Assay-Specific Decision Limits for Two New Automated Parathyroid Hormone and 25-Hydroxyvitamin D Assays

Determination of parathyroid hormone: from radioimmunoassay to LCMS/MS

Parathyroid hormone (PTH) determination is of paramount importance for the exploration of diseases related with calcium metabolism and for the follow-up of patients suffering from bone and mineral disorders associated with chronic kidney diseases (CKD-MBD). Unfortunately, the biologically active form of PTH, i.e. 1-84 PTH, circulates in the blood stream with many fragments and post-translationally modified forms, which decreases the specificity of immunoassays. The assays used to measure PTH, either from 2nd or 3rd generation, are not standardised, which may lead to interpretation errors and clinical consequences. Reference ranges for PTH have neither been always correctly established and the stability of the peptide is also a matter of concern. Fortunately, these last years, newer techniques using mass spectrometry (either high resolution or triple quadripole) coupled with liquid chromatography have been developed, which will help to standardise the different assays. Indeed, PTH assays standardisation is one of the task of the IFCC Committee for Bone Metabolism. Such standardisation will allow a better consistency in the interpretation of the results and will promote studies aiming at the establishment of correct reference ranges.

Serum PTH reference values in an adult Brazilian population: implications for the diagnosis of hyperparathyroidism

OBJECTIVE

To define serum parathyroid hormone (PTH) reference values in carefully selected subjects following the recommended pre-analytical guidelines.

SUBJECTS AND METHODS

First, 676 adults who would be submitted to thyroidectomy were evaluated. Patients using interfering medications or with malabsorption syndrome, hypomagnesemia, hyper- or hypophosphatemia, hypo- or hypercalcemia, 25-hydroxyvitamin D < 30 ng/dL, estimated glomerular filtration rate < 60 mL/min/1.73 m2, urinary calcium/creatinine ratio ≥ 0.25, thyroid dysfunction, parathyroid adenoma detected during surgery were excluded. The sample consisted of 312 subjects.

RESULTS

The median, minimum, maximum, and 2.5th and 97.5th percentiles of the PTH values obtained were 30, 7.2, 78, 10.1, and 52 pg/mL, respectively. Thus, the reference range was 10 to 52 pg/mL. PTH > 65 pg/mL, the upper limit of normal according to the manufacturer of the kit, was observed in only one subject (0.3%). Considering the upper limit proposed by the kit's manufacturer, 1/6 hypercalcemic patients and 4/8 normocalcemic patients with PHPT had normal PTH. Using the upper limit established in this study, only one normocalcemic patient had normal PTH. Thus, the sensitivity of PTH in detecting asymptomatic primary hyperparathyroidism (PHPT) using the values recommended by the kit and established in this study was 64% and 93%, respectively (50% versus 87.5% for normocalcemic PHPT).

CONCLUSION

The upper reference limit of PTH obtained for a rigorously selected sample was 20% lower than that provided by the assay, which increased its sensitivity in detecting PHPT.

Relationship between serum 25-hydroxyvitamin D and red blood cell indices in German adolescents

Since the impact of vitamin D on red blood cell formation has not been well studied, we aimed at assessing the putative link between serum 25-hydroxyvitamin D (25[OH]D) concentrations and hematological markers of erythropoiesis in a large cohort of German adolescents aged 11 to 17 years. In total, 5066 participants from the population-based, nationally representative KiGGS study (Kinder- und Jugendgesundheitssurvey, German Health Interview and Examination Survey for Children and Adolescents) were grouped into either tertiles or clinically accepted cutoff levels for serum 25(OH)D. Results demonstrated significant and inverse correlations between 25(OH)D levels and several hematological parameters including hemoglobin concentration (r = - 0.04, p = 0.003), mean corpuscular hemoglobin (r = - 0.11, p < 0.001), red blood cell count (r = - 0.04, p = 0.002), and soluble transferrin receptor (r = - 0.1, p < 0.001), whereas, in contrast, serum 25(OH)D was positively correlated to the mean corpuscular volume of erythrocytes (r = 0.08, p < 0.001). Multinomial regression models adjusted for clinically relevant confounders confirmed statistically significant differences between the two strata of 25(OH)D groups with respect to red blood cell markers (hemoglobin concentration, red blood cell count, mean corpuscular volume, and corpuscular hemoglobin, as well as iron and soluble transferrin receptor).

CONCLUSIONS

The link between serum 25(OH)D and several important hematological parameters may point to an inhibitory role of vitamin D in the regulation of erythropoiesis in adolescents. What is Known: • The physiological effects of vitamin D on calcium homeostasis and bone metabolism have been established. • However, much less is known about the impact of circulating vitamin D on erythropoiesis. What is New: • Data from the KiGGS study in German adolescents demonstrated significant associations between serum vitamin D concentrations and red blood cell indices. • Further studies should be conducted to decipher the underlying mechanisms of vitamin D on erythropoiesis.

How the reference values for serum parathyroid hormone concentration are (or should be) established?

Well-validated reference values are necessary for a correct interpretation of a serum PTH concentration. Establishing PTH reference values needs recruiting a large reference population. Exclusion criteria for this population can be defined as any situation possibly inducing an increase or a decrease in PTH concentration. As recommended in the recent guidelines on the diagnosis and management of asymptomatic primary hyperparathyroidism, PTH reference values should be established in vitamin D-replete subjects with a normal renal function with possible stratification according to various factors such as age, gender, menopausal status, body mass index, and race. A consensus about analytical/pre-analytical aspects of PTH measurement is also needed with special emphasis on the nature of the sample (plasma or serum), the time and the fasting/non-fasting status of the blood sample. Our opinion is that blood sample for PTH measurement should be obtained in the morning after an overnight fast. Furthermore, despite longer stability of the PTH molecule in EDTA plasma, we prefer serum as it allows to measure calcium, a prerequisite for a correct interpretation of a PTH concentration, on the same sample. Once a consensus is reached, we believe an important international multicentre work should be performed to recruit a very extensive reference population of apparently healthy vitamin D-replete subjects with a normal renal function in order to establish the PTH normative data. Due to the huge inter-method variability in PTH measurement, a sufficient quantity of blood sample should be obtained to allow measurement with as many PTH kits as possible.

Considerations in parathyroid hormone testing

Parathyroid hormone (PTH) is a major player in phosphocalcic metabolism and its measurement is very important for the correct diagnosis and treatment of several diseases. PTH determination represents the paradigm of quality in laboratory medicine as many variables in the pre-, intra-, and post-analytical phases strongly affect the value of the clinical information. Analytical determination of PTH has been rendered difficult by the presence, in the circulation, of truncated fragments that can cross-react with the antibodies used for its determination. In addition, pre-analytical phase is complicated by the lack of stability of the peptide and the best sample to use for its determination remains controversial, as well as sample handling and storage. PTH secretion is also affected by circadian and seasonal rhythms and by physical exercise. Finally, from the post-analytical perspective, establishment of reliable reference ranges requires further efforts as the selection criteria for reference subjects should take into consideration new variables such as gender, race and vitamin D levels. Finally, clinical guidelines have recently revised and improved the criteria for a correct interpretation of PTH values.

Association of circulating 25-hydroxyvitamin D with mental well-being in a population-based, nationally representative sample of German adolescents

OBJECTIVE

Numerous studies have linked vitamin D to health-related quality of life (hrQoL) in chronically ill adults or elderly subjects. The aim of this study was to evaluate the association between 25-hydroxyvitamin D (25(OH)D) and hrQoL in a population-based sample of German adolescents.

METHODS

A total of n = 5066 study participants from the nationwide, representative German Health Interview and Examination Survey for Children and Adolescents (Kinder- und Jugendgesundheitssurvey) aged 11-17 years were included in this post hoc analysis. HrQoL was measured using the well-validated self- and parent-rated Children's Quality of Life questionnaires (KINDL-R), while the level of distress was assessed using the self- and proxy version of the Strengths and Difficulties Questionnaire (SDQ). Serum 25(OH)D concentrations were determined using a commercially available chemiluminescence immunoassay.

RESULTS

Bivariate analyses demonstrated a significant positive association between 25(OH)D and hrQoL for both self- [estimate (E) = 0.82, 95 % confidence interval (95 % CI) 0.35-1.30, p = 0.001] and parent ratings (E = 1.33, 95 % CI 0.83-1.83, p < 0.001). In addition, we found negative correlations between 25(OH)D and self- (E = -0.34, 95 % CI -0.58 to -0.11, p = 0.005) and parent-reported total SDQ scores (E = -0.70, 95 % CI -1.03 to -0.37, p < 0.001). Generalized linear models adjusted for age, sex, body mass index, systolic blood pressure, migration background, socio-economic status, and sedentary screen time confirmed that 25(OH)D independently and significantly predicted better hrQoL (p ≤ 0.004).

CONCLUSIONS

These findings linking 25(OH)D to better well-being in a nationally representative sample of German children and adolescents suggest beneficial effects of vitamin D on mental health. However, recommendations for vitamin supplementation in healthy children and adolescents are not warranted from our data.

Establishment of a normal reference value of parathyroid hormone in a large healthy Chinese population and evaluation of its relation to bone turnover and bone mineral density

A normal reference value of parathyroid hormone (PTH) was established for the first time in a large sample of healthy Chinese subjects by completely excluding interference of vitamin D deficiency. A high PTH level correlated significantly with an elevated bone turnover and a reduced bone mineral density (BMD).

INTRODUCTION

The aims of this study are to establish a normal reference value for serum PTH and to evaluate the effect of parathyroid gland status on bone turnover and BMD.

METHODS

Our cross-sectional study included 1436 healthy individuals from 5 different Chinese cities. Concentrations of serum PTH, 25-hydroxyvitamin D (25OHD), procollagen I N-terminal peptide (P1NP, a bone formation marker), and carboxyl-terminal telopeptide of type I collagen (β-CTX, a bone resorption marker) were measured by electrochemiluminescence immunoassay. BMD was measured by dual-energy X-ray absorptiometry. The relation of PTH concentration to age, gender, height, and weight was examined. Reference values of PTH were established for all subjects and for subjects categorized by serum 25OHD concentrations. Correlations of PTH levels with bone turnover biomarkers and BMD were statistically analyzed.

RESULTS

Reference values of PTH were 8.84-69.95 pg/mL in all the subjects and 7.48-60.73 and 5.83-56.78 pg/mL in the subjects with serum 25OHD concentrations of ≥20 and ≥30 ng/mL, respectively. Serum PTH showed a negative linear correlation with 25OHD, and the breakpoint was 18.21 ng/mL, below which the PTH level rapidly increased. The increase in PTH levels with age showed a positive linear correlation with P1NP and β-CTX concentrations and a negative linear correlation with BMD at the lumbar spines and the femoral neck.

CONCLUSIONS

A reference value of PTH was established in a large sample of healthy Chinese subjects according to 25OHD status, gender, and age. A high PTH level correlated significantly with an elevated bone turnover and a reduced BMD.

Serum PTH reference values established by an automated third-generation assay in vitamin D-replete subjects with normal renal function: consequences of diagnosing primary hyperparathyroidism and the classification of dialysis patients

OBJECTIVE

To determine parathyroid hormone (PTH) reference values in French healthy adults, taking into account serum 25-hydroxyvitamin D (25OHD), renal function, age, gender, and BMI.

PARTICIPANTS AND MAIN BIOLOGICAL MEASUREMENTS

We studied 898 healthy subjects (432 women) aged 18-89 years with a normal BMI and estimated glomerular filtration rate (eGFR), 81 patients with surgically proven primary hyperparathyroidism (PHPT), and 264 dialysis patients. 25OHD and third-generation PTH assays were implemented on the LIAISON XL platform.

RESULTS

Median PTH and 25OHD values in the 898 healthy subjects were 18.8  ng/l and 23.6  ng/ml respectively. PTH was lower in subjects with 25OHD ≥30  ng/ml than in those with lower values. Among the 183 subjects with 25OHD ≥30  ng/ml, those aged ≥60 years (n=31) had higher PTH values than younger subjects, independent of 25OHD, BMI, and eGFR (P<0.001). Given the small number of subjects aged ≥60 years, we adopted the 95% CI of PTH values for the entire group of 183 vitamin D-replete subjects (9.4-28.9  ng/l) as our reference values. With 28.9  ng/l as the upper limit of normal (ULN) rather than the manufacturer's ULN of 38.4  ng/l, the percentage of PHPT patients with 'high' PTH values rose to 90.1% from 66.6% (P<0.001), and 18.6% of the dialysis patients were classified differently in view of the KDIGO target range (two to nine times the ULN).

CONCLUSION

When only subjects with 25OHD ≥30  ng/ml were included in the reference population, the PTH ULN fell by 22.4%, diagnostic sensitivity for PHPT improved, and the classification of dialysis patients was modified.

Interpretation of plasma PTH concentrations according to 25OHD status, gender, age, weight status, and calcium intake: importance of the reference values

CONTEXT

Reference values for plasma PTH assessment were generally established on small samples of apparently healthy subjects, without considering their 25-hydroxyvitamin D (25OHD) status or other potential modifiers of PTH concentration.

OBJECTIVE

Our objective was to assess ranges of plasma PTH concentration in a large sample of adults, stratifying by 25OHD status, age, gender, weight status, and calcium intake.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional survey is based on 1824 middle-aged Caucasian adults from the Supplémentation en Vitamines et Minéraux Antioxydants study (1994).

MAIN OUTCOME MEASURES

Plasma PTH and 25OHD concentrations were measured by an electrochemoluminescent immunoassay. Extreme percentiles of plasma PTH concentrations were assessed specifically in subjects who had plasmatic values of 25OHD of 20 ng/mL or greater and 30 ng/mL or greater.

RESULTS

Among subjects with 25OHD status of 20 ng/mL or greater, the 97.5th percentile of plasma PTH concentration was 45.5 ng/L. By using this value as a reference, 5% of the subjects with plasma 25OHD less than 20 nmol/L had a high plasma PTH level, reflecting secondary hyperparathyroidism. Among vitamin D-replete subjects (25OHD status of 20 ng/mL or greater), the 97.5th percentile of plasma PTH was higher in overweight/obese subjects (51.9 vs 43.5 ng/L among normal weight subjects).

CONCLUSIONS

The reference value for plasma PTH defined in this vitamin D-replete population was far below the value currently provided by the manufacturer (65 ng/L) and varied according to overweight status. These results may contribute to improve the diagnosis of primary and secondary hyperparathyroidism and subsequent therapeutic indication.

[Clinical value of two methods to measure parathyroid hormone in chronic renal insufficiency, considering vitamin D metabolism]

INTRODUCTION

Parathyroid hormone levels provide important information in chronic renal failure.

AIM

To compare parathyroid hormone levels measured by two assays in correlation with vitamin D supply.

METHOD

Parathyroid hormone and 25-hydroxi-vitamin-D were determined in 104 patients (31 patients with chronic renal failure without renal replacement therapy, 36 patients treated with peritoneal dialysis and 37 patients treated with hemodialysis).

RESULTS

Good correlation was found between results of the two parathyroid hormone methods, but the intact parathyroid hormone levels were higher than the biointact values. 87% and 13% of the patients had vitamin-D deficiency and insufficiency, respectively. The frequency of serious vitamin-D deficiency was higher in the peritoneal dialysis than in the hemodialysis group. Intact parathyroid hormone levels were different in dialysed patients having vitamin-D-deficiency and insufficiency, and the difference was higher for the biointact than intact values. Negative correlation was detected between biointact parathyroid hormone and 25-hydroxivitamin-D in the hemodialysis group.

CONCLUSIONS

Biointact parathyroid hormone levels better reflect the vitamin D supply and bone metabolism than intact levels, especially in hemodialysed patients.

Pathology consultation on vitamin D testing

The last decade has seen a substantial increase in clinical interest in vitamin D deficiency and of laboratory testing for vitamin D status. Many clinical laboratories in the United States have seen requests for vitamin D testing increase 100% or more in the last 5 years. The most common laboratory test to assess vitamin D nutritional status is total 25-hydroxyvitamin D serum concentrations. Laboratory professionals are often confronted with challenges related to vitamin D testing, including controversy over optimal and target vitamin D concentrations, variable reference ranges across marketed assays and reference laboratories, lack of standardization of vitamin D assays, and misordering of 1,25-dihydroxyvitamin D testing. This article presents a common clinical case scenario regarding vitamin D and an up-to-date discussion and review of the literature on vitamin D testing.

The relationship between PTH and 25-hydroxy vitamin D early in pregnancy

OBJECTIVE

Measure serum PTH and 25(OH)D in a cross-sectional sample of pregnant women at 11th through 13th weeks' gestation to examine vitamin D status and consider implications.

DESIGN

Observational: we retrieved residual sera stored at -20 °C after routine first trimester Down's syndrome screening, distributed over 12 months.

PATIENTS

430 African American women and 586 Caucasian women.

MEASUREMENTS

PTH and 25-hydroxy vitamin D [25(OH)D] immunoassays.

RESULTS

PTH medians were: 1·33 pmol/l (African American women); 1·20 pmol/l (Caucasian women) (t = 0·43, P = 0·7). Concentrations were highest in winter and decreased significantly in spring, fall, and summer. There was a direct PTH/weight relationship in Caucasian (t = 3·12, P < 0·002), but not African American women (t = 1·34, P = 0·18). Median 25(OH)D concentrations were 47·5 nmol/l (African American women) and 65 nmol/l (Caucasian women) (t = 13·7, P < 0·001). Concentrations were lowest in winter and rose significantly in spring, fall, and summer. Reciprocal 25(OH)D/weight relationships existed for both racial groups (t =-4·31 P < 0·001; t = 4·54, P < 0·001, respectively). Among 68 Caucasian women who smoked, median PTH and 25(OH)D concentrations were somewhat lower (P = ns). In separate regression models with PTH and 25(OH)D [dependent variables] and season, weight and smoking [independent variables], the only qualifying interactive term was in the Caucasian PTH model (season*1/weight). A regression model applied to adjusted scatter plots of PTH vs 25(OH)D indicated a weak relationship.

CONCLUSIONS

The PTH/25(OH)D relationship is weaker during early pregnancy than in non-pregnant adults, making it unreliable for estimating vitamin D sufficiency. A suitable reference point for sufficiency might be the maternal 25(OH)D level considered sufficient for adequate transfer to neonates.

Performance characteristics of six intact parathyroid hormone assays

The aim of this study was to evaluate the performance characteristics of 6 intact parathyroid hormone assays: Access 2 (Beckman Coulter, Fullerton, CA), ARCHITECT i2000(SR) (Abbott Diagnostics, Abbott Park, IL), ADVIA Centaur (Siemens Healthcare Diagnostics, Deerfield, IL), Modular E170 (Roche Diagnostics, Indianapolis, IN), IMMULITE 2000 (Siemens Healthcare Diagnostics), and LIAISON (DiaSorin, Stillwater, MN). Sample collection tubes and storage conditions were compared. Imprecision studies were performed using commercial quality control materials. Linearity was assessed using pools prepared from samples. For method comparison, serum and EDTA plasma samples were tested by all methods, and the ARCHITECT was used as the comparison method. Reference intervals were determined using various vitamin D cutoffs. The types of collection tubes and storage conditions are more important for some methods than others. Total coefficients of variation were 10.9% or less. The maximum deviation from the target recovery for linearity ranged from 5.0% to 82.2%. Bland-Altman plots demonstrated percentage biases ranging from -36.3% to 24.4%. The lower limit of the reference interval was not influenced by vitamin D status, whereas the upper reference limit was affected.

Assessing vitamin D status: pitfalls for the unwary

The use of vitamin D testing has grown rapidly in the recent times as a result of increased interest in the role of vitamin D in health. Although the generally accepted measure of vitamin D status is circulating 25(OH)D concentration, there is little consensus on which assay method should be used. Commonly used assays include competitive protein-binding assay, RIA, enzyme immunoassay, chemiluminescence immunoassays, HPLC, and LC-MS/MS, each with its own advantages and disadvantages. However, there is significant interassay and interlaboratory variability in measurements. Our simulation of the published data showed that using a deficiency cut-point of 50 nmol/L, 57% of samples assessed using a chemiluminescence immunoassay were classified as deficient compared with 41% of samples assessed using LC-MS/MS; a 20% misclassification rate. Similar rates of misclassification were seen at 75 nmol/L. This has implications for clinical practice and decision limits for vitamin D supplementation, suggesting that cut-points should be assay specific rather than universal and that greater harmonization between laboratories is required. Newer assays using alternative biological samples to determine the circulating 25(OH)D have been proposed and advances in the genetics of vitamin D and the role of vitamin D-binding protein may improve future assay accuracy.

Vitamin D and primary hyperparathyroidism (PHPT)

Vitamin D deficiency and PHPT are two common conditions, especially in postmenopausal women. Vitamin D deficiency is said to be even more frequent in PHPT patients than in the general population due to an accelerated conversion of 25OHD into calcitriol or 24-hydroxylated compounds. Although several studies have reported worsening of PHPT phenotype (larger tumours, higher PTH levels, more severe bone disease) when vitamin D deficiency coexists whereas vitamin D supplementation in PHPT patients with a serum calcium level<3 mmol/L has been shown to be safe (no increase in serum or urinary calcium) and to decrease serum PTH concentration, that many physicians are afraid to give vitamin D to already hypercalcemic PHPT patients. On the other hand, it is possible that, in some patients, a persistent vitamin D deficiency induces, in the long-term, an autonomous secretion of PTH (i.e. tertiary hyperparathyroidism). The mechanism by which this could occur is unclear however. Finally, as many, otherwise normal, subjects with vitamin D insufficiency may have an increased serum PTH level we believe that those with vitamin D insufficiency should be excluded from a reference population for serum PTH levels. By doing that, we found that the upper normal limit for serum PTH was 25-30% lower than in the whole population.

Measurement of 25-hydroxyvitamin D in the clinical laboratory: current procedures, performance characteristics and limitations

In this review we describe procedures, performance characteristics and limitations of methods available for the measurement of 25-hydroxyvitamin (25OHD) since the year 2000. The two main types of methods are competitive immunoassay and those based on chromatographic separation followed by non-immunological direct detection (HPLC, LC-MS/MS). Lack of a reference standard for 25OHD has, until recently, been a major issue resulting in poor between-method comparability. Fortunately this should soon improve due to the recent introduction of a standard reference material in human serum (SRM 972) from the National Institute of Standards and Technology (NIST). For immunoassay, specificity can be an issue especially in relation to the proportion of 25OHD2 that is quantified whereas HPLC and LC-MS/MS methods are able to measure the two major vitamin D metabolites 25OHD2 and 25OHD3 independently. HPLC and LC-MS/MS require more expensive equipment and expert staff but this can be offset against lower reagent costs. Increasingly procedures are being developed to semi-automate or automate HPLC and LC-MS/MS but run times remain considerably longer than for immunoassays especially if performed on automated platforms. For most HPLC and LC-MS/MS methods extraction and procedural losses are corrected for by the inclusion of an internal standard which, in part, may account for higher results compared to immunoassay. In general precision of immunoassay, HPLC and LC-MS/MS are comparable and all have the required sensitivity to identify severe vitamin D deficiency. Looking to the future it is hoped that the imminent introduction of a standard reference method (or methods) for 25OHD will further accelerate improvements in between method comparability.

The impact of delayed blood centrifuging, choice of collection tube, and type of assay on 25-hydroxyvitamin D concentrations

Studies have examined the associations between cancers and circulating 25-hydroxyvitamin D [25(OH)D], but little is known about the impact of different laboratory practices on 25(OH)D concentrations. We examined the potential impact of delayed blood centrifuging, choice of collection tube, and type of assay on 25(OH)D concentrations. Blood samples from 20 healthy volunteers underwent alternative laboratory procedures: four centrifuging times (2, 24, 72, and 96 h after blood draw); three types of collection tubes (red top serum tube, two different plasma anticoagulant tubes containing heparin or EDTA); and two types of assays (DiaSorin radioimmunoassay [RIA] and chemiluminescence immunoassay [CLIA/LIAISON((R))]). Log-transformed 25(OH)D concentrations were analyzed using the generalized estimating equations (GEE) linear regression models. We found no difference in 25(OH)D concentrations by centrifuging times or type of assay. There was some indication of a difference in 25(OH)D concentrations by tube type in CLIA/LIAISON((R))-assayed samples, with concentrations in heparinized plasma (geometric mean, 16.1 ng ml(-1)) higher than those in serum (geometric mean, 15.3 ng ml(-1)) (p = 0.01), but the difference was significant only after substantial centrifuging delays (96 h). Our study suggests no necessity for requiring immediate processing of blood samples after collection or for the choice of a tube type or assay.

Specifications for trueness and precision of a reference measurement system for serum/plasma 25-hydroxyvitamin D analysis

BACKGROUND

The divergence in analytical quality of serum/plasma 25-hydroxy-vitamin D analysis calls for defining specifications for a reference measurement system.

METHODS

Fundamentally, in a reference measurement system, there should be a relationship between the analytical specifications for higher- (reference) and lower-order (routine) measurements. Therefore, when setting specifications, we started with limits for routine imprecision (CV(rou)) and bias (B(rou)) using 4 models: (1) the misclassifications in diagnosis, (2) biological variation data (reference interval (RI) and monitoring), (3) expert recommendations, and (4) state-of-the-art performance. Then, we used the derived goals to tailor those for reference measurements and certified reference materials (CRMs) for calibration by setting the limits for CV(ref) at 0.5 CV(rou), B(ref) at 0.33 B(rou)(,) max. uncertainty (U(max)) at 0.33 B(ref).

RESULTS

The established specifications ranged between CV(rou)<or=22%, B(rou)<or=10%, CV(ref)<or=11%, B(ref)<or=3.3%, U(max) 1.1% (model 3) and CV(rou)<or=4%, B(rou)<or=2.6%, CV(ref)<or=2%, B(ref)<or=0.9%, U(max) 0.3% (model 2, monitoring).

CONCLUSIONS

Model 2 (monitoring) gave the most stringent goals, model 3, the most liberal ones. Accounting for state-of-the-art performance and certification capabilities, we used model 2 (RI) to recommend achievable goals: for routine testing, CV(rou)<or=10%, B(rou)<or=5%, for reference measurements, CV(ref)<or=5%, B(ref)<or=1.7%, and for CRMs, U(max) 0.6%.

Prevalence and correlates of vitamin D status in African American men

Background

Few studies have examined vitamin D insufficiency in African American men although they are at very high risk. We examined the prevalence and correlates of vitamin D insufficiency among African American men in Philadelphia.

Methods

Participants in this cross-sectional analysis were 194 African American men in the Philadelphia region who were enrolled in a risk assessment program for prostate cancer from 10/96–10/07. All participants completed diet and health history questionnaires and provided plasma samples, which were assessed for 25-hydroxyvitamin D (25(OH)D) concentrations. We used linear regression models to examine associations with 25(OH)D concentrations and logistic regression to estimate odds ratios (OR) for having 25(OH)D ≥ 15 ng/mL.

Results

Mean 25(OH)D was 13.7 ng/mL, and 61% of men were classified as having vitamin D insufficiency (25(OH)D <15 ng/mL). Even among men with vitamin D intake ≥ 400 IU/day, 55% had 25(OH)D concentrations <15 ng/mL. In multivariate models, 25(OH)D concentrations were significantly associated with supplemental vitamin D intake (OR 4.3, 95% confidence interval (CI) 1.5, 12.4) for >400 vs. 0 IU/day), milk consumption (OR 5.9, 95% CI 2.2, 16.0 for ≥ 3.5 vs. <1 time per week), and blood collection in the summer. Additionally, 25(OH)D concentrations increased with more recreational physical activity (OR 1.3, 95% CI 1.1, 1.6 per hour). A significant inverse association of body mass index with 25(OH)D concentrations in bivariate analyses was attenuated with adjustment for season of blood collection.

Conclusion

The problem of low vitamin D status in African American men may be more severe than previously reported. Future efforts to increase vitamin D recommendations and intake, such as through supplementation, are warranted to improve vitamin D status in this particularly vulnerable population.

Analytical validation of the new version of the Liaison N-Tact PTH assay

We performed analytical validation of the new version of the Liaison N-Tact PTH (parathormone) assay according to NCCLS guidelines and compared this new generation of reagent with the Roche Elecsys PTH assay. We showed that this new version is a sensitive and precise method with good recovery. Significant carryover was observed, but with limited clinical implications. We demonstrated that the new version of the Liaison PTH is in reasonably good agreement with the Roche Elecsys PTH assay, and as we observed no differences in a hemodialyzed population, moving from one method to the other should not affect the daily follow-up of patients. However, one should be cautious with the highest values (>500 pg/mL). We established reference intervals of 12-54 pg/mL for the Liaison and 14-52 pg/mL for the Elecsys assay.

Nouvelles définitions de l'insuffisance vitaminique D, retentissement sur les normes de PTH

SCOPE

Knowledge concerning vitamin D has greatly improved during the past few years. Vitamin D can no longer be considered only as a preventive therapy for rickets-osteomalacia. Indeed, beside its role in the prevention of osteoporotic fractures in the elderly, many data suggest that it may be involved in the prevention of various diseases including cancers and auto-immune diseases.

CURRENT SITUATION AND SALIENT POINTS

Vitamin D status may be easily assessed by the measurement of 25OHD serum concentration. However, many specialists in the field regard most 25OHD reference values as being too low, and believe that the 25OHD threshold below which vitamin D status can be considered as insufficient is somewhere between 50 and 100 nmol/L (20 to 40 ng/mL). It then appears that usually recommended amounts of supplemental vitamin D may be too low to reach these 25OHD concentrations, and thus need to be revised. We have proposed that PTH reference values should be established in healthy subjects with a normal vitamin D status. This supposes that 25OHD is measured in the reference population beforehand, and that the subjects with vitamin D insufficiency are eliminated from the reference group.

PERSPECTIVES

Although more complicated than the usual way to establish normative data, we have shown that it decreases the upper limit of normal by 25-35%, enhancing thus the diagnostic sensitivity for hyperparathyroidism without a decrease in specificity.

Practical considerations in PTH testing

New knowledge concerning PTH biology have accumulated during the past few years. The finding that the so-called "intact" PTH assays measure a "non-1-84" PTH fragment in addition to full-length PTH has led to the development of new assays. These new assays, which were initially thought to measure 1-84 PTH only, have been shown to recognize also another PTH species called "amino-PTH". As the various names given to the different assay methods are highly confusing, there is a need for a simplified nomenclature. A simple way would be to identify the older "intact" PTH assays as second-generation assays and the new assays (Whole, CAP, BioIntact) as third-generation assays. Although of considerable potential interest for the comprehension of PTH physiology, the third-generation PTH assays have not yet proved to be superior to the second-generation assays in clinical practice. There is thus currently no recommendation to switch from the second-generation to the third-generation assays in clinical practice, or to use a ratio derived from the concommitent measurement of PTH with both assay-generation. Because second- and third-generation PTH assays are usually highly correlated, significant differences in the clinical information provided by these methods are unlikely. However, our opinion is that more definitive bone biopsy studies in dialyzed patients selected according to their bone- and calcium-related treatment are still needed to reach a consensus. Finally, we have proposed that PTH reference values should be established in healthy subjects with a normal vitamin D status. This supposes that 25OHD is measured in the reference population beforehand, and that the subjects with vitamin D insufficiency are eliminated from the reference group. Although more complicated than the usual way to establish normative data, we have shown that it decreases the upper limit of normal by 25-35%, enhancing thus the diagnostic sensitivity for hyperparathyroidism without a decrease in specificity.

HPLC method for 25-hydroxyvitamin D measurement: comparison with contemporary assays

BACKGROUND

The concentration of 25-hydroxyvitamin D [25(OH)D] in serum has been designated the functional indicator of vitamin D (VitD) nutritional status. Unfortunately, variability among 25(OH)D assays limits clinician ability to monitor VitD status, supplementation, and toxicity.

METHODS

We developed an HPLC method that selectively measures 25-hydroxyvitamin D2 [25(OH)D2] and D3 [25(OH)D3] and compared this assay with a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method, a competitive protein-binding assay (CPBA) on the Nichols Advantage platform, and an RIA from Diasorin.

RESULTS

For the new HPLC assay, between-run CVs were 2.6%-4.9% for 25(OH)D3 and 3.2%-13% for 25(OH)D2; recoveries were 95%-102%; and the assay was linear from 5 microg/L to at least 200 microg/L. Comparison data were as follows: for HPLC vs LC-MS/MS, y = 1.01x - 4.82 microg/L (Sy/x = 4.93 microg/L; r = 0.996) for 25(OH)D3, and y = 0.902x - 0.566 microg/L (Sy/x = 2.56 microg/L; r = 0.9965 for 25(OH)D2; for HPLC vs Diasorin RIA, y = 0.709x - 5.86 microg/L (Sy/x = 7.35 microg/L; r = 0.7509); and for HPLC vs Nichols Advantage CPBA, y = 1.00x - 3.60 microg/L (Sy/x = 32.7 microg/L; r = 0.6823).

CONCLUSIONS

The new HPLC method is reliable, robust, and has advantages compared with the Nichols Advantage CPBA and the Diasorin RIA. The Nichols Advantage CPBA overestimated or underestimated 25(OH)D concentrations predicated on the prevailing metabolite present in patients' sera.