Volume 61, Issue 9

C-reactive protein (CRP) is an acute-phase protein which is synthesized by the liver in response to the secretion of several inflammatory cytokines including interleukin 6 (IL-6), IL-1 and tumor necrosis factor (TNF). CRP was the first acute-phase protein to be described and adopted in clinical laboratories as an exquisitely sensitive systemic marker of inflammation and tissue damage. The measurement of CRP is widely used for the diagnosis and monitoring of inflammatory conditions, including sepsis, trauma, and malignancies. In the last decades, impressive advances in analytical methods (from qualitative to high-sensitivity assays), automation and availability of results in a short time, not only translated in an increasing demand for the right management of systemic inflammatory diseases, but also in evaluating subclinical inflammatory processes underlying atherothrombotic events. CRP measurement is one of the most requested laboratory tests for both the wide range of clinical conditions in which it may assure a valuable information and some analytical advantages due to the evidence that it is a “robust biomarker”. Even recently, the measurement of CRP received new interest, particularly as a biomarker of severity of Coronavirus disease 2019 (COVID-19), and it deserves further concern for improving demand appropriateness and result interpretation.

C-reactive protein (CRP) is a cytokine-mediated acute phase reactant with a recognized role in inflammatory conditions and infectious disease. In coronavirus disease 2019 (COVID-19), elevated CRP concentrations in serum were frequently detected and significantly associated with poor outcome in terms of disease severity, need for intensive care, and in-hospital death. For these reasons, the marker was proposed as a powerful test for prognostic classification of COVID-19 patients. In most of available publications, there was however confounding information about how interpretative criteria for CRP in COVID-19 should be derived, including quality of employed assays and optimal cut-off definition. Assuring result harmonization and controlling measurement uncertainty in terms of performance specifications are fundamental to allow worldwide application of clinical information according to specific CRP thresholds and to avoid risk of patient misclassification.

From External Quality Assessment data, current harmonization of CRP measuring systems appears to be satisfactory, the inter-assay CV being well below 10%. The inter-method variability is even better (close to 3%) when the widely used measuring systems are compared at CRP concentrations employed as cut-off for detecting sub-clinical infection (i.e., 10.0 mg/L) and measurement variability estimated, according to ISO 20914:2019 Technical Specification, from the intermediate within-lab reproducibility of 6-month consecutive measurement data. According to the state-of-the-art model (which is better suited for CRP), the maximum allowable measurement uncertainty (MAU) for CRP measurement on clinical samples with 10.0 mg/L concentrations is 3.76% (desirable quality). As measurement uncertainty (MU) of the only available reference material (ERM-DA474/IFCC) is ∼3%, to fulfil desirable MAU on clinical samples, IVD manufacturers should work to keep the contribution of remaining MU sources (commercial calibrator and intermediate within-lab reproducibility) lower than 2.3%.

The Joint Committee for Traceability in Laboratory Medicine (JCTLM) currently lists the secondary commutable certified reference material (CRM) ERM DA-474/IFCC (DA-474) “C-Reactive Protein in Human Serum” and two generic immunoassay-based method principles as the basis for implementing the metrological traceability of C-reactive protein (CRP) measurements by end-user measurement procedures used by medical laboratories. The current metrological traceability has produced well harmonized results for clinical samples among different end-user measurement procedures. New higher-order pure substance and secondary commutable CRMs have been nominated for listing by the JCTLM. However, the data supporting performance of these new candidate CRMs, including use of new mass spectrometry based candidate reference measurement procedures (RMPs), was not clear regarding the influence that introducing these new CRMs would have on the current well harmonized results achieved with the existing metrological traceability to DA-474. The clinically relevant CRP measurand in blood serum or plasma is a pentamer of identical subunits, which adds complexity to the application of higher-order CRMs and RMPs. The JCTLM convened a workshop in December 2022 to review the appropriate implementation of metrological traceability of CRP measurements. The workshop consensus was that the extent-of-equivalence data must include considerations about the impact of a new CRM when used for its intended purpose in the calibration hierarchies of existing end-user measuring systems; and that a new RMP must compare results with another existing well validated candidate RMP or with a globally available end-user measurement system.

Objectives The exploration of the metabolites in the degradation pathways of vitamin D (VTD) has gained importance in recent years and simultaneous quantitation of twenty-five-hydroxy vitamin D (25(OH)D) mass concentration together with 24,25-dihydroxyvitamin D (24,25(OH)2D) has been proposed as a newer approach to define VTD deficiency. Yet, no data are available on 24,25(OH)2D biological variation (BV). In this study, we evaluated 24,25(OH)2D’s BV on the European Biological Variation Study (EuBIVAS) cohort samples to determine if analytical performance specifications (APS) for 24,25(OH)2D could be generated. Methods Six European laboratories recruited 91 healthy participants. 25(OH)D and 24,25(OH)2D concentrations in K 3 -EDTA plasma were examined weekly for up to 10 weeks in duplicate with a validated LC-MS/MS method. The Vitamin D Metabolite Ratio (24,25(OH)2D divided by 25(OH)D × 100) was also calculated at each time point. Results Linear regression of the mean 24,25(OH)2D concentrations at each blood collection showed participants were not in steady state. Variations of 24,25(OH)2D over time were significantly positively associated with the slopes of 25(OH)D concentrations over time and the concentration of 25(OH)D of the participant at inclusion, and negatively associated with body mass index (BMI), but not with age, gender, or location of the participant. The variation of the 24,25(OH)2D concentration in participants over a 10 weeks period was 34.6%. Methods that would detect a significant change linked to the natural production of 24,25(OH)2D over this period at p<0.05 would need a relative measurement uncertainty ( u %)<14.9% while at p<0.01, relative measurement uncertainty should be <10.5%. Conclusions We have defined for the first time APS for 24,25(OH)2D examinations. According to the growing interest in this metabolite, several laboratories and manufacturers might aim to develop specific methods for its determination. The results presented in this paper are thus necessary prerequisites for the validation of such methods.

Objectives In the digital age, the metaverse has emerged with impressive potential for many segments of society. The metaverse could be presented as a parallel dimension able to enhance the physical world as well as our actions and decisions in it with the objective to use a coalition between the natural and virtual worlds for value creation. Our aim was to elaborate on the impact of the metaverse on laboratory medicine. Methods Based on the available evidence, literature and reports, we analyzed the different perspectives of the metaverse on laboratory medicine and the needs for an efficient transition. Results The convergence and integration of technologies in the metaverse will participate to the reimagination of laboratory medicine services with augmented services, users’ experiences, efficiency, and personalized care. The revolution around the metaverse offers different opportunities for laboratory medicine but also open multiple related challenges that are presented in this article. Conclusions Scientific societies, multidisciplinary teams and specialists in laboratory medicine must prepare the integration metaverse and meta-medical laboratories, raise the awareness, educate, set guidance to obtain a maximum of value and mitigate potential adverse consequences.

The International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) Committee on Point-of-Care Testing (C-POCT) supports the use of point-of-care testing (POCT) outside of the hospital setting performed by healthcare professionals without formal laboratory education because of its numerous benefits. However, these benefits are associated with risks that must be managed, to ensure the provision of reliable test results and minimize harm to the patient. Healthcare professionals, local regulatory bodies, accredited laboratories as well as manufacturers should actively be engaged in education, oversight and advice to ensure that the healthcare professional selects the appropriate equipment and is able to analyze, troubleshoot and correctly interpret the point-of-care (POC) test results.

Objectives Alzheimer’s disease (AD) is considered the most common cause of dementia in older people. Recently, blood-based markers (BBM) Aβ1-42, Aβ1-40, and phospho Tau181 (p-Tau181) have demonstrated the potential to transform the diagnosis and prognostic assessment of AD. Our aim was to investigate the effect of different storage conditions on the quantification of these BBM and to evaluate the interchangeability of plasma and serum samples. Methods Forty-two individuals with some degree of cognitive impairment were studied. Thirty further patients were retrospectively selected. Aβ1-42, Aβ1-40, and p-Tau181 were quantified using the LUMIPULSE-G600II automated platform. To assess interchangeability between conditions, correction factors for magnitudes that showed strong correlations were calculated, followed by classification consistency studies. Results Storing samples at 4 °C for 8–9 days was associated with a decrease in Aβ fractions but not when stored for 1–2 days. Using the ratio partially attenuated the pre-analytical effects. For p-Tau181, samples stored at 4 °C presented lower concentrations, whereas frozen samples presented higher ones. Concerning classification consistency in comparisons that revealed strong correlations (p-Tau181), the percentage of total agreement was greater than 90 % in a large number of the tested cut-offs values. Conclusions Our findings provide relevant information for the standardization of sample collection and storage in the analysis of AD BBM in an automated platform. This knowledge is crucial to ensure their introduction into clinical settings.

Objectives We evaluated the long-term stability of thyroid peroxidase antibody (anti-TPO). Methods In the Danish General Suburban Population Study (GESUS), serum samples were biobanked at −80 °C during 2010–2013. In a paired design with 70 subjects, we compared anti-TPO (30–198 U/mL) measured on fresh serum on Kryptor Classic in 2010–2011 (anti-TPO fresh ) with anti-TPO remeasured on frozen serum (anti-TPO frozen ) on Kryptor Compact Plus in 2022. Both instruments used the same reagents and the anti-TPO n automated immunofluorescent assay, which was calibrated against the international standard NIBSC 66/387, based on the Time Resolved Amplified Cryptate Emission (TRACE) technology from BRAHMS. Values greater than 60 U/mL are regarded as positive in Denmark with this assay. Statistical comparisons included Bland-Altman, Passing-Bablok regression, and Kappa statistic. Results The mean follow-up time was 11.9 years (SD: 0.43). For anti-TPO frozen vs. anti-TPO fresh , the line of equality was within the confidence interval of the absolute mean difference [5.71 (−0.32; 11.7) U/mL] and the average percentage deviation [+2.22% (−3.89%; +8.34%)]. The average percentage deviation of 2.22% did not exceed analytical variability. Passing-Bablok regression revealed both a statistically significant systematic and proportional difference: Anti-TPO frozen =−22.6 + 1.22*(anti-TPO fresh ). Frozen samples were correctly classified as positive in 64/70 (91.4%; Kappa=71.8%). Conclusions Anti-TPO serum samples in the range 30–198 U/mL were stable after 12-years of storage at −80 °C with an estimated nonsignificant average percentage deviation of +2.22%. This comparison is based on Kryptor Classic and Kryptor Compact Plus, which used identical assays, reagents, and calibrator, but for which the agreement in the range 30–198 U/mL is unclarified.

Objectives To find suitable external quality assessment (EQA) materials for serum C-peptide, we evaluated the commutability of five types of processed materials. Methods Seventy-four individual serum samples and 12 processed samples including three EQA samples currently in use, frozen human serum pools (FHSP), and three other kinds of processed samples were prepared by dissolving WHO International Standard Reagent for C-peptide (WHO ISR 13/146) in three different matrixes: 0.05 % bovine serum albumin, fetal bovine serum and human serum pools. Samples were analyzed using the isotope dilution liquid chromatography-tandem mass spectrometry (ID-LC-MS/MS) method and six widely used immunoassays. The commutabilities of processed materials were assessed according to the difference in bias approach recommended by the IFCC. And the short- and long-term stability of FHSP samples at different temperatures were also evaluated. Results Out of the five kinds of processed materials, FHSP samples were commutable on most assays. In contrast, the EQA materials currently in use were only commutable on a few immunoassays. Additionally, processed materials derived from WHO ISR 13/146 were found to be un-commutable on over half of immunoassays. The FHSP samples could be stably stored at 4 and −20 °C for at least 16 days, and at −80 °C for at least 1 year, but at room temperature only for 12 h. Conclusions With clarified commutability and stability information, the human serum pool samples along with the developed ID-LC-MS/MS method could be used in the EQA program to promote the comparability among laboratories for C-peptide measurement in China.

Objectives Free thyroxine (FT4) in serum is routinely measured in clinical practice to diagnose and monitor thyroid disease. Due to its concentration in picomolar range and the delicate equilibrium of free and protein-bound T4, accurate measurement is challenging. As a consequence, large inter-method differences in FT4 results exists. Optimal method design and standardization of the FT4 measurement is therefore necessary. The IFCC Working Group for Standardization of Thyroid Function Tests proposed a reference system with a conventional reference measurement procedure (cRMP) for FT4 in serum. In this study, we describe our FT4 candidate cRMP and its validation in clinical samples. Methods This candidate cRMP is based on equilibrium dialysis (ED) combined with determination of T4 with an isotope-dilution liquid chromatography tandem mass-spectrometry (ID-LC-MS/MS) procedure and was developed according to the endorsed conventions. Its accuracy, reliability, and comparability was investigated using human sera. Results It was shown that the candidate cRMP adhered to the conventions and its accuracy, precision, and robustness were adequate in serum of healthy volunteers. Conclusions Our candidate cRMP measures FT4 accurately and performs well in serum matrix.

Objectives In clinical practice, steroid measurements are performed mainly by direct, non-extraction immunoassays adapted to high throughput, automated immunoassay platforms and employing secondary calibrators. The accuracy of such steroid immunoassays is limited by cross-reactivity with structurally related steroids and nonspecific matrix interference as well as the metrological traceability of manufacturer supplied calibrators. The accuracy of steroid immunoassay calibrators has been little investigated by independent chemical methods. Methods Steroid concentrations of 41 calibrators (4–6 replicates per calibrator) supplied by four manufacturers for use in testosterone (T), estradiol (E 2 ), and progesterone (P 4 ) commercial immunoassays were measured by ultra-pressure liquid chromatography-mass spectrometry (UPLC-MS). Results Among 14 non-zero T calibrators, six (43 %) deviated significantly from the label concentration with 29 % outside 20 % of it. Among 14 E 2 calibrators, eight (57 %) deviated significantly, whereas seven (50 %) were outside 20 % of the label concentration. Among 11 P 4 calibrators, eight (73 %) deviated significantly whereas four (36 %) were outside within 20 % of the label concentration. Conclusions We conclude that inaccurate calibration of manufacturer’s supplied standards may contribute to inaccuracy of commercial direct steroid immunoassays.

Objectives Rheumatoid arthritis (RA) is a systemic autoimmune disease characterised by the presence of autoantibodies that are used for classification of the disease. Though routine diagnostics is commonly restricted to measuring rheumatoid factor (RF) and anti-citrullinated protein antibodies, detection of RF IgM, IgG and IgA isotypes, may increase the power of RA serodiagnosis by reducing the number of seronegative patients as well as provide prognostic information. The agglutination-based RF assays, such as nephelometry or turbidimetry, are unable to differentiate isotypes. We compared three different immunoassays used in current laboratory practice to detect RF isotypes. Methods We tested 117 consecutive serum samples that were positive for total RF at nephelometry, from 55 RA and 62 non-RA subjects. IgA, IgG, and IgM isotypes of RF were tested by immunoenzymatic (ELISA, Technogenetics), fluoroenzymatic (FEIA, ThermoFisher) and chemiluminescence (CLIA, YHLO Biotech Co.) immunoassays. Results Diagnostic performance differed considerably between the assays, especially with regard to RF IgG isotype. Agreement among methods by Cohen’s kappa ranged from 0.05 (RF IgG CLIA vs. FEIA) to 0.846 (RF IgM CLIA vs. FEIA). Conclusions The poor agreement observed in this study indicates substantial lack of comparability among assays for RF isotypes. Harmonization of these tests requires further efforts before their measurement can be used in clinical practice.

Objectives Define the cutoff thresholds of the Kappa (K) and Lambda (L) free light chains (FLC) indices for the detection of intrathecal immunoglobulin synthesis (IIS) using the new K and L FLC ELISA from SEBIA. The reference technique, which is not readily standardized between laboratories, is based on the demonstration of oligoclonal banding (OCB) in cerebrospinal fluid (CSF) which is absent in serum. For the past 6 years, we have also routinely calculated the K FLC index using The Binding Site (TBS) reagents on an Optilite instrument, an approach increasingly used as an alternative and/or a complement to electrophoretic analysis. Methods We analyzed 391 serum/CSF pairs divided into three groups. The first group were cases without OCB and with normal albumin CSF/serum ratio (n=174). The second group were cases with specific OCB (n=73). The last group included patients with increased albumin CSF/sera ratio without OCB (n=142). Results Analysis of the first group determined that the cutoffs for detection of IIS are respectively 2.55 and 1.02 for the K FLC and L FLC indices. Of the 73 cases with IIS, only 2 had a K FLC index below this threshold (sensitivity of 97.26%), while 16 out of 73 cases (78.08%) and 13 out of 72 cases (81.94%) had an IgG and L FLC index below the cutoffs, respectively. Additionally, we illustrate equivalent performances for prediction of the presence of OCB between SEBIA and TBS methods. Conclusions Sebia K FLC and L FLC assays are adequate alternative methods for the diagnosis of IIS.

Objectives Placental growth factor (PlGF) is used for first-trimester preeclampsia screening and could be combined with other biochemical markers for Down syndrome screening. We aim to estimate the predictive value of the combination of pregnancy-associated plasma protein (PAPP-A), free β-human chorionic gonadotropin (free β-hCG), placental growth factor (PlGF) and α-fetoprotein (AFP) with and without nuchal translucency. Methods Singleton pregnancies recruited at 11–14 weeks and followed until delivery. The four maternal markers were measured using Kryptor (ThermoFisher-BRAHMS) and adjusted for gestational age and maternal characteristics. The risk of Down syndrome was calculated using the Fetal Medicine Foundation algorithm and multivariate linear regression analyses in all cases and in 2,200 controls. Receiver-operator characteristic (ROC) curves were used to calculate the detection and false-positive rates. Results Twenty-six (0.2%) cases of Down syndrome were diagnosed among 13,386 participants. The combination of the four biomarkers could have detected 88% (95% CI: 72–97%) of the cases at a false-positive rate of 13% (95% CI: 12–15%). The addition of nuchal translucency would have increased the detection rate to 96% (95% CI: 82–99%) at a false-positive rate of 4% (95% CI: 4–5%) using a 1:300 cut-off and to 100% (95% CI: 89–100%) at a false-positive rate of 6% (95% CI: 5–8%) using a 1:500 cut-off. Conclusions First-trimester screening using biochemical markers allows the identification of approximately 88% of Down syndrome cases for a false-positive rate of 13%. The addition of nuchal translucency raises the detection rate above 95% with a false-positive rate below 5%.

Objectives This study assesses the clinical relevance of dipeptidyl peptidase 4 (DPP4) membrane exopeptidase as a biomarker of inflammatory bowel disease (IBD). A spike-and-recovery approach of DPP4 in fecal samples was used to compare two different methods for protein extraction, followed by a stability assessment. Methods Fecal samples of healthy volunteers spiked with known concentrations of recombinant DPP4 were processed using a standard manual extraction protocol and the CALEX ® protocol. The two methods were compared by quantification of fecal DPP4 by ELISA, followed by Bland-Altman analysis. For the stability assays DPP4 was extracted from fecal samples and stored under different conditions of temperature and time after collection. Results In general, the levels of spiked DPP4 in stool samples were lower with the manual protocol than in those obtained with the CALEX ® method; this trend was corroborated by Bland-Altman analysis. Nonetheless, variability was within the acceptable limits for both protocols. In the stability assessment, no statistically significant differences were found between the results obtained under the different storage conditions. Conclusions Both manual and CALEX ® protocols provided equal extraction ability of DPP4 from stool samples. In addition, DPP4 provided flexibility in terms of sample storage enabling the accurate assessment of samples delivered up to a week before analysis.

Objectives NT-proBNP is frequently used for ruling out heart failure. Different cut-offs are used depending on the clinical context, e.g. an acute or chronic condition. Medical decision limits have been suggested at 125 and 300 ng/L or 400 ng/L in international guidelines. However, there is limited standardization between NT-proBNP methods and using the same blood sample might cause different treatment of patients. Methods Data from the external quality assessment program for NT-proBNP from Equalis, Sweden, were extracted for the period 2011–2021, and categorized according to manufacturer. Manufacturer median NT-proBNP values were compared to total median values. CV% was calculated for each manufacturer and in comparison to different levels of NT-proBNP. Results Roche was the most common method, and its median results were closest to the median consensus results. When looking at the total CV at NT-proBNP levels in the range of 0–500 ng/L, the total CV varied from 4 to 27%. During 2019–2021, Siemens (Immulite, Centaur, Atellica) yielded results 16–20% above the consensus median depending on sample level. Similarly, Abbott was 5–7% above, while Roche and Siemens Stratus were 1% respectively 6–10% below the consensus median. Conclusions The introduction of new manufacturers and methods in 2017 have caused the agreement between manufacturers to decline. This highlights the need for a common calibrator and reference materials, particularly since medical decision limits in guidelines, e.g. European Society of Cardiology 2021, which are mostly based on Roche methods, do not take these method differences into account.

Objectives The SARS-CoV-2 immune response is mediated by both humoral and cellular immunity. In this study, SARS-CoV-2 specific cellular immunity was tested by a novel direct real-time PCR (dRT-PCR) assay, targeting mRNA of CXCL10 , and compared with respect to an ELISA measuring interferon gamma (IFN-γ) release. Methods Whole blood (Li–He) and serum samples were collected from 92 healthcare workers (HCW), with three doses of homologous (Pfizer/BioNTech, n=74) or heterologous (Pfizer/BioNTech and Vaxzevria or Moderna, n=18) vaccinations. Li–He samples were incubated with SCV2 PANEL-1-T-ACTIVATION (Hyris srl, Lodi, Italy), or CoV-2 IGRA TUBE ELISA (Euroimmune, Lubeck, Germany). CXCL10 mRNA expression was analyzed by bCube/bApp (Hyris), while IFN-γ was evaluated by quant-T-Cell SARS-CoV-2 ELISA (Euroimmune). Anti-SARS-CoV-2 S-RBD IgG levels were measured in sera using a CLIA assay (Snibe, Shenzen, China). Results Imprecision of dRT-PCR assay was found to be satisfactory, and the two methods for measuring T cell immunity to SARS-CoV-2 peptides agreed in 82/87 (94.2%) of results. At qualitative dRT-PCR analyses, 81 subjects (93.2%) resulted as reactive to SARS-CoV-2 peptides, 3 (3.4%) were borderline and 3 were negative (3.4%). At univariate and multivariate analyses of quantitative dRT-PCR mRNA of CXCL10 and IFN-γ release results showed no difference between HCW with previous infection, homologous/heterologous vaccination, or demographical features. Anti-SARS-CoV-2 S-RBD IgG was associated with the previous infection and the time between the last vaccination or positivity. Conclusions Direct RT-PCR appeared accurate for determining the presence or absence of immunoreactivity of SARS-CoV-2 specific T cells, especially when rapid analyses are required, such as for organ transplantation.

Objectives Laboratory testing for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has played an important role in the effort to prevent and contain local outbreaks. The aim of this study was to assess the diagnostic accuracy of a new fully automated SARS-CoV-2 laboratory-based antigen assay (CoV2Ag) and to explore the efficiency of a diagnostic algorithm combining antigen and conventional high-throughput molecular assays to address potential future challenges of the SARS-CoV-2 pandemic. Methods One thousand two hundred and twenty four consecutive nasopharyngeal swabs were tested using RT-PCR and CoV2Ag assay. Results The overall sensitivity and specificity of CoV2Ag were 79.1 and 97.8%, respectively. When the analysis was restricted to cases with Ct values ≤30, the sensitivity of the assay improved to 98.1%. Acceptable sensitivity was found when the analysis was limited to patients presenting within one or two to four days of symptom onset (80.5 and 84.8%, respectively). A retrospective analysis of the use of a two-step diagnostic approach combining the CoV2Ag assay and RT-PCR during an acute pandemic phase of 97 days showed a potential reduction in the number of RT-PCR tests by 36.1%, corresponding to savings in reagent costs and technician workload of approximately €8,000 and 10.5 h per day, respectively. Conclusions Our data show that the proposed algorithm represents a valid alternative diagnostic approach to increase testing efficiency during future pandemic phases with high positivity rates (>20%) and elevated numbers of RT-PCR test requests.

Objectives The BNT162b2 messenger RNA vaccine is highly effective in reducing COVID‐19 infection, hospitalization and death. However, many subjects developed a breakthrough infection despite a full vaccination scheme. Since the waned efficacy of mRNA vaccines is correlated with the decrease of antibodies occurring over time, we aimed at evaluating whether lower levels of antibodies were associated with an increased risk of breakthrough infection in a cohort of breakthrough subjects that received three vaccine doses. Methods Total binding antibodies against the RBD of the S1 subunit (Roche Diagnostics, Machelen, Belgium) and neutralizing antibodies using the Omicron B.1.1.529 variant pseudovirus were measured. Based on individual kinetic curves, the antibody titer of each subject was interpolated just before the breakthrough infection and compared to a matched-control group that did not develop a breakthrough infection. Results Lower levels of total binding and neutralizing antibodies were observed compared to the control group (6.900 [95% CI; 5.101–9.470] vs. 11.395 BAU/mL [8.627–15.050] [p=0.0301] and 26.6 [18.0–39.3] vs. 59.5 dilution titer −1 [32.3–110] [p=0.0042], respectively). The difference between breakthrough and control subjects was mostly observed for neutralizing antibodies before three months after the homologous booster administration (46.5 [18.2–119] vs. 381 [285–509] [p=0.0156]). Considering the measurement of total binding antibodies before 3 months, there was no significant difference (p=0.4375). Conclusions In conclusion, our results showed that subjects that developed a breakthrough infection had lower levels of neutralizing and total binding antibodies compared to controls. The difference was mostly noticeable considering neutralizing antibodies, especially for infections occurring before 3 months after the booster administration.