Issues in Immunoassay Standardization: The ARCHITECT Folate Model for Intermethod Harmonization

Comparison of four different immunoassays and a rapid isotope-dilution liquid chromatography-tandem mass spectrometry assay for serum folate

OBJECTIVES

Accurate measurement of serum folate is essential for the diagnosis and management of various disorders. This study aims to investigate the between-method differences of four immunoassays and a rapid isotope-dilution liquid chromatography-tandem mass spectrometry (ID-LC-MS/MS) method.

METHODS

Roche Cobas (USA), Abbott Alinity i2000 (USA), Beckman Coulter Access (USA), Mindray CL-6000i (China), and the ID-LC-MS/MS method were compared using 46 human serum samples. The results were analysed by Passing-Bablok regressions and Bland-Altman plots. A bias of 13.31% based on biological variation was used as the bias criterion.

RESULTS

All the within-run and total coefficients of variation (CVs) met the specification. The folate concentrations determined by all the assays were significantly different (p=0.0028). All assays had correlation coefficients over 0.97 with each other. The 95% confidence intervals (CIs) for the slope seldom contained 1 and few 95% CIs for the intercept contained 0 in the regression equations. Compared to ID-LC-MS/MS, the biases of all assays ranged from -20.91 to 13.56 nmol/L, and the mean relative biases ranged from -9.85 to 40.33%. The predicted mean relative biases at the medical decision levels rarely met the criterion.

CONCLUSIONS

Assays for serum folate had good correlations with each other but lacked good agreement. The accuracy and consistency of assays for serum folate should be measured and assessed routinely. Standardization work to improve the accuracy of serum folate assays, such as the extension of traceability to reference methods or materials, calibration standardization efforts, and assay-adjusted cut-offs should be promoted.

A computational solution to improve biomarker reproducibility during long-term projects

Biomarkers are fundamental to basic and clinical research outcomes by reporting host responses and providing insight into disease pathophysiology. Measuring biomarkers with research-use ELISA kits is universal, yet lack of kit standardization and unexpected lot-to-lot variability presents analytic challenges for long-term projects. During an ongoing two-year project measuring plasma biomarkers in cancer patients, control concentrations for one biomarker (PF) decreased significantly after changes in ELISA kit lots. A comprehensive operations review pointed to standard curve shifts with the new kits, an analytic variable that jeopardized data already collected on hundreds of patient samples. After excluding other reasonable contributors to data variability, a computational solution was developed to provide a uniform platform for data analysis across multiple ELISA kit lots. The solution (ELISAtools) was developed within open-access R software in which variability between kits is treated as a batch effect. A defined best-fit Reference standard curve is modelled, a unique Shift factor “S” is calculated for every standard curve and data adjusted accordingly. The averaged S factors for PF ELISA kit lots #1–5 ranged from -0.086 to 0.735, and reduced control inter-assay variability from 62.4% to <9%, within quality control limits. S factors calculated for four other biomarkers provided a quantitative metric to monitor ELISAs over the 10 month study period for quality control purposes. Reproducible biomarker measurements are essential, particularly for long-term projects with valuable patient samples. Use of research-use ELISA kits is ubiquitous and judicious use of this computational solution maximizes biomarker reproducibility.

Simultaneous assessment of iodine, iron, vitamin A, malarial antigenemia, and inflammation status biomarkers via a multiplex immunoassay method on a population of pregnant women from Niger

Deficiencies of vitamin A, iron, and iodine are major public health concerns in many low- and middle-income countries, but information on their status in populations is often lacking due to high costs and logistical challenges associated with assessing micronutrient status. Accurate, user-friendly, and low-cost analytical tools are needed to allow large-scale population surveys on micronutrient status. We present the expansion of a 7-plex protein microarray tool for the simultaneous measurement of up to seven biomarkers with relevance to the assessment of the key micronutrients iron, iodine, and vitamin A, and inflammation and malaria biomarkers: α-1-acid glycoprotein, C-reactive protein, ferritin, retinol binding protein 4, soluble transferrin receptor, thyroglobulin, and histidine-rich protein II. Assay performance was assessed using international reference standards and then verified by comparing the multiplexed and conventional immunoassay results on a training panel of plasma samples collected from US adults. These data were used to assign nominal concentrations to the calibrators of the assay to further improve performance which was then assessed by interrogating plasma samples from a cohort of pregnant women from Niger. The correlation between assays for each biomarker measured from this cohort was typically good, with the exception of thyroglobulin, and the sensitivity ranged from 74% to 93%, and specificity from 81% to 98%. The 7-Plex micronutrient assay has the potential for use as an affordable tool for population surveillance of vitamin A, iron, and iodine deficiencies as well as falciparum malarial parasitemia infectivity and inflammation. The assay is easy-to-use, requires minimal sample volume, and is scalable, rapid, and accurate—needing only a low-cost reader and basic equipment present in most reference laboratory settings and so may be employed by low and middle income countries for micronutrient surveillance to inform on status in key populations. Micronutrient deficiencies including iron, iodine, and vitamin A affect a significant portion of the world’s population. Efforts to assess the prevalence of these deficiencies in vulnerable populations are challenging, partly due to measurement tools that are inadequate for assessing multiple micronutrients in large-scale population surveys. We have developed a 7-plex immunoassay for the simultaneous measurement of seven biomarkers relevant to assessing iodine, iron, and vitamin A status, inflammation and Plasmodium falciparum parasitemia by measuring levels of thyroglobulin, ferritin, soluble transferrin receptor, retinol binding protein 4, α-1-acid glycoprotein, C-reactive protein, and histidine-rich protein II. This 7-plex immunoassay technique has potential as a rapid and effective tool for use in large-scale surveys and assessments of nutrition intervention programs in low- and middle-income countries.

Tackling serum folate test in European countries within the health technology assessment paradigm: request appropriateness, assays and health outcomes

Several authors have recently claimed an excess in serum folate test ordering, suggesting phasing out it from clinical use. According to studies performed in countries undergoing folic acid fortification policies, it is indeed no more cost-effective to test folate in the face of deficiency prevalence <1%. In this paper, we sought to evaluate request appropriateness, analytical issues, and cost-effectiveness of serum folate determination for clinical purposes in the European context, considering if evidence retrieved in fortified countries may be generalized. Studies performed in non-fortified countries have generally reported a suboptimal folate intake and suggest a remarkable prevalence of folate deficiency. Our internal data suggest that ~20%–25% of the subjects undergoing serum folate test are at risk for deficiency. However, a reliable evaluation of the risk for deficiency implies the knowledge of all issues related to the total testing process of folate measurement as well as the identification of the appropriate population in which to perform the test. The cost-effectiveness of the test is maximized when the request is oriented to subjects suggestive/at risk for deficiency, becoming low if the test is used as a screening tool or for monitoring of vitamin intake/supplementation. Because the individual folate status has a key role in ensuring normal development, physiologic growth, and maintenance of optimal health, the evaluation of its serum levels has to be retained in the clinical use in non-fortified countries, boosting for more appropriate request, and evidence from countries following fortification policies should be cautionary interpreted.

Biomarkers of Nutrition for Development-Folate Review

The Biomarkers of Nutrition for Development (BOND) project is designed to provide evidence-based advice to anyone with an interest in the role of nutrition in health. Specifically, the BOND program provides state-of-the-art information and service with regard to selection, use, and interpretation of biomarkers of nutrient exposure, status, function, and effect. To accomplish this objective, expert panels are recruited to evaluate the literature and to draft comprehensive reports on the current state of the art with regard to specific nutrient biology and available biomarkers for assessing nutrients in body tissues at the individual and population level. Phase I of the BOND project includes the evaluation of biomarkers for 6 nutrients: iodine, iron, zinc, folate, vitamin A, and vitamin B-12. This review represents the second in the series of reviews and covers all relevant aspects of folate biology and biomarkers. The article is organized to provide the reader with a full appreciation of folate's history as a public health issue, its biology, and an overview of available biomarkers (serum folate, RBC folate, and plasma homocysteine concentrations) and their interpretation across a range of clinical and population-based uses. The article also includes a list of priority research needs for advancing the area of folate biomarkers related to nutritional health status and development.

From prediction to function using evolutionary genomics: human-specific ecotypes of Lactobacillus reuteri have diverse probiotic functions

The vertebrate gut symbiont Lactobacillus reuteri has diversified into separate clades reflecting host origin. Strains show evidence of host adaptation, but how host–microbe coevolution influences microbial-derived effects on hosts is poorly understood. Emphasizing human-derived strains of L. reuteri, we combined comparative genomic analyses with functional assays to examine variations in host interaction among genetically distinct ecotypes. Within clade II or VI, the genomes of human-derived L. reuteri strains are highly conserved in gene content and at the nucleotide level. Nevertheless, they share only 70–90% of total gene content, indicating differences in functional capacity. Human-associated lineages are distinguished by genes related to bacteriophages, vitamin biosynthesis, antimicrobial production, and immunomodulation. Differential production of reuterin, histamine, and folate by 23 clade II and VI strains was demonstrated. These strains also differed with respect to their ability to modulate human cytokine production (tumor necrosis factor, monocyte chemoattractant protein-1, interleukin [IL]-1β, IL-5, IL-7, IL-12, and IL-13) by myeloid cells. Microarray analysis of representative clade II and clade VI strains revealed global regulation of genes within the reuterin, vitamin B₁₂, folate, and arginine catabolism gene clusters by the AraC family transcriptional regulator, PocR. Thus, human-derived L. reuteri clade II and VI strains are genetically distinct and their differences affect their functional repertoires and probiotic features. These findings highlight the biological impact of microbe:host coevolution and illustrate the functional significance of subspecies differences in the human microbiome. Consideration of host origin and functional differences at the subspecies level may have major impacts on probiotic strain selection and considerations of microbial ecology in mammalian species.

Isotope Dilution–LC-MS/MS Reference Method Assessment of Serum Folate Assay Accuracy and Proficiency Testing Consensus Mean

BACKGROUND

Current methods for measuring folates in clinical laboratories are competitive folate binding protein assays. These assays show a considerable lack of agreement that has implications for the comparability of data across studies as well as for long-term population studies. The development of isotope dilution–liquid chromatography–tandem mass spectrometry (ID-LC-MS/MS) reference methods permitted the evaluation of method accuracy and consistency over time.

METHODS

We measured 3 pools of human serum by ID-LC-MS/MS, calculated values for total folate, and distributed the same pools to participants in a national External Quality Assessment scheme. We used linear regression to compare the all-laboratory and method data with reference method values. The exercise was repeated after 18 months to assess the stability of the all-laboratory and method data.

RESULTS

The distributed serum pools had mass spectrometry values for folate species typical of those found in healthy individuals from populations not receiving dietary folic acid fortification. There was good agreement of the all-laboratory data set with the reference method (y =0.86x + 0.91 μg/L) at both time points. Linear regression demonstrated that the Abbott Architect showed the closest agreement with the reference method. The Roche Elecsys method was nonlinear and showed a calibration offset of 2.6 μg/L (4.57 nmol/L).

CONCLUSIONS

Calibration of serum folate assays traceable to higher-order reference methods increases method accuracy and improves consistency. The all-laboratory consensus mean proved sufficiently accurate and stable to be used as the target for monitoring laboratory performance.

Assessment and interpretation of micronutrient status during pregnancy

Accurate assessment of maternal micronutrient status is critical to the prevention of suboptimal micronutrient status and anaemia during pregnancy. Measurement of Fe, folate and vitamin B12 status is complicated by adaptive changes to maternal and placental physiology that markedly affect concentrations of circulating micronutrients and their functional biomarkers. Validation of new assessment methods by comparison with gold standards is often prevented by ethical considerations. Antenatal screening in the UK is predominantly concerned with the detection of anaemia, although estimation of maternal Fe stores by serum ferritin at the start of antenatal care may be a more effective preventive strategy. Functional assessment of maternal anaemia is highly problematic, so instead reference data are used for its definition. The effect of mild-to-moderate anaemia on pregnancy outcome is unclear because of the crude nature of its assessment and the influence of confounding factors. Fe-deficient erythropoiesis may be detected by assessment of erythrocyte Zn protoporphyrin and reticulocyte Hb, although such measures may be unavailable in many clinical laboratories. Serum soluble transferrin receptor is highly responsive to tissue Fe deficiency and is less affected by inflammation than most other indicators. Direct inter-assay comparison of serum and erythrocyte folate values is inadvisable since recovery rates differ greatly between methods. Serum total homocysteine is a useful functional biomarker of both folate and vitamin B12 status but during pregnancy is influenced by other factors that reduce its sensitivity. Isotope-dilution liquid chromatography-tandem MS and serum holo-transcobalamin provide new opportunities to gain detailed data of folate species and vitamin B12 fractions in large samples.