disease progression in patients with Chronic Kidney Disease (CKD) [2], [5], [6], [7]. To overcome these problems either a standardization program should be started or assay specific reference values or assay specific decision limits should be used [8], [9], [4]. In order to define assay specific reference values a proper description of the population characteristics such as age, gender, race, BMI and vitamin D levels is required [10]. These characteristics are often poorly described in the package inserts [9]. Recent studies have shown that 1–14% of the variation in PTH levels is explained by vitamin D status [11], [12]. Given the inverse relationship between PTH and 25-hydroxyvitamine D (25OHD), 25OHD levels should be simultaneously assessed while defining PTH reference values [10], [13]. Or alternatively, only vitamin D sufficient subjects should be included in the reference population [9], [11].
The aim of the present study was to examine the association between 25OHD levels and PTH in two large, healthy cohorts and calculate assay specific PTH reference levels on the Architect (Abbott) in EDTA plasma in relation to 25OHD levels.
Section snippets
Subjects
The LASA study is based on an age and sex-stratified representative healthy sample of Dutch, mainly Caucasian, older population as described earlier [14], [15]. In this study the baseline measurements of the second cohort were included. The samples from the LASA were collected in 2002. Blood samples were obtained in the morning after light breakfast without dairy products. The samples were centrifuged and stored at − 20 °C until analysis. PTH and vitamin D measurements were carried out in 2009 as
Study characteristics
Since PTH measurements were available in these large, well defined cohorts we used them in the current study to define PTH reference values. To establish PTH reference levels data from 738 subjects of the LASA cohort (mean age 60.0 ± 2.9 years) and 633 subjects of the NESDA cohort (mean age 41.0 ± 14.6 years), were included in the analysis (Table 1). Vitamin D deficiency or insufficiency (25OHD < 50 nmol/L) [22] was present in 41% and 29% of the LASA and NESDA cohorts, respectively.
Determinants of PTH
As shown in Table 2,
Discussion
In the current study we defined reference values for the Architect PTH assay while taking 25OHD concentrations and other possible confounding factors into account. In agreement with previous studies, higher PTH reference values were obtained in vitamin D deficient and insufficient subjects [9], [11], [12]. The 97.5th percentile obtained from our cohorts is 11.1 and 9.4 pmol/L, respectively which is significantly different from 7.2 pmol/L as stated by Abbott. To our knowledge this is the first
Acknowledgments
The Longitudinal Aging Study Amsterdam is largely supported by a grant from the Netherlands Ministry of Health Welfare and Sports, Directorate of Nursing Care and Older Persons. We would like to acknowledge our colleagues from the clinical laboratory of the Isala Klinieken (Weezenlanden) and Ziekenhuis Bernhoven in Oss and Veghel for measuring creatinine levels and Evelien Sohl for constructing the spline plots. The infrastructure for the NESDA study (www.nesda.nl) is funded through the
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