Analysis of vitamin D metabolites by liquid chromatography-tandem mass spectrometry

Measurement of Serum Free Vitamin D Concentrations: Importance, Challenges, and the Emerging Role of Mass Spectrometry

BACKGROUND

Serum total 25-hydroxyvitamin D [25(OH)D] concentration is the most widely used clinical biomarker for vitamin D status. Under certain physiological and pathological conditions, however, total 25(OH)D may not always be the best index for vitamin D status. Instead, the nonprotein-bound (free) fraction of total 25(OH)D has been suggested as a more appropriate marker in certain clinical situations.

CONTENT

Free 25(OH)D levels can either be calculated or measured directly. Calculated free 25(OH)D depends on the concentrations of total serum 25(OH)D, vitamin D binding protein (VDBP), and albumin, as well as the affinity between analyte and binding proteins. Differences in VDBP concentrations are observed between populations as a result of health status, gene polymorphisms, and the assay used for determination. Direct measurement methods for free 25(OH)D are often complicated (dialysis, ultrafiltration) or susceptible to interferences, cross-reactivity, and type of antibody (immunoassays). Therefore, it is very important to develop tools that allow either accurate and precise measurement of VDBP or direct measurement of free 25(OH)D. For the latter, liquid chromatography combined with tandem mass spectrometry (LC-MS/MS) has recently shown promise for analysis of free vitamin D. In the current review, we present the importance and challenges regarding free 25(OH)D determination and the role of LC-MS-based methods in future studies.

SUMMARY

More research is required to determine the role of free 25(OH)D in the assessment of vitamin D status in healthy subjects and in various clinical conditions. Recent advances in technology, including mass spectrometry, can provide the required assays for this purpose.

Development and Validation of Novel HPLC Methods for Quantitative Determination of Vitamin D3 in Tablet Dosage Form

In the present work, an efficient isocratic HPLC method was developed for the precise and accurate estimation of vitamin D3 in tablet form. The chromatographic conditions comprised an L3 silica column (5 µm in particle size, 4.6 mm × 250 mm) with a mobile phase n-hexane/ethyl acetate (85:15 v/v) with a flow rate of 2.0 mL/min and a detection wavelength of 292 nm. The new methodology was validated for accuracy, precision, specificity, robustness, and quantification limits according to an official monograph of USP/BP and ICH guidelines. The peak areas of the six replicates of the homogeneous sample were recorded. The mean value obtained was 67,301, and the relative standard deviation (RSD) was 0.1741. The linearity and range were in the acceptable bounds, i.e., 0.999, which was calculated using regression line analysis. The results show that the method is truly acceptable as the RSD, as the flow rate was 0.81%, while for the mobile phase composition, it was 0.72%, which lies in the acceptable range. The limit of detection (LOD) and the limit of quantification (LOQ) values were 0.0539 µg/mL and 0.1633 µg/mL, respectively. The % RSD of the intra and inter-day precision of the method was deemed acceptable according to the international commission for harmonization guidelines. The developed method has potential to be used for the detection and quantification of vitamin D3 during routine analysis for tablets in dosage form.

Accurate Clinical Detection of Vitamin D by Mass Spectrometry: A Review

Vitamin D deficiency is thought to be associated with a wide range of diseases, including diabetes, cancer, depression, neurodegenerative diseases, and cardiovascular and cerebrovascular diseases. This vitamin D deficiency is a global epidemic affecting both developing and developed countries and therefore qualitative and quantitative analysis of vitamin D in a clinical context is essential. Mass spectrometry has played an increasingly important role in the clinical analysis of vitamin D because of its accuracy, sensitivity, specificity, and the ability to detect multiple substances at the same time. Despite their many advantages, mass spectrometry-based methods are not without analytical challenges. Front-end and back-end challenges such as protein precipitation, analyte extraction, derivatization, mass spectrometer functionality, must be carefully considered to provide accurate and robust analysis of vitamin D through a well-designed approach with continuous control by internal and external quality control. Therefore, the aim of this review is to provide a comprehensive overview of the development of mass spectrometry methods for vitamin D accurate analysis, including emphasis on status markers, deleterious effects of biological matrices, derivatization reactions, effects of ionization sources, contribution of epimers, standardization of assays between laboratories.

Optimised liquid chromatography tandem mass spectrometry method for the simultaneous quantification of serum vitamin D analogues while also accounting for epimers and isobars

The accurate quantification of multiple vitamin D analogues simultaneously is challenging. This study set out to use liquid chromatography-tandem mass spectrometry (LC-MS/MS) to develop a method capable of measuring a comprehensive vitamin D profile, encompassing twelve vitamin D analogues (vitamin D2, D3, 25(OH)D2, 25(OH)D3, 1,25(OH)2D2, 1,25(OH)2D3, 24,25(OH)2D2, 24,25(OH)2D3, 3-epi-25(OH)D2, 3-epi-25(OH)D3, 7αC4 and1α(OH)D3) in a single run. Serum samples were prepared using double liquid-liquid extraction and analysed on an Agilent 6460 QQQ LC-MS/MS equipped with a Pursuit 3 Pentafluorophenyl (4.6 x 100 mm, 3 μm) column. Recovery rates for all analytes were above 95 % with a coefficient of variation (CV) below 10 %. The method exhibited good linearity (r > 0.995) and had a range of detection limits between 0.01 and 0.35 ng/mL and quantification limits between 0.15 and 0.96 ng/mL. Repeatability and within-lab precision were acceptable, with CV values below 10 % and 15 %, respectively. Method accuracy was excellent, with a systematic error below 6.60 %. additionally, all analytes-maintained stability for 48 h following sample preparation, and no interferences were observed among co-eluting analytes. Lastly, this method achieved "world-class" status according to the Sigma metric scale specifications, requiring minimal quality control to ensure data quality. This successfully validated method has the potential not only for improving vitamin D profiling procedures but also for aiding in the diagnosis of other genetic disorders where measuring beyond 25(OH)D is crucial.

An Overview of Different Vitamin D Compounds in the Setting of Adiposity

A large body of research shows an association between higher body weight and low vitamin D status, as assessed using serum 25-hydroxyvitamin D concentrations. Vitamin D can be metabolised in adipose tissue and has been reported to influence gene expression and modulate inflammation and adipose tissue metabolism in vitro. However, the exact metabolism of vitamin D in adipose tissue is currently unknown. White adipose tissue expresses the vitamin D receptor and hydroxylase enzymes, substantially involved in vitamin D metabolism and efficacy. The distribution and concentrations of the generated vitamin D compounds in adipose tissue, however, are largely unknown. Closing this knowledge gap could help to understand whether the different vitamin D compounds have specific health effects in the setting of adiposity. This review summarises the current evidence for a role of vitamin D in adipose tissue and discusses options to accurately measure vitamin D compounds in adipose tissue using liquid chromatography tandem mass spectrometry (LC/MS-MS).

Mass Spectrometry Based on Chemical Derivatization Has Brought Novel Discoveries to Lipidomics: A Comprehensive Review

Lipids, as one of the most important organic compounds in organisms, are important components of cells and participate in energy storage and signal transduction of living organisms. As a rapidly rising field, lipidomics research involves the identification and quantification of multiple classes of lipid molecules, as well as the structure, function, dynamics, and interactions of lipids in living organisms. Due to its inherent high selectivity and high sensitivity, mass spectrometry (MS) is the "gold standard" analysis technique for small molecules in biological samples. The combination chemical derivatization with MS detection is a unique strategy that could improve MS ionization efficiency, facilitate structure identification and quantitative analysis. Herein, this review discusses derivatization-based MS strategies for lipidomic analysis over the past decade and focuses on all the reported lipid categories, including fatty acids and modified fatty acids, glycerolipids, glycerophospholipids, sterols and saccharolipids. The functional groups of lipids mainly involved in chemical derivatization include the C=C group, carboxyl group, hydroxyl group, amino group, carbonyl group. Furthermore, representative applications of these derivatization-based lipid profiling methods were summarized. Finally, challenges and countermeasures of lipid derivatization are mentioned and highlighted to guide future studies of derivatization-based MS strategy in lipidomics.

Improved quantitative LC-MS/MS analysis of vitamin D metabolites in serum after one-pot double derivatization

In this study, we report a one-pot double derivatization scheme, which used acetylation after a Diels-Alder reaction using 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) to improve separation efficiency and provide baseline separations of the five vitamin D metabolites 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), 24,25-dihydroxyvitamin D3 (24,25(OH)2D3), 3β-25-hydroxyvitamin D3 (3β-25(OH)D3), 3α-25-hydroxyvitamin D3 (3α-25(OH)D3) and vitamin D3 on a C-18 stationary phase. Vitamin D metabolites are often very challenging to measure quantitatively using mass spectrometry, due to their low serum concentration levels and low ionization efficiencies. Moreover, some of these species are isomers with virtually identical mass spectral dissociation behavior. To overcome the low ionization efficiency and unspecific fragmentation behavior, derivatization using Diels-Alder reactions with Cookson-type reagents such as PTAD are common. These derivatization reactions generally result in more complicated liquid chromatography separations, because both 6R- and 6S-isomers are formed during Diels-Alder reactions. It has been shown that separations have been particularly challenging for the 3α-25(OH)D3 and 3β-25(OH)D3 epimers. Here, we optimized the PTAD derivatization and the esterification using acetic anhydride. By utilizing the esterification catalyst 4-dimethylaminopyridine, we avoided quenching and evaporation between the two derivatization steps, but were also able to perform the esterification at room temperature without heating. The optimized one-pot double derivatization LC-MS/MS assay was validated with respect to inter/intra-day precision, accuracy, recovery and linear dynamic range and applied to metabolic fingerprinting of vitamin D3 metabolites in serum samples. The metabolites 3α-25(OH)D3, 3β-25(OH)D3 and 24,25(OH)2D3, were readily quantified in all investigated samples. The method was, in principle, also fit for purpose for quantification of the native vitamin D3 species; the relatively high blank concentration of the commercial vitamin D-depleted serum used for calibration, however, limited the limits of quantification for this metabolite. The method provided insufficient limits of quantification for serum levels of 1,25(OH)2D3.

Comparing derivatization reagents for quantitative LC-MS/MS analysis of a variety of vitamin D metabolites

The present study systematically compares the sensitivity and selectivity of the analysis of multiple vitamin D metabolites after chemical derivatization using different reagents for liquid chromatography-tandem mass spectrometry (LC-MS/MS). Generally, chemical derivatization is applied to vitamin D metabolites to increase the ionization efficiency, which is particularly important for very low abundant metabolites. Derivatization can also improve the selectivity of the LC separation. A wide variety of derivatization reagents has been reported in recent years, but information on their relative performance and applicability to different vitamin D metabolites is, unfortunately, not available in the literature. To fill this gap, we investigated vitamin D₃, 3β-25-hydroxyvitamin D₃ (3β-25(OH)D₃), 3α-25-hydroxyvitamin D₃ (3α-25(OH)D₃), 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), and 24,25-dihydroxyvitamin D₃ (24,25(OH)₂D₃) and compared response factors and selectivity after derivatizing with several important reagents, including four dienophile reagents (4-phenyl-1,2,4-triazoline-3,5-dione (PTAD), 4-[2-(6,7-dimethoxy-4-methyl-3-oxo-3,4-dihydroquinoxalinyl)ethyl]-1,2,4-triazoline-3,5-dione (DMEQ-TAD), Amplifex, 2-nitrosopyridine (PyrNO)) as well as two reagents targeting hydroxyl groups: isonicotinoyl chloride (INC) and 2-fluoro-1-methylpyridinium-p-toluenesulfonate (FMP-TS). In addition, a combination of dienophiles and hydroxyl group reagents was examined. For LC separations, reversed-phase C-18 and mixed-mode pentafluorophenyl HPLC columns using different compositions of the mobile phase were compared. With respect to detection sensitivity, the optimum derivatization reagent for the profiling of multiple metabolites was Amplifex. Nevertheless, FMP-TS, INC, PTAD, or PTAD combined with an acetylation reaction showed very good performance for selected metabolites. These reagent combinations provided signal enhancements on the order of 3- to 295-fold depending on the compound. Chromatographic separation of the dihydroxylated vitamin D₃ species was readily achieved using any of the derivatization reactions, while for 25(OH)D₃ epimers, only PyrNO, FMP, INC, and PTAD combined with acetylation enabled complete separation. In conclusion, we believe this study can serve as a useful reference for vitamin D laboratories, to help analytical and clinical scientists decide which derivatization reagent to choose for their application.

Quantification of 25OHD in serum by ID-LC-MS/MS based on oriented immobilization of antibody on magnetic materials

Magnetic nanomaterials are widely used, but co-adsorption of impurities will lead to saturation. In this study, the aim was to prepare a magnetic nano-immunosorbent material based on orienting immobilization that can purify and separate 25-hydroxyvitamin D (25OHD) from serum and provides a new concept of sample pretreatment technology. Streptococcus protein G (SPG) was modified on the surface of the chitosan magnetic material, and the antibody was oriented immobilized using the ability of SPG to specifically bind to the Fc region of the monoclonal antibody. The antigen-binding domain was fully exposed and made up for the deficiency of the antibody random immobilization. Compared with the antibody in the random binding format, this oriented immobilization strategy can increase the effective activity of the antibody, and the amount of antibody consumed is saved to a quarter of the former. The new method is simple, rapid, and sensitive, without consuming a lot of organic reagents, and can enrich 25OHD after simple protein precipitation. Combining with liquid chromatography-tandem mass spectrometry (LC-MS/MS), the analysis can be completed in less than 30 min. For 25OHD₂ and 25OHD₃, the LOD was 0.021 and 0.017 ng mL^-1, respectively, and the LOQ was 0.070 and 0.058 ng mL^-1, respectively. The results indicated that the magnetic nanomaterials based on oriented immobilization can be applied as an effective, sensitive, and attractive adsorbent to the enrichment of serum 25OHD.

Ferrocene tagged primary antibody generates electrochemical signal: An electrochemical immunosensing platform for the monitoring of vitamin D deficiency in clinical samples

In this paper, a new kind of ultrasensitive and low-cost electrochemical immunosensing probe was designed to monitor vitamin D deficiency using 25(OH)D₃ as a clinical biomarker. Ferrocene carbaldehyde conjugated on Ab-25(OH)D₃ antibodies was used as an electrochemical probe for generating signals. The graphene nanoribbon-modified electrode (GNRs) was used to immobilize the (Ab-25(OH)D₃-Fc) conjugate. The high electron transferability, greater surface area, and effective biocompatibility of GNRs enabled the capture of the greater number of primary antibodies (Ab-25(OH)D₃). The developed probe was structurally and morphologically characterized. The step-wise modification was investigated by electrochemical techniques. The direct electrochemistry of ferrocene enabled 25(OH)D₃ biomarker detection with excellent sensitivity. The reduction in peak current was proportional to the concentrations of 25(OH)D₃ in the range of 1-100 ng mL^-1 with a 0.1 ng mL^-1 limit of detection. The probe was tested in terms of reproducibility, repeatability, and stability. Finally, the developed immunosensing probe was applied in serum samples for 25(OH)D₃ quantification, and no significant difference was noticed in the assay results when compared with the standard chemiluminescent immunoassay (CLIA) method. The developed detection strategy has a wider scope for future potential clinical diagnostics applications.

Determination of vitamin D3 conjugated metabolites: a complementary view on hydroxylated metabolites

Experts typically define vitamin D deficiency levels by the determination of a circulating 25-hydroxyvitamin D₃-calcifediol prohormone. A large part of the population is characterized by deficient vitamin D levels (calcifediol < 20 ng mL^-1) despite individuals not being affected by any disorder. Cholecalciferol (vitamin D₃) and/or calcifediol supplementation is a common practice for vitamin D-deficient individuals as recommended by international scientific societies and official agencies. In the last few years, several studies have reported the presence of conjugated vitamin D₃ metabolites, mainly glucuronidation and sulfation derivatives, although simultaneous quantitative measurements involving phase I and II vitamin D metabolites have not been carried out. A quantitative method based on tandem mass spectrometry detection is proposed here for the combined determination of phase I and phase II vitamin D₃ metabolites in human serum. As phase I and phase II metabolites are preferentially ionized in different modes, a switching polarity mode was adopted to determine both groups of compounds in serum at high sensitivity levels (pg mL^-1). The validation of this proposal was successfully accomplished by following the Center for Drug Evaluation and Research (CDER) guidelines. Its applicability was tested in a cohort of volunteers with mostly deficient baseline levels. Considering the sulfated form of calcifediol, the sum of its concentrations showed sufficient baseline vitamin D levels in all individuals, suggesting that this could be a novel strategy for vitamin D deficiency definition. Therefore, phase II metabolites are proposed to be included when evaluating the vitamin D status since they provide more information about the overall status of the vitamin D endocrine system. Nevertheless, further studies are required to confirm the biological activity of these conjugated metabolites and the suitability of this strategy for the description of vitamin D deficiency.

Effect of Serum Level of Vitamin D on External Apical Root Resorption in Maxillary Anterior Teeth in Patients under Fixed Orthodontic Treatment

Introduction

This study aimed to assess the effect of serum level of vitamin D on external apical root resorption (EARR) in maxillary anterior teeth in patients under fixed orthodontic treatment.

Materials and Methods

This retrospective cohort was conducted on patients under fixed orthodontic treatment who were between 12 to 30 years of age. All patients underwent the same treatment technique by the same orthodontist using a 0.022 MBT system. EARR in maxillary anterior teeth was evaluated on pre- and postoperative panoramic radiographs. Blood samples were also collected from patients, and their serum level of vitamin D was measured after the completion of treatment. Data were analyzed by independent t-test and Chi-square test (alpha = 0.05).

Results

A reduction in root length was noted in all patients, which was significant (P < 0.0001); 75% of patients showed EARR in at least one maxillary incisor. EARR had no significant correlation with the serum level of vitamin D (P=0.423).

Conclusions

Serum level of vitamin D had no significant correlation with the occurrence of EARR. However, the high prevalence of EARR calls for measures to minimize it.

Differentiation of Dihydroxylated Vitamin D3 Isomers Using Tandem Mass Spectrometry

Vitamin D compounds are a group of secosteroids derived from cholesterol that are vital for maintaining bone health in humans. Recent studies have shown extraskeletal effects of vitamin D, involving vitamin D metabolites such as the dihydroxylated vitamin D₃ compounds 1,25-dihydroxyvitamin D₃ and 24,25-dihydroxyvitamin D₃. Differentiation and characterization of these isomers by mass spectrometry can be challenging due to the zero-mass difference and minor structural differences between them. The isomers usually require separation by liquid chromatography (LC) prior to mass spectrometry, which adds extra complexity to the analysis. Herein, we investigated and revisited the use of fragmentation methods such as collisional induced dissociation (CID), infrared multiphoton dissociation (IRMPD), electron induced dissociation (EID), and ultraviolet photodissociation (UVPD), available on a 12T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) to generate characteristic fragments for the dihydroxylated vitamin D₃ isomers that can be used to distinguish between them. Isomer-specific fragments were observed for the 1,25-dihydroxyvitamin D₃, which were clearly absent in the 24,25-dihydroxyvitamin D₃ MS/MS spectra using all fragmentation methods mentioned above. The fragments generated due to cleavage of the C-6/C-7 bond in the 1,25-dihydroxyvitamin D₃ compound demonstrate that the fragile OH groups were retained during fragmentation, thus enabling differentiation between the two dihydroxylated vitamin D₃ isomers without the need for prior chromatographic separation or derivatization.

Sample preparation techniques for extraction of vitamin D metabolites from non-conventional biological sample matrices prior to LC-MS/MS analysis

The determination of vitamin D metabolites as status marker or for diagnostic purposes is almost entirely conducted from blood serum or plasma. Other biological matrices, however, have also interested researchers, for two main reasons: (1) alternative matrices may allow non-invasive sampling, permit easier sample transfer and require less demanding storage conditions; and (2) the levels of vitamin D metabolites in other body compartments may further aid the understanding of vitamin D metabolism and function. Thus, the development of reliable and efficient sample preparation protocols for sample matrices other than serum/plasma, which will remove potential interferences and selectively extract the targeted metabolites, is of great importance. This review summarizes sample preparation methods for measurement of vitamin D metabolites using liquid chromatography-(tandem)mass spectrometry in more than ten different human tissues, including hair, saliva, adipose tissue, brain and others.

An ultrasensitive UHPLC-ESI-MS/MS method augmented with a controlled microwave derivatization reaction for quantitation of vitamin D3 and its major metabolites in COVID-19 patients

It is established that vitamin D deficiency is correlated with the disease severity in COVID-19 patients. However, the reliable and sensitive quantitation of vitamin D3 (D3) and its metabolites remains a difficult challenge. Herein, a novel ultrasensitive and reliable UHPLC-ESI-MS/MS method was developed and validated for the quantitation of D3 and its major metabolites in COVID-19 patients. The mass spectral sensitivity was augmented via controlled microwave-assisted derivatization reaction (CMDR) with 2-nitrosopyridine (Pyr-NO) at 65 °C for 2 min. CMDR hyphenation with UHPLC-MS/MS improves detection sensitivity while shortening separation and derivatization reaction times. The precursor to product ion transitions for D3, 25-hydroxy D3 (25(OH)D3), 1,25-dihydroxy D3 (1,25-(OH)2D3) and calcipotriol (CPT) as an internal standard were m/z 493.4 → 231.3, m/z 509.4 → 231.3, m/z 525.4 → 247.3, and m/z 521.4 → 247.3; respectively. The separation of the formed derivatives was conducted using a gradient elution mode with mobile phase A: formic acid (0.1%) in water and mobile phase B: formic acid (0.1%) in acetonitrile. The elution started with 40% (v/v) of B for 0.3 min then increased linearly to 90% (v/v) at 2 min on an Agilent EclipsePlus C18 (50 × 2.1 mm, 1.8 μm) column at a flow rate of 0.3 mL min⁻¹. The method was validated using FDA standards for bioanalytical method validation over a concentration range of 0.02–50 ng mL⁻¹ with correlation coefficient ≥0.9987 and the lower limit of quantitation (LLOQ) were 0.02–0.05 ng mL⁻¹ in human plasma. The developed method has demonstrated excellent comparability to a well-established chemiluminescent immunoassay (CLIA) method for the analysis of D3 metabolites in human samples. The developed UHPLC-ESI-MS/MS method was implemented for routine and reliable quantitation of D3 and its major metabolites in COVID-19 patients.

Quantitative NMR (qNMR) spectroscopy based investigation of the absolute content, stability and isomerization of 25-hydroxyvitamin D2/D3 and 24(R),25-dihydroxyvitamin D2 in solution phase

Vitamin D is an important parameter, in serum/plasma based diagnostic analysis, for the determination of optimal regulation of calcium and phosphate homeostases in the human body, vital for the monitoring/progression of osteomalacia and rickets. Particularly, the quantification of 25-hydroxyvitamin D2, 25-hydroxyvitamin D3 and 24R,25-dihydroxyvitamin D in blood is an excellent indicator for the vitamin D status of a patient. For this purpose, LC–MS/MS methods, based on appropriate vitamin D reference standards, are considered to be ‘gold standard’ for such measurements. We have utilized quantitative NMR spectroscopy to determine the absolute content of these molecules, available as non-certified chemicals, and have determined the stability of these callibrators in borderline polar solvents at room temperature. We have observed significant isomerization of the analytes, which can play a big role in quantification of these analytes by hyphenated LC and GC analytical techniques. Appropriate explanations are given for the observation of new impurities with time in solution phase. The spin system selected for quantitation was determined using relevant 1D and 2D NMR pulse sequences. The advantage of the qNMR approach is that it is based on the quantification of atoms rather than molecular properties (e.g., quantitation by LC/UV, GC, etc.). Since the signals in an NMR spectrum are different nuclear spin-systems dispersed precisely in a magnetic environment, with the intensity being directly proportional to the amount of a particular type of nuclear spin, this technique delivers unparalleled information about the chemical structure and the absolute content.

Analysis of vitamin D metabolic markers by mass spectrometry: Recent progress regarding the "gold standard" method and integration into clinical practice

Liquid chromatography/tandem mass spectrometry is firmly established today as the gold standard technique for analysis of vitamin D, both for vitamin D status assessments as well as for measuring complex and intricate vitamin D metabolic fingerprints. While the actual mass spectrometry technology has seen only incremental performance increases in recent years, there have been major, very impactful changes in the front- and back-end of MS-based vitamin D assays; for example, the extension to new types of biological sample matrices analyzed for an increasing number of different vitamin D metabolites, novel sample preparation techniques, new powerful chemical derivatization reagents, as well the continued integration of high resolution mass spectrometers into clinical laboratories, replacing established triple-quadrupole instruments. At the same time, the sustainability of mass spectrometry operation in the vitamin D field is now firmly established through proven analytical harmonization and standardization programs. The present review summarizes the most important of these recent developments.

Vitamin D3 levels in women and factors contributing to explain metabolic variations

The elucidated metabolism of vitamin D₃ in humans has been the support to explain the high involvement of this liposoluble vitamin in physiological functions. Clinical studies have associated levels of vitamin D₃ metabolites with several disorders. Despite this knowledge, there is a controversy regarding the estimation of deficiency and the physiological and supraphysiological levels of vitamin D₃ metabolites. The association between serum concentrations of vitamin D₃ metabolites and several potentially influential factors (namely, age and anthropometric, seasonal, spatial and metabolic factors) is analyzed in this study. For this purpose, 558 women were recruited and interviewed in several Spanish provinces before blood sampling. Serum vitamin D₃ and its metabolites were determined using an SPE-LC-MS/MS platform. The concentration range for vitamin D₃ was 1.7-21.1 nmol/L and was influenced by body mass index (BMI), waist-to-hip ratio (WHR) and seasonal period. 25-hydroxyvitamin D₃ levels were within 4.8-147.2 nmol/L and were related to WHR, season, latitude and calcium intake. The range of 24,25-dihydroxyvitamin D₃, 0.3-15.0 nmol/L, was associated to BMI, WHR, season, latitude and calcium intake. Finally, energy intake influenced the vitamin D 25-hydroxylase through the 25-hydroxyvitamin D₃/vitamin D₃ ratio, which regulates the synthesis of the circulating form. According to these results, it is worth emphasizing the relevance of all these factors to explain the variability in serum levels of vitamin D₃ and its metabolites. All these factors should be considered in future studies assessing the alteration of vitamin D₃ metabolism.

Bad Prognosis in Critical Ill Patients with COVID-19 during Short-Term ICU Stay regarding Vitamin D Levels

BACKGROUND AND AIMS

Vitamin D inadequacy may be involved in the mechanisms of SARS-CoV-2 infection and in potential risk factors for disease propagation or control of coronavirus disease 2019 (COVID-19). This study assessed a short-term evolution of vitamin D status and its influence upon different clinical parameters in critically ill patients with COVID-19.

METHODS

A prospective analytical study in which 37 critically ill volunteers between 41 and 71 years of age with COVID-19 were evaluated at baseline and three days of intensive care unit (ICU) stay. 25-OH-D₃ and 25-OH-D₂ were analyzed by liquid chromatography-tandem mass spectrometry and total 25-OH-D levels were calculated as the sum of both.

RESULTS

All patients presented low 25-OH-D levels at baseline, decreasing total 25-OH-D (p = 0.011) mainly through 25-OH-D₂ (p = 0.006) levels during ICU stay. 25-OH-D₂ levels decreased a mean of 41.6% ± 89.6% versus 7.0% ± 23.4% for the 25-OH-D₃ form during the ICU stay. Patients who did not need invasive mechanical ventilation presented higher levels of 25-OH-D₂ at baseline and follow-up. Lower 25-OH-D and 25-OH-D₃ levels were associated with higher D-dimer at baseline (p = 0.003; p = 0.001) and at follow up (p = 0.029), higher procalcitonin levels (p = 0.002; p = 0.018) at follow up, and lower percentage lymphocyte counts (p = 0.044; p = 0.040) during ICU stay.

CONCLUSIONS

Deficient vitamin D status in critical patients was established at the admission and further worsened after three days of stay. Lower vitamin D levels were related to key altered clinical and biochemical parameters on patients with SARS-CoV-2 infection. Given the different response of the 25-OH-D₃ and 25-OH-D₂ forms, it would be useful to monitor them on the evolution of the critically ill patient.

Effectiveness of eight-week zinc supplementation on vitamin D3 status and leptin levels in a population of postmenopausal women: a double-blind randomized trial

BACKGROUND

The menopausal period is characterized by hormonal imbalance related to the alteration of parameters involved in lipid metabolism. In addition, menopause increases the risk of deficiencies of key vitamins and minerals such as vitamin D and zinc in such women. The present study investigates the influence of zinc supplementation on the status of vitamin D₃ and other lipid parameters in postmenopausal women.

METHODS

Fifty-one healthy postmenopausal women aged 44-76 years from the province of Granada (Spain) were divided into two groups (placebo and zinc) of 25 and 26 women, respectively. The zinc group was supplemented with 50 mg/day of zinc for 8 weeks. Nutrient intake assessment was performed by means of a 24 -h reminder. Zinc was determined by flame atomic absorption spectrophotometry. Vitamin D was analyzed by liquid chromatography - tandem mass spectrometry. Leptin was determined by enzyme immunoassay.

RESULTS

Zinc supplementation improved the initial vitamin D₃ status of the postmenopausal population (p = 0.049). Plasma levels of 25-OH-D₃ increased significantly after Zn supplementation in women with lower age at menopause (p = 0.045). Both intake and plasma zinc levels were inversely correlated to serum leptin levels (p = 0.044 and p = 0.033, respectively), being significantly lower in lower age at menopause (p < 0.001).

CONCLUSION

Zinc supplementation improved vitamin D₃ status and was associated to low leptin levels in the postmenopausal women of the study.

A Review of Analytical Methods for Calcium Salts and Cholecalciferol in Dietary Supplements

Dietary supplements composed by the combination of a calcium salt with cholecalciferol (vitamin D₃) are widely used for improving bone health in conditions caused by the deficiency of these compounds in the body. Historically, these supplements have been linked to quality and safety issues. In the case of calcium salts, the presence of potentially toxic contaminants such as lead (Pb) has already been alerted by health authorities from different countries. Meanwhile, cholecalciferol is very unstable under inadequate manufacturing and storage conditions. The content of both compounds in commercial dietary supplements is often found to be in disagreement with the label claims, which can lead to a deficient or excessive nutrient intake by consumers. In this scenario, analyzing these compounds is still a difficult and time-consuming task, which usually requires specific pretreatment procedures and multiple analytical methods due to the inorganic nature of calcium and the organic nature of cholecalciferol. Therefore, this article reviews the analytical methods, described in official compendia and scientific literature, for the determination of calcium salts and cholecalciferol in dietary supplement formulations. We also approached the sample preparation procedures highly required due to the matrix complexity of these materials.

Vitamin D Metabolism and Profiling in Veterinary Species

The demand for vitamin D analysis in veterinary species is increasing with the growing knowledge of the extra-skeletal role vitamin D plays in health and disease. The circulating 25-hydroxyvitamin-D (25(OH)D) metabolite is used to assess vitamin D status, and the benefits of analysing other metabolites in the complex vitamin D pathway are being discovered in humans. Profiling of the vitamin D pathway by liquid chromatography tandem mass spectrometry (LC-MS/MS) facilitates simultaneous analysis of multiple metabolites in a single sample and over wide dynamic ranges, and this method is now considered the gold-standard for quantifying vitamin D metabolites. However, very few studies report using LC-MS/MS for the analysis of vitamin D metabolites in veterinary species. Given the complexity of the vitamin D pathway and the similarities in the roles of vitamin D in health and disease between humans and companion animals, there is a clear need to establish a comprehensive, reliable method for veterinary analysis that is comparable to that used in human clinical practice. In this review, we highlight the differences in vitamin D metabolism between veterinary species and the benefits of measuring vitamin D metabolites beyond 25(OH)D. Finally, we discuss the analytical challenges in profiling vitamin D in veterinary species with a focus on LC-MS/MS methods.

Improved sample preparation method for fast LC-MS/MS analysis of vitamin D metabolites in serum

Despite the fact that more than 90% of vitamin D analysis are performed using immuno-enzymatic techniques, it is liquid chromatography coupled with tandem mass spectrometry that is currently the reference method. It allows for specific and selective analysis of all relevant vitamin D metabolites from a variety of biological materials, including serum or a dried blood spot. This paper presents development of a fast, cheap and high-throughput method of serum sample preparation using protein precipitation. For this purpose, organic solvent is used. Several substances were tested, including acetonitrile, methanol and their mixtures with zinc sulfate. However, the highest recovery values for the vitamin D metabolites were obtained for acetonitrile, with an organic solvent to serum ratio of 8:1. The preparation of a sample is carried out in 96-well plates and takes an hour and a half, together with a derivatization reaction using Cookson-type reagent 4-(4'-dimethylaminophenyl)-1,2,4-triazoline-3,5-dione. Due to the fact that vitamin D metabolites are bound to proteins, the relationship between the content of organic solvent in the sample preparation process and their release from the protein complex was examined. The results indicate that the organic solvent content should be 30-70% in order to completely release the tested compounds from the proteins. In addition, the developed chromatographic method has eliminated false positive signals for the 24,25(OH)₂D₃ metabolite. Total analysis time is 5.5 min., while maintaining resolution necessary to separate the analyzed compounds.

Response of Vitamin D after Magnesium Intervention in a Postmenopausal Population from the Province of Granada, Spain

Menopause is a stage of hormonal imbalance in women which, in addition to other physiopathological consequences, poses a risk of deficiency of key micronutrients such as magnesium and vitamin D. A study was made of the influence of a magnesium intervention upon vitamin D status in a postmenopausal population from the province of Granada (Spain). Fifty-two healthy postmenopausal women between 44–76 years of age were included. Two randomized groups—placebo and magnesium (500 mg/day)—were treated during eight weeks. Nutrient intake was assessed using questionnaires based on 72-h recall. Vitamin D was analyzed by liquid chromatography—tandem mass spectrometry. Baseline vitamin D proved deficient in over 80% of the subjects. The administration of magnesium resulted in significantly increased vitamin D levels in the intervention group versus the controls (p < 0.05). Magnesium supplementation improved vitamin D status in the studied postmenopausal women.

Current trends in the analytical determination of vitamin D

Introduction

Vitamin D is a micronutrient that plays a large role in bone disease, and researchers are now discovering that it also does so in non-skeletal disease, thus making high-quality analytical determination necessary. To make this determination, a series of immunochemical and physical methods are used. These methods present a series of different ways of handling samples as well as different methodologies that bring a series of advantages and limitations based on the scope of work in which the vitamin D analysis methodology is applied. Although the Liquid Chromatography-tandem Mass Spectrometry (LC-MS/MS) is the gold standard method of analytical vitamin D determination, and is the only one to offer a more complete and accurate view of all metabolites of this vitamin, it is necessary to standardize all the analysis methodologies that allow accurate, reliable and quality analytical determination, since it is essential to obtain results that can reliably be extrapolated to the population, and that can be decisive in assessing a large number of pathologies.

Development and application of a LC-MS/MS assay for simultaneous analysis of 25-hydroxyvitamin-D and 3-epi-25-hydroxyvitamin-D metabolites in canine serum

Hypovitaminosis D and hypervitaminosis D are well recognised disorders in dogs. Hypovitaminosis D can occur following consumption of a diet inadequately supplemented with vitamin D or as a sequelae of severe intestinal disease. Hypervitaminosis D may occur as a result of consuming proprietary dog foods over-supplemented with vitamin D or through ingestion of vitamin D containing medicinal products or rodenticides. Consequently, there is a clear need to establish a methodology that can accurately quantify vitamin D metabolites across a broad dynamic range in dogs. The existence of C3-epimers of vitamin D metabolites has yet to be elucidated in dogs, yet are known to interfere with the analysis of vitamin D and have unknown biological activity in other species. Here, we describe the development and validation of a sensitive, specific and robust analytical liquid chromatography tandem mass spectrometry (LC-MS/MS) assay capable of separating and accurately measuring 25-hydroxyvitamin-D_2/3 (25(OH)D_2/3) and 3-epi-25-hydroxyvitamin-D_2/3 (3-epi-25(OH)D_2/3). We describe a simplified workflow utilising supported liquid extraction (SLE) without derivatization that provides good linearity (mean r > 0.996) and accuracy across a broad dynamic range of 4-500 nmol/L for D₃ metabolites and 7.8-500 nmol/L for D₂ metabolites. Upon application of this assay to 117 canine serum samples, 25(OH)D₃ was detectable in all samples with a median concentration of 82.1 nmol/L (inter-quartile range (IQR) 59.7-101.8 nmol/L). 3-epi-25(OH)D₃ could be detected in 87.2 % of the study population, with a median concentration of 5.2 nmol/L (2.4-8.1 nmol/L). However, 3-epi-25(OH)D₃ was quantified below the LLOQ in 40.2 % of these samples. 3-epi-25(OH)D₃ contributed on average 6.3 % to 25(OH)D₃ status (contribution ranges from 0 to 23.8%) and a positive correlation was detected between 25(OH)D₃ and 3-epi-25(OH)D₃ concentrations. Free 25(OH)D was also measured using an immunoassay with a median concentration of 15.2 pmol/L (12.5-23.2 pmol/L), and this metabolite was also positively correlated to both 3-epi-25(OH)D₃ and 25(OH)D₃ concentrations. D₂ metabolites were not detected in canine serum as expected. Vitamin D metabolite concentrations were variable between individuals, and research into the causes of this variation should include factors such as breed, age, sex and neuter status to determine the impact of genetic and hormonal factors. Given the clinical importance of vitamin D in dogs, and the immense potential for utilising this species as a model for human disease, further elucidation of the vitamin D pathway in this species would provide immense clinical and research benefit.

Association between Body Fatness and Vitamin D3 Status in a Postmenopausal Population

Vitamin D is a micronutrient that plays a key role in phosphocalcic metabolism. The postmenopausal population presents a risk of deficiency in this vitamin due to hormonal alterations which, in the case of obesity, would be exacerbated. The objective was to assess the status of vitamin D in a postmenopausal population and determine the relationship of 25-hydroxivitamin D [25(OH)D] and its metabolites with anthropometric parameters. The study included 78 healthy postmenopausal women aged from 44 to 76. The nutrient intake assessment was carried out using the 24 h reminder (R24h). 25(OH)D was analyzed using ultra-high-performance liquid chromatography (UHPLC). A total of 80% and 68% of the women studied did not reach sufficient values of 25(OH)D and 25-hydroxivitamin D₃ [25(OH)D₃], respectively, which was inversely correlated with Body Mass Index (BMI) (r = −0.25, p = 0.04), hip perimeter (r = −0.26 and r = −0.24, all p < 0.05), arm circumference (r = −0.29, p = 0.01) and fat mass (r = −0.28 and r = −0.26, all p < 0.05). 25(OH)D₃ is the metabolite that contributed most to this association. In conclusion, 25(OH)D₃ levels are related to anthropometric parameters in the postmenopausal women in this study, confirming insufficient status in the majority of the population. Approach strategies are necessary to correct and avoid this risk in order to ensure future quality of life.

Re-certification of hydroxyvitamin D standards by isotope pattern deconvolution

BACKGROUND

Vitamin D testing in analytical clinical laboratories has been experiencing a rapid increase of demand over the last years, as it plays a key role in several disorders. Due to the narrow ranges of medical significance regarding its concentration levels in human serum, accurate and precise determinations of vitamin D metabolites are required.

METHODS

We present an isotope dilution mass spectrometry quantification method for the re-certification of routine commercial standards used in method validation steps, isotope pattern deconvolution (IPD) based on LC-MS/MS.

RESULTS

IPD allowed to compensate for the observed biases of +4.7% for 25(OH)D3, -29% for 25(OH)D2 and -30% for 24,25(OH)₂D₃ standard concentrations, respectively in an easy, cheap and straightforward way.

CONCLUSIONS

Is has been observed that, in some cases, discrepancies may exist between stated purity or amount of routinely used commercial standards and actual values, which would lead to unwanted bias in the developed methodologies. The present correction has helped meeting the regulations established by international standardization programs, including Vitamin D Standardization Program (VDSP).

Acylation derivatization based LC-MS analysis of 25-hydroxyvitamin D from finger-prick blood

Vitamin D metabolite analysis possessed significant clinical value for the pediatric department. However, invasive venipuncture sampling and high blood consumption inflicted much suffering on patients. For alleviation, we carried out a LC-MS method for 25-hydroxyvitamin D quantification in only 3 μl of plasma from the considerably less invasive finger-prick blood samples. To improve sensitivity, acylation on C3-hydroxyl (by isonicotinoyl chloride) rather than Diels-Alder adduction on s-cis-diene structure was for the very first time introduced into vitamin D metabolite derivatization. Compared with the existing derivatization approaches, this novel strategy not only prevented isomer interference, but also exhibited higher reacting throughput. For certification, the methodology was systematically validated and showed satisfying consistency with SRM927a. During clinical application, we found a convincing correlation between 25-hydroxyvitamin D and indirect/total bilirubin in jaundiced newborns. Such an observation indicated that vitamin D supplementation may help to achieve optimal outcomes in neonatal jaundice.

Development of a method for multiple vitamin D metabolite measurements by liquid chromatography coupled with tandem mass spectrometry in dried blood spots

There are two forms of vitamin D which are essential to the human body, i.e. vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). The inactive metabolites of vitamin D are commonly used for quantitative analysis because of their longer half-life, stability, and relatively high blood concentrations. This paper presents the development of a high-throughput and sensitive method for determining four vitamin D metabolites in dried blood spots using liquid chromatography coupled with tandem mass spectrometry. This method allows for the determination of 25(OH)D2 and 25(OH)D3 concentrations, as well as the epimeric form 3-epi-25(OH)D3 and 24,25(OH)2D3. The analyzed material is capillary blood taken from the fingertip, deposited on filter paper. Four different chromatographic columns were tested to separate all compounds, in particular, the epimeric form. The column of choice was F5 (Phenomenex, Torrance, CA, USA). In order to prove the consistency between the results for DBS, used as an alternative biological matrix, and serum, comparative studies of these two materials were carried out in nearly 100 individuals. The results indicated their positive correlation. The evaluation of short-term stability of metabolites in DBS within the month showed no change in metabolite concentration. During the validation, the impact of the matrix on the ionization of the tested compounds was evaluated. Capillary blood and venous blood collected for different anticoagulants were also compared. The smallest differences in the results were obtained for citrate. In order to achieve a limit of quantitation of 0.2 ng ml-1, sample preparation involved derivatization using a Cookson-type reagent, 4-(4'-dimethylaminophenyl)-1,2,4-triazoline-3,5-dione (DAPTAD).

Effectiveness of early identification and electronic interventions for teens with risk factors for the development of heart disease and diabetes: Phase II findings

BACKGROUND AND PURPOSE

Serum and physical risk factors for the development of heart disease and diabetes are detectable long before adulthood. The purpose of this two-part study was to assess the prevalence and associations of these risk factors in teens and determine the effectiveness of a customizable two-part electronic education program on minimizing identified risks.

METHODS

Data were collected from teens (n = 168) from two high schools (one urban and one rural) in the mid-Atlantic region. After baseline data were collected, the two-part electronic education program was initiated. Serum and physical risk factors were rechecked at 12-week intervals, and results were analyzed.

CONCLUSIONS

Significant serum and physical risk factor associations were identified and remained present among teens over the course of the study. High-density lipoproteins showed significant, steady improvement. Low-density lipoproteins were positively associated with body mass index, diastolic blood pressure, and thyroid-stimulating hormone level.

IMPLICATIONS FOR PRACTICE

Although serum and physical risk factors are identifiable in teens, routine screening of this age group and younger is not an established standard of care. Health care providers need effective, innovative methods to counteract these risks. Through increased awareness of the presence of risk factors in young patients, advanced nurse practitioners may implement earlier interventions to counteract these risks.

A fast and simple method for simultaneous measurements of 25(OH)D, 24,25(OH)2D and the Vitamin D Metabolite Ratio (VMR) in serum samples by LC-MS/MS

BACKGROUND

Rapid, easy and reliable measurement of the major vitamin D metabolites is required in order to fulfill the needs of a clinical routine laboratory. To overcome these challenges, we have developed and validated a LC-MS/MS method for the quantification of 25-hydroxyvitamin D₂ and D₃, epi-25-hydroxyvitamin D₃ and 24,25-dihydroxyvitamin D₃.

METHODS

Sample preparation was based on precipitation and centrifugation of 100μL of patient serum, followed by injection into the LC-MS/MS system. Samples from Vitamin D Standardization Program (n=80) and patient samples (n=281) have been compared with a reference LC-MS/MS method. For the analytical validation NIST and Labquality quality control materials were used.

RESULTS

Mean intra-assay and inter-assay imprecision were <6.0 and 6.4% and mean recoveries were within 95-104%. LOQ's were 0.5μg/L for 24,25(OH)₂D₃, 1.1μg/L for 25(OH)D₃ and epi-25(OH)D₃ and 1.7μg/L for 25(OH)D₂. A 3% bias obtained between the proposed and the reference method satisfies Vitamin D Standardization Program recommendations.

CONCLUSIONS

We present a rapid, easy, reliable and cost-effective method completely adequate for routine testing, which permits the measurement of the ratio of 24,25-dihydroxyvitamin D to 25-hydroxyvitamin D, Vitamin D Metabolite Ratio (VMR), in serum samples.