Evaluation of the necessity and the feasibility of the standardization of procalcitonin measurements: Activities of IFCC WG-PCT with involvement of all stakeholders

Discrepancies between two total IgE assays and difference in reference intervals in healthy adults

OBJECTIVE

To compare total immunoglobulin (Ig) E assay performance characteristics between Abbott Architect and Siemens Immulite test systems. Reference intervals were also determined for both platforms in an American population of healthy adults.

METHODS

Agreement of the two total IgE assays was evaluated in a cohort of 331 subjects with normal complete blood count (CBC) and comprehensive metabolic panel (CMP) results. Reference intervals were established in 302 subjects after exclusion of atopic individuals on the Abbott Architect and Siemens Immulite test systems.

RESULTS

We demonstrated a 32% positive bias for total IgE quantitation on the Siemens Immulite platform compared to the Abbott Architect, despite both methods calibrated against the same WHO international reference material (75/502), Furthermore, the upper limit of the reference interval (95th percentile) was determined to be higher for the Siemens Immulite assay compared to the Abbott Architect (132 and 102 IU/mL, respectively).

CONCLUSION

Despite the use of a common WHO reference material for total IgE assay calibration, significant differences in quantitation was observed between two FDA-cleared test systems. Given that, it is warranted for clinical laboratories to verify vendor established reference intervals and adjust accordingly based on internal assessment of the normal range.

Diagnostic accuracy of procalcitonin for bacterial infection in the Emergency Department: a systematic review

INTRODUCTION AND OBJECTIVE

The care of patients with a suspected infectious process in hospital emergency departments (ED) accounts for 15%-35% of all daily care in these healthcare areas in Spain and Latin America. The early and adequate administration of antibiotic treatment (AB) and the immediate making of other diagnostic-therapeutic decisions have a direct impact on the survival of patients with severe bacterial infection. The main objective of this systematic review is to investigate the diagnostic accuracy of PCT to predict bacterial infection in adult patients treated with clinical suspicion of infection in the ED, as well as to analyze whether the different studies manage to identify a specific value of PCT as the most relevant from the diagnostic point of view of clinical decision that can be recommended for decision making in ED.

METHOD

A systematic review is carried out following the PRISMA regulations in the database of PubMed, Web of Science, EMBASE, Lilacs, Cochrane, Epistemonikos, Tripdatabase and ClinicalTrials.gov from January 2005 to May 31, 2023 without language restriction and using a combination of MESH terms: "Procalcitonin", "Infection/Bacterial Infection/Sepsis", "Emergencies/Emergency/Emergency Department", "Adults" and "Diagnostic". Observational cohort studies (diagnostic performance analyses) were included. The Newcastle-Ottawa Scale (NOS) was used to assess the quality of the method used and the risk of bias of the included articles. Observational cohort studies were included. No meta-analysis techniques were performed, but results were compared narratively.

RESULTS

A total of 1,323 articles were identified, of which 21 that met the inclusion criteria were finally analyzed. The studies include 10,333 patients with 4,856 bacterial infections (47%). Eight studies were rated as high, 9 as moderate, and 4 as low. The AUC-ROC of all studies ranges from 0.68 (95% CI: 0.61-0.72) to 0.99 (95% CI: 0.98-1). The value of PCT 0.2-0.3 ng/ml is the most used and proposed in up to twelve of the works included in this review whose average estimated performance is an AUC-ROC of 0.79. If only the results of the 5 high-quality studies using a cut-off point of 0.2-0.3 ng/ml PCT are taken into account, the estimated mean AUC-COR result is 0.78 with Se:69 % and Es:76%.

CONCLUSIONS

PCT has considerable diagnostic accuracy for bacterial infection in patients treated in ED for different infectious processes. The cut-off point of 0.25 (0.2-0.3) ng/ml has been positioned as the most appropriate to predict the existence of bacterial infection and can be used to help reasonably rule it out.

Efficient expression and purification of tag-free recombinant human procalcitonin (hPCT) with precise sequence in E.coli

We present an efficient method for expression and purification of recombinant human procalcitonin (hPCT) in E. coli T7 express LysY/Iq cells, ensuring precise N- and C-terminal amino acid sequences. Our method involves fusing codon-optimized cDNA with two distinct tag sequences: eXact tag and chitin binding domain (CBD) tag. To purify the protein, we employ a two-step affinity chromatography process. Firstly, we utilize the N-terminal Profinity eXact tag and purify the protein through Profinity eXact-affinity column chromatography using a resin on which a mutant subtilisin protease was immobilized. The eXact tag was removed by adding NaF to activate the enzyme. Subsequently, the digested sample containing C-terminal CBD tag is directly loaded for the second step of chitin affinity chromatography. Elution is achieved through dithiothreitol (DTT)-catalyzed self-cleavage of the intein sequence from the fusion protein. As a result, the target protein is selectively recovered in the flow-through, completely tag-free, with a purity exceeding 95%. To ensure high purity and eliminate potential contaminants, we effectively remove E. coli host DNA and endotoxins through a combination of streptomycin sulfate, Triton X-114, and ammonium sulfate treatment. The exceptional level of purity obtained eliminates the need for further purification steps in most applications. This highly purified hPCT can be used as a calibrator in procalcitonin or calcitonin immunoassays. Notably, our approach effectively manages small peptides that are prone to degradation by E. coli host proteases, offering a robust solution for various research and application requirements.

Establishing metrological traceability for small molecule measurands in laboratory medicine

For molecules that can be well described metrologically in the sense of the definition of measurands, and which can also be recorded analytically as individual substances, reference measurement service traceability to a metrologically sound foundation is a necessity. The establishment of traceability chains must be initiated by National Metrology Institutes (NMIs) according to applicable standards; they are at the top and leading position in this concept. If NMIs are not in the position to take up this task, alternative approaches must be sought. Traceability initiatives established by in vitro device industry or academia must meet the quality standards of NMIs. Adherence to International Organization for Standardization (ISO) procedure 15193 must be a matter of course for the establishment of reference measurement procedures (RMPs). Certified reference material (CRM) characterization must be thorough, e.g., by the application of quantitative nuclear magnetic resonance measurements and by adherence to ISO 15194. Both for RMPs and CRMs Joint Committee for Traceability in Laboratory Medicine (JCTLM) listing must be the ultimate goal. Results must be shared in a transparent manner to allow other stakeholders including NMIs to reproduce and disseminate the reference measurement procedures.

Immunoaffinity LC-MS/MS Quantification of the Sepsis Biomarker Procalcitonin Using Magnetic- and Polystyrene-Bead Immobilized Polyclonal Antibodies

Procalcitonin (PCT) is a biomarker for bacterial sepsis, and accurate quantification of PCT is critical for sepsis diagnosis and treatment. Immunological PCT quantification methods are routinely used in clinical laboratories, yet there is a need for harmonization of PCT quantification protocols. An orthogonal method to clinical immunological assays, such as LC-MS/MS, is required. In this study, a highly sensitive and robust immunoaffinity LC-MRM quantitative method for detecting procalcitonin in human serum has been developed. An initial comparison of immunocapture of PCT with a polyclonal anti-PCT antibody immobilized on polystyrene nanoparticles (Latex) and magnetic beads demonstrated superior performance with magnetic beads. Three tryptic PCT peptides from the N- and C-terminal regions of PCT were selected for LC-MS/MS quantification. For PCT quantification, an LLOQ of 0.25 ng/mL of PCT in human serum was achieved using a sample volume of 1 mL. The method's trueness and precision consistently lie within the 15% margin. The parallel measurement of three PCT peptides may allow future differentiation of intact PCT vs other PCT forms originating from potential degradation, processing, or polymorphisms. An established and validated LC-MRM-based quantification of PCT will be relevant as an orthogonal method for harmonization and standardization of clinical assays for PCT.

Commutability evaluation of candidate reference materials and ERM-DA470k/IFCC for immunoglobulin M using two international approaches

BACKGROUND

This study aimed to assess the commutability of frozen pooled human serum (PHS), high concentration of Immunoglobulin M (IgM) pure diluted materials (HPDM), commercialized pure materials (CPM), and dilutions of ERM-DA470k/IFCC in IgM detection using the CLSI and IFCC approaches, to support standardization or harmonization of IgM measurement.

METHODS

Twenty-four serum samples, relevant reference materials (PHS, HPDM, CPM), and different ERM-DA470k/IFCC dilutions were analyzed in triplicate using six routine methods. The commutability of the relevant reference materials was carried out following CLSI EP30-A and IFCC bias analysis.

RESULTS

According to the CLSI approach, low, medium, and high concentrations of PHS, HPDM, and CPM were commutable on 10, 13, 15, 13, and 8 of 15 assay combinations, respectively. Using the IFCC approach, low, medium, and high concentrations of PHS, HPDM, and CPM were commutable on 10, 11, 9, 15, and 10 of 15 assay combinations, respectively. The ERM-DA470k/IFCC dilutions with D-PBS and RPMI-1640 Medium were commutable on 13 of 15 assay combinations according to CLSI and were commutable on all 15 assay combinations using IFCC approach.

CONCLUSIONS

High concentration of PHS were commutable on all six detection systems using the CLSI approach. Low and medium concentration of PHS showed unsatisfied commutability. HPDM, not CPM have good commutability, has the potential to become reference materials. ERM-DA470k/IFCC diluted with different medium showed different commutability.

Comprehensive Comparison of the Capacity of Functionalized Sepharose, Magnetic Core, and Polystyrene Nanoparticles to Immuno-Precipitate Procalcitonin from Human Material for the Subsequent Quantification by LC-MS/MS

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection. The fast and accurate diagnosis of sepsis by procalcitonin (PCT) has emerged as an essential tool in clinical medicine. Although in use in the clinical laboratory for a long time, PCT quantification has not yet been standardized. The International Federation of Clinical Chemistry working group on the standardization of PCT (IFCC-WG PCT) aims to provide an LC-MS/MS-based reference method as well as the highest metrological order reference material to address this diagnostic need. Here, we present the systematic evaluation of the efficiency of an immuno-enrichment method, based on functionalized Sepharose, magnetic-core, or polystyrene (latex) nano-particles, to quantitatively precipitate PCT from different human sample materials. This method may be utilized for both mass spectrometric and proteomic purposes. In summary, only magnetic-core nano-particles functionalized by polyclonal PCT antibodies can fulfil the necessary requirements of the international standardization of PCT. An optimized method proved significant benefits in quantitative and specific precipitation as well as in the subsequent LC-MS/MS detection of PCT in human serum samples or HeLa cell extract. Based on this finding, further attempts of the PCT standardization process will utilize a magnetic core-derived immuno-enrichment step, combined with subsequent quantitative LC-MS/MS detection.

AACC Guidance Document on the Clinical Use of Procalcitonin

BACKGROUND

Procalcitonin (PCT), a peptide precursor of the hormone calcitonin, is a biomarker whose serum concentrations are elevated in response to systemic inflammation caused by bacterial infection and sepsis. Clinical adoption of PCT in the United States has only recently gained traction with an increasing number of Food and Drug Administration-approved assays and expanded indications for use. There is interest in the use of PCT as an outcomes predictor as well as an antibiotic stewardship tool. However, PCT has limitations in specificity, and conclusions surrounding its utility have been mixed. Further, there is a lack of consensus regarding appropriate timing of measurements and interpretation of results. There is also a lack of method harmonization for PCT assays, and questions remain regarding whether the same clinical decision points may be used across different methods.

CONTENT

This guidance document aims to address key questions related to the use of PCT to manage adult, pediatric, and neonatal patients with suspected sepsis and/or bacterial infections, particularly respiratory infections. The document explores the evidence for PCT utility for antimicrobial therapy decisions and outcomes prediction. Additionally, the document discusses analytical and preanalytical considerations for PCT analysis and confounding factors that may affect the interpretation of PCT results.

SUMMARY

While PCT has been studied widely in various clinical settings, there is considerable variability in study designs and study populations. Evidence to support the use of PCT to guide antibiotic cessation is compelling in the critically ill and in some lower respiratory tract infections but is lacking in other clinical scenarios, and evidence is also limited in the pediatric and neonatal populations. Interpretation of PCT results requires guidance from multidisciplinary care teams of clinicians, pharmacists, and clinical laboratorians.

Comparison of a novel automated DiaSys procalcitonin immunoassay with four different BRAHMS-partnered immunoassays

Objectives

Procalcitonin (PCT) is an important biomarker of sepsis and respiratory infections. Various automated immunoassays for measuring PCT in patient plasma are available in medical laboratories. However, due to a lack of international reference material for PCT, the assays are not always comparable.

Design and methods

In this study, we compared a new turbidimetric immunoassay from DiaSys, measured on the Abbott Architect c16000 and Alinity c, with four BRAHMS-associated chemiluminescence immunoassays (Abbott Architect i2000SR, Alinity i, Roche Cobas e411 and DiaSorin Liaison XL) using 120 random patient plasma samples from the clinical laboratory routine at the University Medical Center Goettingen.

Results

The DiaSys assay showed clear differences as compared to the BRAHMS-associated assays when measured on Architect c: i.e. 58% positive mean bias vs. Architect i, 67% vs. Cobas and 23% vs. Liaison. As a result, additional 19% our patients would have a suspected bacterial infection, when using PCT values from the DiaSys assay and commonly accepted decision limits. A crosscheck of the DiaSys calibrator on the BRAHMS-associated systems showed a low recovery of the calibrator material (approx. 50%).

Conclusions

Overall, this study shows significant differences between the DiaSys and BRAHMS-associated assays. This could be attributed to a potential DiaSys calibrator problem. This highlights the need for an international reference material for harmonization of the PCT assays.

Candidate High-Resolution Mass Spectrometry-Based Reference Method for the Quantification of Procalcitonin in Human Serum Using a Characterized Recombinant Protein as a Primary Calibrator

Procalcitonin (PCT) is a widely used biomarker for rapid sepsis diagnosis and antibiotic stewardship. Variability of results in commercial assays has highlighted the need for standardization of PCT measurements. An antibody-free candidate reference measurement procedure (RMP) based on the isotope dilution mass spectrometry and protein calibration approach was developed and validated to quantify PCT in human serum. The method allows quantification of PCT from 0.25 to 13.74 μg/L (R > 0.998) with extension up to 132 μg/L after dilution of samples with PCT concentration above 13.74 μg/L. Intraday bias was between −3.3 and +5.7%, and interday bias was between −3.0 and −0.7%. Intraday precision was below 5.1%, and interday precision was below 4.0%. The candidate RMP was successfully applied to the absolute quantification of PCT in five frozen human serum pools. A recombinant PCT used as a primary calibrator was characterized by high-resolution mass spectrometry and amino acid analysis to establish traceability of the results to the SI units. This candidate RMP is fit to assign target values to secondary certified reference materials (CRMs) for further use in external quality assessment schemes to monitor the accuracy and comparability of the commercially available immunoassay results and to confirm the need for improving the harmonization of PCT assays. The candidate RMP will also be used to evaluate whether the correlation between the candidate RMP and immunoassays is sufficiently high. Overall, this candidate RMP will support reliable sepsis diagnosis and guide treatment decisions, patient monitoring, and outcomes.

Rapid FRET-based homogeneous immunoassay of procalcitonin using matched carbon dots labels

A novel method for the detection of procalcitonin in a homogeneous system by matched carbon dots (CDs) labeled immunoprobes was proposed based on the principle of FRET and double antibody sandwich method. Blue-emitting carbon dots with a strong fluorescence emission range of 400-550 nm and red-emitting carbon dots with the best excitation range of 410-550 nm were prepared before they reacted with procalcitonin protoclone antibody pairs to form immunoprobes. According to the principles of FRET, blue-emitting carbon dots were selected as the energy donor and red-emitting carbon dots as the energy receptor. The external light source excitation (310 nm) could only cause weak luminescence of CDs. However, once procalcitonin was added, procalcitonin and antibodies would be combined with each other quickly (≤20 min). Here, blue-emitting carbon dots acquired energy could be transferred to red-emitting carbon dots efficiently, causing the emitted fluorescence enhancement of red-emitting carbon dots. The fluorescence detection results in PBS buffer solution and diluted rabbit blood serum showed that the fluorescence intensity variation was linear with the concentration of procalcitonin. There was a good linear relationship betweenF/F0 and procalcitonin concentrations in PBS buffer solution that ranged from 0 to 100 ng ml^-1, and the linear equation wasF/F0 = 0.004 *C_pct + 0.98359. Detection in the diluted rabbit serum led to the results that were linear in two concentration ranges, including 0-40 ng ml^-1and 40-100 ng ml^-1, and the detection limit based on 3σK^-1was 0.52 ng ml^-1. It is likely that this matched CDs labeled immunoprobes system can provide a new mode for rapid homogeneous detection of disease markers.

Harmonization status of procalcitonin measurements: what do comparison studies and EQA schemes tell us?

Sepsis represents a global health priority because of its high mortality and morbidity. The key to improving prognosis remains an early diagnosis to initiate appropriate antibiotic treatment. Procalcitonin (PCT) is a recognized biomarker for the early indication of bacterial infections and a valuable tool to guide and individualize antibiotic treatment. To meet the increasing demand for PCT testing, numerous PCT immunoassays have been developed and commercialized, but results have been questioned. Many comparison studies have been carried out to evaluate analytical performance and comparability of results provided by the different commercially available immunoassays for PCT, but results are conflicting. External Quality Assessment Schemes (EQAS) for PCT constitute another way to evaluate results comparability. However, when making this comparison, it must be taken into account that the variety of EQA materials consist of different matrices, the commutability of which has not yet been investigated. The present study gathers results from all published comparison studies and results from 137 EQAS surveys to describe the current state-of-the-art harmonization of PCT results. Comparison studies globally highlight a significant variability of measurement results that nonetheless seem to have a moderate impact on medical decision-making. For their part, EQAS for PCT provides highly discrepant estimates of the interlaboratory CV. Due to differences in commutability of the EQA materials, the results from different peer groups could not be compared. To improve the informative value of the EQA data, the existing limitations such as non-harmonized conditions and suboptimal and/or unknown commutability of the EQA materials have to be overcome. The study highlights the need for commutable reference materials that could be used to properly evaluate result comparability and possibly standardize calibration, if necessary. Such an initiative would further improve the safe use of PCT in clinical routine.

Development of an antibody-free ID-LC MS method for the quantification of procalcitonin in human serum at sub-microgram per liter level using a peptide-based calibration

The quantification of low abundant proteins in complex matrices by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) remains challenging. A measurement procedure based on optimized antibody-free sample preparation and isotope dilution coupled to LC-MS/MS was developed to quantify procalcitonin (PCT) in human serum at sub-microgram per liter level. A combination of sodium deoxycholate-assisted protein precipitation with acetonitrile, solid-phase extraction, and trypsin digestion assisted with Tween-20 enhanced the method sensitivity. Linearity was established through peptide-based calibration curves in the serum matrix (0.092-5.222 μg/L of PCT) with a good linear fit (R2 ≥ 0.999). Quality control materials spiked with known amounts of protein-based standards were used to evaluate the method's accuracy. The bias ranged from -2.6 to +4.3%, and the intra-day and inter-day coefficients of variations (CVs) were below 2.2% for peptide-based quality controls. A well-characterized correction factor was determined and applied to compensate for digestion incompleteness and material loss before the internal standards spike. Results with metrological traceability to the SI units were established using standard peptide of well-characterized purity determined by peptide impurity corrected amino acid analysis. The validated method enables accurate quantification of PCT in human serum at a limit of quantification down to 0.245 μg/L (bias -1.9%, precision 9.1%). The method was successfully applied to serum samples obtained from patients with sepsis. Interestingly, the PCT concentration reported implementing the isotope dilution LC-MS/MS method was twofold lower than the concentration provided by an immunoassay.