Interlaboratory comparison of 25-hydroxyvitamin D assays: Vitamin D Standardization Program (VDSP) Intercomparison Study 2 - Part 2 ligand binding assays - impact of 25-hydroxyvitamin D(2) and 24R,25-dihydroxyvitamin D(3) on assay performance

Analytical performance evaluation and optimization of serum 25(OH)D LC-MS/MS measurement

OBJECTIVES

Measuring serum 25-hydroxyvitamin D is key in clinical labs, but inter-laboratory variations risk diagnostic errors. This study evaluates the performance of current in-house liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods used in top Chinese clinical laboratories and proposes an optimized method for improving serum 25(OH)D measurement accuracy and reliability.

METHODS

Seven serum pools with different concentrations of 25(OH)D were prepared and sent to 12 participating laboratories for multiple repeat analysis with their current in-house LC-MS/MS methods and then an optimized LC-MS/MS method. Precision was assessed in terms of coefficient of variance (CV), and trueness was assessed in terms of bias referring to the U.S. National Institute of Standards and Technology (NIST) reference measurement procedure (RMP). The analytical performances of the two methods were compared and evaluated.

RESULTS

Eighty percent and 90 % of the laboratories achieved the defined performance criteria (CV, <12.5 %; mean bias, <8.3 %) with the optimized method for the measurement of 25(OH)D2 and 25(OH)D3, compared with 43 % and 57 % of the laboratories meeting the criteria with their in-house methods, respectively. Precision and trueness improved after applying the optimized method. Although the optimized method didn't not ensure that all laboratory samples meet the measurement uncertainty (MU) criteria (MU<13.6 %), particularly for low-concentration samples, it significantly reduced the MU compared to the in-house method.

CONCLUSIONS

Precision, trueness and MU improved after applying the optimized method. Nonetheless, more efforts are needed to ensure the reliability and accuracy of 25(OH)D measurements in clinical laboratories in China.

Treatment response variations to a single large bolus of enteral cholecalciferol in vitamin D deficient critically Ill children: Metabolomic insights for precision nutrition

Vitamin D deficiency (VDD) is prevalent globally and in pediatric intensive care units, where it represents a modifiable risk factor that may impact patient recovery during hospitalization. Herein, we performed a retrospective analysis of serum samples from a phase-II randomized placebo-controlled trial involving a single large bolus of 10,000 IU/kg vitamin D3 ingested by critically ill children with VDD (25-OH-D < 50 nmol/L). Targeted and untargeted methods were used to comprehensively measure 6 vitamin D metabolites, 239 lipids, 68 polar metabolites, and 4 electrolytes using a multi-step data workflow for compound authentication. Complementary statistical methods classified circulating metabolites/lipids associated with vitamin D repletion following high-dose vitamin D3 intake (n = 20) versus placebo (n = 11) comprising an optional standard of care maintenance dose (< 1000 IU/day). There was a striking increase in median serum concentrations of 25-OH-D3 (4.7-fold), 3-epi-25-OH-D3 (24-fold) and their C3-epimer ratio (6.7-fold) in treated patients on day 3, whereas serum vitamin D3 peaked on day 1 (128-fold) unlike placebo. Treatment response differences were attributed to D3 bioavailability and C3-epimerase activity without evidence of hypercalcemia. For the first time, we report the detection of circulating 3-epi-D3 that was strongly correlated with vitamin D3 uptake (r = 0.898). Metabolomic studies revealed that vitamin D sufficiency (serum 25-OH-D >75 nmol/L) coincided with lower circulating levels of 3-methylhistidine, cystine, S-methylcysteine, uric acid, and two lysophosphatidylcholines 7 days after treatment. Rapid correction of VDD was associated with indicators of lower oxidative stress, inflammation, and muscle protein turn-over that may contribute clinical benefits in high-risk critically ill children.

Commutability assessment of new standard reference materials (SRMs) for determining serum total 25-hydroxyvitamin D using ligand binding and liquid chromatography-tandem mass spectrometry (LC-MS/MS) assays

Commutability is where the measurement response for a reference material (RM) is the same as for an individual patient sample with the same concentration of analyte measured using two or more measurement systems. Assessment of commutability is essential when the RM is used in a calibration hierarchy or to ensure that clinical measurements are comparable across different measurement procedures and at different times. The commutability of three new Standard Reference Materials® (SRMs) for determining serum total 25-hydroxyvitamin D [25(OH)D], defined as the sum of 25-hydroxyvitamin D2 [25(OH)D2] and 25-hydroxyvitamin D3 [25(OH)D3], was assessed through an interlaboratory study. The following SRMs were assessed: (1) SRM 2969 Vitamin D Metabolites in Frozen Human Serum (Total 25-Hydroxyvitamin D Low Level), (2) SRM 2970 Vitamin D Metabolites in Frozen Human Serum (25-Hydroxyvitamin D2 High Level), and (3) SRM 1949 Frozen Human Prenatal Serum. These SRMs represent three clinically relevant situations including (1) low levels of total 25(OH)D, (2) high level of 25(OH)D2, and (3) 25(OH)D levels in nonpregnant women and women during each of the three trimesters of pregnancy with changing concentrations of vitamin D-binding protein (VDBP). Twelve laboratories using 17 different ligand binding assays and eight laboratories using nine commercial and custom liquid chromatography-tandem mass spectrometry (LC-MS/MS) assays provided results in this study. Commutability of the SRMs with patient samples was assessed using the Clinical and Laboratory Standards Institute (CLSI) approach based on 95% prediction intervals or a pre-set commutability criterion and the recently introduced International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) approach based on differences in bias for the clinical and reference material samples using a commutability criterion of 8.8%. All three SRMs were deemed as commutable with all LC-MS/MS assays using both CLSI and IFCC approaches. SRM 2969 and SRM 2970 were deemed noncommutable for three and seven different ligand binding assays, respectively, when using the IFCC approach. Except for two assays, one or more of the three pregnancy levels of SRM 1949 were deemed noncommutable or inconclusive using different ligand binding assays and the commutability criterion of 8.8%. Overall, a noncommutable assessment for ligand binding assays is determined for these SRMs primarily due to a lack of assay selectivity related to 25(OH)D2 or an increasing VDBP in pregnancy trimester materials rather than the quality of the SRMs. With results from 17 different ligand binding and nine LC-MS/MS assays, this study provides valuable knowledge for clinical laboratories to inform SRM selection when assessing 25(OH)D status in patient populations, particularly in subpopulations with low levels of 25(OH)D, high levels of 25(OH)D2, women only, or women who are pregnant.

Discordant Health Implications and Molecular Mechanisms of Vitamin D in Clinical and Preclinical Studies of Prostate Cancer: A Critical Appraisal of the Literature Data

Clinical and preclinical studies have provided conflicting data on the postulated beneficial effects of vitamin D in patients with prostate cancer. In this opinion piece, we discuss reasons for discrepancies between preclinical and clinical vitamin D studies. Different criteria have been used as evidence for the key roles of vitamin D. Clinical studies report integrative cancer outcome criteria such as incidence and mortality in relation to vitamin D status over time. In contrast, preclinical vitamin D studies report molecular and cellular changes resulting from treatment with the biologically active vitamin D metabolite, 1,25-dihydroxyvitamin D3 (calcitriol) in tissues. However, these reported changes in preclinical in vitro studies are often the result of treatment with biologically irrelevant high calcitriol concentrations. In typical experiments, the used calcitriol concentrations exceed the calcitriol concentrations in normal and malignant prostate tissue by 100 to 1000 times. This raises reasonable concerns regarding the postulated biological effects and mechanisms of these preclinical vitamin D approaches in relation to clinical relevance. This is not restricted to prostate cancer, as detailed data regarding the tissue-specific concentrations of vitamin D metabolites are currently lacking. The application of unnaturally high concentrations of calcitriol in preclinical studies appears to be a major reason why the results of preclinical in vitro studies hardly match up with outcomes of vitamin D-related clinical studies. Regarding future studies addressing these concerns, we suggest establishing reference ranges of tissue-specific vitamin D metabolites within various cancer entities, carrying out model studies on human cancer cells and patient-derived organoids with biologically relevant calcitriol concentrations, and lastly improving the design of vitamin D clinical trials where results from preclinical studies guide the protocols and endpoints within these trials.

Post-diagnosis serum 25-hydroxyvitamin D concentrations in women treated for breast cancer participating in a lifestyle trial in Italy

OBJECTIVE

To report cross-sectionally serum levels of 25-hydroxyvitamin D [25(OH)D] in women living in Italy within 12 months from breast cancer (BC) diagnosis.

METHODS

Baseline data were obtained from 394 women diagnosed with primary BC, enrolled from 2016 to 2019 in a lifestyle trial conducted in Italy. Subjects' characteristics were compared between two 25(OH)D concentrations (hypovitaminosis D<20 and ≥20 ng/mL) with the Chi-squared test or Fisher's exact test for small-expected counts. Using multiple logistic regression-adjusted models, we estimated odds ratios (ORs) of hypovitaminosis D with 95% confidence intervals (CIs) in the total sample and in the unsupplemented subgroup.

RESULTS

Hypovitaminosis D was found in 39% of all subjects, 60% in unsupplemented subjects, and 10% in supplemented subjects. Increasing ORs of hypovitaminosis D were found with increasing body mass index, 25-30, >30, and ≥35 versus <25 kg/m2 (ORs: 2.50, 4.64, and 5.81, respectively, in the total cohort and ORs: 2.68, 5.38, and 7.08 in the unsupplemented); living in the most southern Italian region (OR 2.50, 95%CI 1.22-5.13); and with hypertriglyceridemia (OR 2.46; 95%CI 1.16-5.22), chemotherapy history (OR 1.86, 95%CI 1.03-3.38), and inversely with anti-estrogenic therapy (OR 0.43, 95%CI 0.24-0.75) in the total sample.

CONCLUSIONS

Hypovitaminosis D in women recently diagnosed with BC and participating in a lifestyle trial in Italy was widespread and highest with obesity, hypertriglyceridemia, and chemotherapy use. Considering that hypovitaminosis D is a risk factor for lower efficacy of bone density treatments and possibly BC mortality, our results suggest the need to promptly address and treat vitamin D deficiency.

Vitamin D Status and Reference Intervals Measured by Liquid Chromatography-Tandem Mass Spectrometry for the Early Adulthood to Geriatric Ages in a South Korean Population during 2017-2022

This study aimed to describe the latest 25-hydroxyvitamin D (25(OH)D) status of the South Korean population aged ≥ 20 years using 25(OH)D concentrations measured by liquid chromatography-tandem mass spectrometry and to determine the factors associated with total 25(OH)D concentrations. This cross-sectional, retrospective study consecutively selected 119,335 subjects with a median age of 57 (20-101) years who underwent health checkups among 13 Korean cities during 2017-2022. The total 25(OH)D concentration was 54.5 ± 24.0 nmol/L (mean ± SD). The 7.6%, 47.5%, and 82.9% of participants had 25(OH)D less than 25, 50, and 75 nmol/L, respectively. The prevalence of 25(OH)D deficiency (<25 nmol/L) was higher in females than in males (8.9% vs. 6.1%) and varied between age groups, decreasing in older subjects. Those aged 20-29 years had the highest prevalence of 25(OH)D deficiency (23.0% in females and 20.1% in males), which also varied between cities. In the adjusted model, female sex, older age, summer and autumn seasons, lower body mass index (<25 kg/m2), and lower high-sensitivity C-reactive protein concentration (<1 mg/L) were associated with higher total 25(OH)D concentrations. This study could provide an exact understanding of the status of vitamin D and help devise strategies to prevent vitamin D deficiency among the Korean population.

Multicenter comparison of analytical interferences of 25-OH vitamin D immunoassay and mass spectrometry methods by endogenous interferents and cross-reactivity with 3-epi-25-OH-vitamin D3

Background

Vitamin D (vit-D) deficiency is highly prevalent in the Korean population, highlighting the need for accurate measurements. In this study, the interferences by endogenous and cross-reactive substances were compared between routine vit-D immunoassays and mass spectrometry (MS) methods.

Methods

Two MS methods and 4 immunoassays from different manufacturers (Abbott, Beckman Coulter, Roche, Siemens) were compared. Residual samples that were icteric, lipemic, hemolyzed, high in rheumatoid factor, from myeloma patients, or patients undergoing hemodialysis were collected. Also, 4 levels of National Institute of Standards and Technology (NIST) Standard Reference Material 972a, and 12 samples serially spiked with 3-epi-25-OH-D3 were prepared.

Results

Significant interferences were observed in hemolytic (Roche), icteric (Beckman and Siemens) and lipemic samples (all 4 immunoassays). Level 4 NIST material and 3-epi-25-OH-D3-spiked samples induced significant cross-reactivity, yielding higher total vit-D measurements in non-epimer-separating MS methods, and both the Beckman and Roche immunoassays.

Conclusion

Most observed interferences were consistent with manufacturers' claims, but overall improvement of immunoassay bias limits is required. Awareness of potential interference is important to increase the accuracy of vit-D measurements. Moreover, care is due when interpreting vit-D results of newborns, infants and less commonly, pregnant women, who are known to have physiologically high levels of the highly cross-reactive 3-epi-25-OH-D3.

Functional Assessment of Vitamin D Status by a Novel Metabolic Approach: The Low Vitamin D Profile Concept

BACKGROUND

Determining serum 25-hydroxyvitamin D [25(OH)D], 24,25-dihydroxyvitamin D [24,25(OH)2D] and the vitamin D metabolite ratio (VMR) allows the identification of individuals with a low vitamin D metabolite profile. Here, we evaluated if such a functional approach provides superior diagnostic information to serum 25(OH)D alone.

METHODS

25(OH)D, 24,25(OH)2D, and the VMR were determined in participants of the DESIRE (Desirable Vitamin D Concentrations, n = 2010) and the LURIC (Ludwigshafen Risk and Cardiovascular Health, n = 2456) studies. A low vitamin D metabolite profile (vitamin D insufficiency) was defined by a 24,25(OH)2D concentration <1.2 ng/mL (<3 nmol/L) and a VMR <4%. Parathyroid hormone (PTH) and bone turnover markers were measured in both cohorts, whereas 10-year mortality data was recorded in LURIC only.

RESULTS

The median age in DESIRE and LURIC was 43.3 and 63.8 years, respectively. Median 25(OH)D concentrations were 27.2 ng/mL (68.0 nmol/L) and 15.5 ng/mL (38.8 nmol/L), respectively. Serum 25(OH)D deficiency, defined as <20.2 ng/mL (<50 nmol/L), was present in 483 (24.0%) and 1701 (69.3%) participants of DESIRE and LURIC, respectively. In contrast, only 77 (3.8%) and 521 (21.2%) participants had a low vitamin D metabolite profile. Regardless of the serum 25(OH)D concentration, a low vitamin D metabolite profile was associated with a significantly higher PTH, accelerated bone metabolism, and higher all-cause mortality than an unremarkable vitamin D metabolite profile.

CONCLUSIONS

The personalized assessment of vitamin D status using a functional approach better identifies patients with accelerated bone metabolism and increased mortality than the use of a fixed 25(OH)D cutoff of 20 ng/mL (50 nmol/L).

Determination of 24,25-dihydroxyvitamin D3 in Vitamin D External Quality Assessment Scheme samples using a reference measurement procedure

Ninety archived human serum samples from the Vitamin D External Quality Assessment Scheme (DEQAS) were analyzed using a reference measurement procedure (RMP) based on isotope dilution liquid chromatography - tandem mass spectrometry (ID LC-MS/MS) for the determination of 24,25-dihydroxyvitamin D3 [24,25(OH)2D3]. These 24,25(OH)2D3 results, in conjunction with concentration values assigned using RMPs for 25-hydroxyvitamin D2 [25(OH)D2] and 25-hydroxyvitamin D3 [25(OH)D3], provide a valuable resource for assessing the accuracy of measurements for 24,25(OH)2D3 and for investigating the relationship between 24,25(OH)2D3 and 25(OH)D3. Results for 24,25(OH)2D3 using the RMP were compared to DEQAS consensus values demonstrating that the consensus values were not sufficient to assess the accuracy of measurements among different laboratories and methods. A multivariable regression analysis approach using historical DEQAS consensus values for various total 25(OH)D assays was used to assess the contribution of 24,25(OH)2D3 concentration on the assay response. The response of several ligand binding assays for total 25(OH)D was shown to be impacted by the presence of 24,25(OH)2D3.

Vitamin D: marker, measurand & measurement

The measurement of vitamin D metabolites aids in assessing vitamin D status and in diagnosing disorders of calcium homeostasis. Most laboratories measure total 25-hydroxyvitamin D (25(OH)D), while others have taken the extra effort to measure 25(OH)D2 and 25(OH)D3 separately and additional metabolites such as 1,25-dihydroxyvitamin D and 24,25-dihydroxyvitamin D. The aim of this review is to provide an updated overview of the main markers of vitamin D metabolism, define the intended measurands, and discuss the advantages and disadvantages of the two most widely used assays, automated assays and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Whether using the easy and fast automated assays or the more complex LC-MS/MS, one should know the pitfalls of the used technique in order to interpret the measurements. In conclusion, automated assays are unable to accurately measure 25(OH)D in all patient groups, including persons using D2. In these cases, an LC-MS/MS method, when appropriately developed and standardized, produces a more reliable measurement.

Determination of a Serum 25-Hydroxyvitamin D Reference Ranges in Japanese Adults Using Fully Automated Liquid Chromatography-Tandem Mass Spectrometry

BACKGROUND

Despite an increasing interest in vitamin D status, a reference range of the nutrient has not been fully established. This is partly due to a paucity of standardized measuring systems with high throughput. In addition, the range may vary by populations and may change with modernization of lifestyles.

OBJECTIVES

This study aims to calculate the current reference concentration of 25-hydroxyvitamin D (25(OH)D) among healthy people living in an urban area in Japan.

METHODS

A newly developed fully automated liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) system was used to measure serum 25(OH)D concentrations. Reproducibility was assessed by measuring standardized samples. Accuracy was validated by comparing with commercially available immunoassays. Then, mass screening was conducted targeting participants who received medical checkups in Tokyo from April 2019 to March 2020, and the reference ranges were calculated.

RESULTS

The coefficients of variations of interoperator and interday reproducibility were 4.1%-8.5% and 3.7%-8.0% for 25-hydroxyvitamin D₂ (25(OH)D2) and 4.7%-7.0% and 4.0%-6.9% for 25-hydroxyvitamine D3, respectively. The measured total 25(OH)D concentrations correlated well with those measured by immunoassays. In total, 5518 participants were measured for 25(OH)D concentrations, among whom 98% showed inadequate concentrations (<30 ng/mL). The reference ranges of total 25(OH)D for female, male, and total participants were 7-30 ng/mL, 5-27 ng/mL, and 6-29 ng/mL, respectively. After excluding those with abnormal renal and liver function, the range was 6-30 ng/mL.

CONCLUSIONS

The high prevalence of vitamin D insufficiency among seemingly healthy population may be attributed to lifestyle characteristics of people living in urban areas of Japan, including spending less time outdoors and lower intake of traditional foods. Longitudinal follow-up and mass screenings targeting different population will help elucidate reasons for discrepancies between official guidelines and the observed concentrations, to which the well-validated measurement system is essential.

Determination of optimal 25-hydroxyvitamin D cutoff values for the evaluation of restless legs syndrome among pregnant women

STUDY OBJECTIVES

Vitamin D deficiency is associated with restless legs syndrome (RLS). However, a cutoff value for serum 25-hydroxyvitamin D (25[OH]D) level associated with RLS has yet to be clearly determined. We evaluated the association between 25(OH)D and RLS in pregnant women.

METHODS

Data from 203 pregnant women were evaluated using blood samples taken in the third trimester. Liquid chromatography-tandem mass spectrometry and ligand binding assays were used to measure 25(OH)D. RLS was diagnosed based on International Classification of Sleep Disorders, third edition, criteria. The cutoff value for serum 25(OH)D associated with RLS was explored using receiver operating characteristic (ROC) curve and classification and regression tree (CART) analyses.

RESULTS

The results of liquid chromatography-tandem mass spectrometry (x) and ligand binding assays (y) for serum 25(OH)D in the RLS (n = 35, 17.2%) and non-RLS (n = 168) groups showed a relationship of y = -2.65 + 0.08x . The RLS group showed lower serum 25(OH)D and folate levels. ROC curve and CART analyses revealed cutoff values of 10-12.7 ng/mL and 6.6-7.2 ng/mL for 25(OH)D and folate, respectively. Of the 5 women with RLS symptoms persisting at a moderate-to-severe level after delivery, 4 had 25(OH)D levels < 10 ng/mL and all had folate levels < 6 ng/mL.

CONCLUSIONS

Vitamin D and folate deficiency were associated with RLS in pregnant women and may be associated with persistent moderate-to-severe postpartum RLS symptomatology; it is essential to examine associations with RLS while accounting for measurement methods and assay systems.

CITATION

Miyazaki A, Takahashi M, Shuo T, Eto H, Kondo H. Determination of optimal 25-hydroxyvitamin D cutoff values for the evaluation of restless legs syndrome among pregnant women. J Clin Sleep Med. 2023;19(1):73-83.

A candidate reference method and multiple commutable control materials for serum 25-hydroxyvitamin D measurement

OBJECTIVES

The aim of the current study was to establish a reliable candidate reference method for serum 25-hydroxyvitamin D [25(OH)D] measurement and to assess the commutability of multiple control materials among liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods.

METHODS

Serum 25-hydroxyvitamin D2 [25(OH)D2] and 25-hydroxyvitamin D3 [25(OH)D3] together with spiked internal standards were extracted with a one-step approach and then analyzed by LC-MS/MS. The commutability assessment for 25(OH)D was conducted according to the Clinical and Laboratory Standards Institute (CLSI) EP14-A3 protocol. 25(OH)D concentrations in 5 levels of unprocessed serum pools, 7 levels of serum pools spiked with 25(OH)D3 or 25(OH)D2, 3 levels of commercial control materials, 2 levels of spiked bovine serum, and 4 levels of external quality assessment (EQA) materials were measured along with 30 single-donor samples using the candidate reference method and two routine LC-MS/MS methods.

RESULTS

The candidate reference method could separate 25(OH)D2 and 25(OH)D3 from 14 potential interfering compounds completely within a 9-min analysis time. Good method precision was obtained, and measurement results on certified reference material NIST SRM 972a were within the uncertainty of the certified values. All candidate materials were assessed commutable for LC-MS/MS methods.

CONCLUSIONS

The candidate reference method for serum 25(OH)D measurement is precise, accurate, and robust against interferences and can provide an accuracy base for routine methods. The multiple alternative control materials with commutability among LC-MS/MS methods will facilitate the further standardization for serum 25(OH)D measurement.

Towards harmonization of directly measured free 25-hydroxyvitamin D using an enzyme-linked immunosorbent assay

The majority of circulating 25-hydroxyvitamin D (25(OH)D) is protein bound and perhaps less available than the free fraction of 25(OH)D; therefore, researchers have proposed that the measurement of free 25(OH)D in human serum may be a better indicator of vitamin D health status than total 25(OH)D. The availability of a new enzyme-linked immunosorbent assay (ELISA) for the determination of free 25(OH)D provides a method for direct measurement of the low levels of non-protein bound 25(OH)D. As an initial step towards harmonization of measurements of free 25(OH)D, the ELISA was used to measure free 25(OH)D in three existing Standard Reference Materials (SRMs): SRM 972a Vitamin D Metabolites in Frozen Human Serum, SRM 2973 Vitamin D Metabolites in Frozen Human Serum (High Level), and SRM 1949 Frozen Prenatal Human Serum. Target values for free 25(OH)D in the nine SRM serum pools, obtained by combining the results from two laboratories, ranged from 3.76 ± 0.36 to 10.0 ± 0.58 pg/mL. Of particular significance is the assignment of free 25(OH)D target values to SRM 1949, which consists of four serum pools from non-pregnant female donors of reproductive age and pregnant women in each of the three trimesters and which also has values assigned for vitamin D binding protein, which increases during pregnancy. The availability of target values for free 25(OH)D in these SRMs will allow researchers to validate new analytical methods and to compare their results with other researchers as an initial step towards harmonization of measurements among different studies and laboratories.

Assessment of serum total 25-hydroxyvitamin D assays for Vitamin D External Quality Assessment Scheme (DEQAS) materials distributed at ambient and frozen conditions

The Vitamin D External Quality Assessment Scheme (DEQAS) distributes human serum samples four times per year to over 1000 participants worldwide for the determination of total serum 25-hydroxyvitamin D [25(OH)D)]. These samples are stored at -40 °C prior to distribution and the participants are instructed to store the samples frozen at -20 °C or lower after receipt; however, the samples are shipped to participants at ambient conditions (i.e., no temperature control). To address the question of whether shipment at ambient conditions is sufficient for reliable performance of various 25(OH)D assays, the equivalence of DEQAS human serum samples shipped under frozen and ambient conditions was assessed. As part of a Vitamin D Standardization Program (VDSP) commutability study, two sets of the same nine DEQAS samples were shipped to participants at ambient temperature and frozen on dry ice. Twenty-eight laboratories participated in this study and provided 34 sets of results for the measurement of 25(OH)D using 20 ligand binding assays and 14 liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods. Equivalence of the assay response for the frozen versus ambient DEQAS samples for each assay was evaluated using multi-level modeling, paired t-tests including a false discovery rate (FDR) approach, and ordinary least squares linear regression analysis of frozen versus ambient results. Using the paired t-test and confirmed by FDR testing, differences in the results for the ambient and frozen samples were found to be statistically significant at p < 0.05 for four assays (DiaSorin, DIAsource, Siemens, and SNIBE prototype). For all 14 LC-MS/MS assays, the differences in the results for the ambient- and frozen-shipped samples were not found to be significant at p < 0.05 indicating that these analytes were stable during shipment at ambient conditions. Even though assay results have been shown to vary considerably among different 25(OH)D assays in other studies, the results of this study also indicate that sample handling/transport conditions may influence 25(OH)D assay response for several assays.

Vitamin D Standardization Program (VDSP) intralaboratory study for the assessment of 25-hydroxyvitamin D assay variability and bias

An intralaboratory study assessing assay variability and bias for determination of serum total 25-hydroxyvitamin D [25(OH)D] was conducted by the Vitamin D Standardization Program (VDSP). Thirteen assays for serum total 25(OH)D were evaluated in a single laboratory including 11 unique immunoassays and one liquid chromatography - tandem mass spectrometry (LC-MS/MS) assay. Fifty single-donor serum samples, including eight samples with high concentrations of 25(OH)D₂ (> 30 nmol/L), were assigned target values for 25(OH)D₂ and 25(OH)D₃ using reference measurement procedures (RMP). Using four replicate measurements for each sample, the mean total percent coefficient of variation (%CV) and mean % bias from the target values were determined for each assay using the 50 single-donor samples and a 42-sample subset, which excluded 8 high 25(OH)D₂ concentration samples, and compared with VDSP performance criteria of ≤ 10 % CV and ≤ ±5 % mean bias. All 12 assays achieved the performance criterion for % CV, and 9 of the 12 assays were within ≤ ±5 % mean bias. The Fujirebio Inc. assay exhibited the lowest %CV and highest percentage of individual measurements within ≤ ±5 % mean bias. Ten immunoassays exhibited changes in response due to the high 25(OH)D₂ samples with Abbott, Biomérieux, DiaSorin, DIAsource, and IDS-iSYS assays having the largest deviations. The Fujirebio Inc. and Beckman Coulter assays were only minimally affected by the presence of the high 25(OH)D₂ samples. Samples with high concentrations of 25(OH)D₂ provided a critical performance test for immunoassays indicating that some assays may not have equal response or recovery for 25(OH)D₂ and 25(OH)D₃.

Interlaboratory comparison of 25-hydroxyvitamin D assays: Vitamin D Standardization Program (VDSP) Intercomparison Study 2 - Part 1 liquid chromatography - tandem mass spectrometry (LC-MS/MS) assays - impact of 3-epi-25-hydroxyvitamin D3 on assay performance

An interlaboratory comparison study was conducted by the Vitamin D Standardization Program (VDSP) to assess the performance of liquid chromatography - tandem mass spectrometry (LC-MS/MS) assays used for the determination of serum total 25-hydroxyvitamin D (25(OH)D), which is the sum of 25-hydroxyvitamin D₂ (25(OH)D₂) and 25-hydroxyvitamin D₃ (25(OH)D₃). A set of 50 single-donor samples was assigned target values for concentrations of 25(OH)D₂, 25(OH)D₃, 3-epi-25-hydroxyvitamin D₃ (3-epi-25(OH)D₃), and 24R,25-dihydroxyvitamin D₃ (24R,25(OH)₂D₃) using isotope dilution liquid chromatography - tandem mass spectrometry (ID LC-MS/MS). VDSP Intercomparison Study 2 Part 1 includes results from 14 laboratories using 14 custom LC-MS/MS assays. Assay performance was evaluated using mean % bias compared to the assigned target values and using linear regression analysis of the test assay mean results and the target values. Only 53% of the LC-MS/MS assays met the VDSP criterion of mean % bias ≤ |±5%|. For the LC-MS/MS assays not meeting the ≤ |±5%| criterion, four assays had mean % bias of between 12 and 21%. Based on multivariable regression analysis using the concentrations of the four individual vitamin D metabolites in the 50 single-donor samples, the performance of several LC-MS/MS assays was found to be influenced by the presence of 3-epi-25(OH)D₃. The results of this interlaboratory study represent the most comprehensive comparison of LC-MS/MS assay performance for serum total 25(OH)D and document the significant impact of the lack of separation of 3-epi-25(OH)D₃ and 25(OH)D₃ on assay performance, particularly with regard to mean % bias.

Assessment of serum total 25-hydroxyvitamin D assay commutability of Standard Reference Materials and College of American Pathologists Accuracy-Based Vitamin D (ABVD) Scheme and Vitamin D External Quality Assessment Scheme (DEQAS) materials: Vitamin D Standardization Program (VDSP) Commutability Study 2

An interlaboratory study was conducted through the Vitamin D Standardization Program (VDSP) to assess commutability of Standard Reference Materials® (SRMs) and proficiency testing/external quality assessment (PT/EQA) samples for determination of serum total 25-hydroxyvitamin D [25(OH)D] using ligand binding assays and liquid chromatography-tandem mass spectrometry (LC-MS/MS). A set of 50 single-donor serum samples were assigned target values for 25-hydroxyvitamin D2 [25(OH)D2] and 25-hydroxyvitamin D3 [25(OH)D3] using reference measurement procedures (RMPs). SRM and PT/EQA samples evaluated included SRM 972a (four levels), SRM 2973, six College of American Pathologists (CAP) Accuracy-Based Vitamin D (ABVD) samples, and nine Vitamin D External Quality Assessment Scheme (DEQAS) samples. Results were received from 28 different laboratories using 20 ligand binding assays and 14 LC-MS/MS methods. Using the test assay results for total serum 25(OH)D (i.e., the sum of 25(OH)D2 and 25(OH)D3) determined for the single-donor samples and the RMP target values, the linear regression and 95% prediction intervals (PIs) were calculated. Using a subset of 42 samples that had concentrations of 25(OH)D2 below 30 nmol/L, one or more of the SRM and PT/EQA samples with high concentrations of 25(OH)D2 were deemed non-commutable using 5 of 11 unique ligand binding assays. SRM 972a (level 4), which has high exogenous concentration of 3-epi-25(OH)D3, was deemed non-commutable for 50% of the LC-MS/MS assays.