Development of a dual antigen lateral flow immunoassay for detecting Yersinia pestis

Background

Yersinia pestis is the causative agent of plague, a zoonosis associated with small mammals. Plague is a severe disease, especially in the pneumonic and septicemic forms, where fatality rates approach 100% if left untreated. The bacterium is primarily transmitted via flea bite or through direct contact with an infected host. The 2017 plague outbreak in Madagascar resulted in more than 2,400 cases and was highlighted by an increased number of pneumonic infections. Standard diagnostics for plague include laboratory-based assays such as bacterial culture and serology, which are inadequate for administering immediate patient care for pneumonic and septicemic plague.

Principal findings

The goal of this study was to develop a sensitive rapid plague prototype that can detect all virulent strains of Y. pestis. Monoclonal antibodies (mAbs) were produced against two Y. pestis antigens, low-calcium response V (LcrV) and capsular fraction-1 (F1), and prototype lateral flow immunoassays (LFI) and enzyme-linked immunosorbent assays (ELISA) were constructed. The LFIs developed for the detection of LcrV and F1 had limits of detection (LOD) of roughly 1–2 ng/mL in surrogate clinical samples (antigens spiked into normal human sera). The optimized antigen-capture ELISAs produced LODs of 74 pg/mL for LcrV and 61 pg/mL for F1 when these antigens were spiked into buffer. A dual antigen LFI prototype comprised of two test lines was evaluated for the detection of both antigens in Y. pestis lysates. The dual format was also evaluated for specificity using a small panel of clinical near-neighbors and other Tier 1 bacterial Select Agents.

Conclusions

LcrV is expressed by all virulent Y. pestis strains, but homologs produced by other Yersinia species can confound assay specificity. F1 is specific to Y. pestis but is not expressed by all virulent strains. Utilizing highly reactive mAbs, a dual-antigen detection (multiplexed) LFI was developed to capitalize on the diagnostic strengths of each target.

Potential false-positive reasons for SARS-CoV-2 antibody testing and its solution

Coronavirus disease 2019 (COVID-19) has brought a huge impact on global health and the economy. Early diagnosis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is essential for epidemic prevention and control. The detection of SARS-CoV-2 antibodies is an important criterion for diagnosing COVID-19. However, SARS-CoV-2 antibody testing also has certain false positives causing confusion in clinical diagnosis. This article summarizes the causes of false-positive detection of SARS-CoV-2 antibodies in clinical practice. The results indicate that the most common endogenous interferences include rheumatoid factor, heterophile antibodies, human anti-animal antibodies, lysozyme, complement, and cross-antigens. The exogenous interference is mainly incomplete coagulation of the specimen, contamination of the specimen, and insufficient optimization of the diagnostic kit's reaction system.

Investigating the presence of human anti-mouse antibodies (HAMA) in the blood of laboratory animal care workers

Background

In the present study, the researchers evaluated the presence of human anti-mouse antibodies (HAMA) in a normal population and laboratory animal care providers for the first time in the world. Also, the cause of HAMA incidence in the human body through a close contact with mice was identified.

Methods

The study population consisted of 40 laboratory animal care providers aged between 24 and 57 years with a close contact with mice (e.g. taking care of mice, feeding mice, etc.) and 40 individuals of the same age as the above group with no contact with mice. HAMA was measured in both the case and control groups using sandwich enzyme-linked immunosorbent assay (ELISA) method. Data were analyzed using SPSS 18. Univariate and multivariate linear regression and independent t-test were used. The significance of results was measured based on p < 0.05.

Results

The present study revealed that the animal care providers had (p = 000) a higher titer of HAMA (4.95 ng/mL) in their blood than the control group (1.67 ng/mL). Also, the individuals in the case group (exposed to mice) were more allergic (43.6%) than those in the control group (15%) (p = 0.003).

Conclusions

The results of this study revealed that exposure to mice in laboratory care centers can cause production of HAMA in the human body but its titer is possibly lower in Iranian working staff than those in the other parts of the world.

Muscle enzyme and fiber type-specific sarcomere protein increases in serum after inertial concentric-eccentric exercise

Muscle damage induced by inertial exercise performed on a flywheel device was assessed through the serum evolution of muscle enzymes, interleukin 6, and fiber type-specific sarcomere proteins such as fast myosin (FM) and slow myosin (SM). We hypothesized that a model of muscle damage could be constructed by measuring the evolution of serum concentration of muscle proteins following inertial exercise, according to their molecular weight and the fiber compartment in which they are located. Moreover, by measuring FM and SM, the type of fibers that are affected could be assessed. Serum profiles were registered before and 24, 48, and 144 h after exercise in 10 healthy and recreationally active young men. Creatine kinase (CK) and CK-myocardial band isoenzyme increased in serum early (24 h) and returned to baseline values after 48 h. FM increased in serum late (48 h) and remained elevated 144 h post-exercise. The increase in serum muscle enzymes suggests increased membrane permeability of both fast and slow fibers, and the increase in FM reveals sarcomere disruption as well as increased membrane permeability of fast fibers. Consequently, FM could be adopted as a fiber type-specific biomarker of muscle damage.

Cytokines as biomarkers in rheumatoid arthritis

RA is a complex disease that develops as a series of events often referred to as disease continuum. RA would benefit from novel biomarker development for diagnosis where new biomarkers are still needed (even if progresses have been made with the inclusion of ACPA into the ACR/EULAR 2010 diagnostic criteria) and for prognostic notably in at risk of evolution patients with autoantibody-positive arthralgia. Risk biomarkers for rapid evolution or cardiovascular complications are also highly desirable. Monitoring biomarkers would be useful in predicting relapse. Finally, predictive biomarkers for therapy outcome would allow tailoring therapy to the individual. Increasing numbers of cytokines have been involved in RA pathology. Many have the potential as biomarkers in RA especially as their clinical utility is already established in other diseases and could be easily transferable to rheumatology. We will review the current knowledge's relation to cytokine used as biomarker in RA. However, given the complexity and heterogeneous nature of RA, it is unlikely that a single cytokine may provide sufficient discrimination; therefore multiple biomarker signatures may represent more realistic approach for the future of personalised medicine in RA.

Heterophilic antibody interference in immunometric assays

Immunometric assays are inherently vulnerable to interference from heterophilic antibodies, endogenous antibodies that bind assay antibodies. The consequences of such interference can be devastating. In this review, we discuss strategies that reduce the damage caused by heterophilic antibodies. Clinicians should only order blood tests that are indicated for the patient and clinical setting at hand, and have the confidence to question laboratory results discordant with the clinical picture. Laboratorians should familiarize themselves with the vulnerability of the assays they offer, and be able to perform and interpret adequate confirmatory measures correctly. When designing immunoassays, the immunoassay industry should invest the necessary resources in specific protective measures against heterophilic antibody interference. Examples include using antibody fragments and the addition of effective blockers to assay reagents. The increasing use of modified monoclonal mouse antibodies both in therapy and diagnostics could present a particular challenge in the future.

Dysthyronemia in normal concentrations of thyrotropin--analytical and clinical consequences

This article discusses the conditions that may lead to a phenomenon called dysthyronemia. Here, the thyroid gland has concentration of thyrotropin in circulation within the reference range, but the concentrations of free or total fractions of thyroid hormones are outside the reference range. Normal values of thyrotropin (TSH) and increased values of THs are referred to as hyperthyroxinemia, while normal values of thyrotropin and decreased values of thyroid hormone are hypothyroxinemia. As shown by our observations, it is a relatively frequent situation in the parallel determinations of TSH and free thyroxine, when results verging on hyperthyroxinemia were found in 7% of cases (6.74%, n=259,590), and also in the parallel sets of TSH and total triiodothyronine when hypotriiodothyroninemia reached 8.5% (8.48%, n=73,143). We are assuming that the main cause of hyperthyroxinemia in the free thyroxine and TSH system is the presence of autoantibodies against thyroxine in patients with autoimmune thyroid disease. The reason of hypotriiodothyroninemia in the system of triiodothyronine and TSH is a decreased concentration of thyroid binding globulin in postmenopausal women. Manufacturers of immunoanalytical kits should take into account the potential adverse effects of autoantibodies against thyroid hormones when measuring the results of immunoassay determination of the free fraction of these hormones.

Avian IgY antibodies and their recombinant equivalents in research, diagnostics and therapy

The generation and use of avian antibodies is of increasing interest in a wide variety of applications within the life sciences. Due to their phylogenetic distance, mechanisms of immune diversification and the way in which they deposit IgY immunoglobulin in the egg yolk, chickens provide a number of advantages compared to mammals as hosts for immunization. These advantages include: the one-step purification of antibodies from egg yolk in large amounts facilitates having a virtually continuous supply; the epitope spectrum of avian antibodies potentially grants access to novel specificities; the broad absence of cross-reactivity with mammalian epitopes avoids assay interference and improves the performance of immunological techniques. The polyclonal nature of IgY antibodies has limited their use since avian hybridoma techniques are not well established. Recombinant IgY, however, can be generated from mammalian monoclonal antibodies which makes it possible to further exploit the advantageous properties of the IgY scaffold. Moreover, cloning and selecting the immune repertoire from avian organisms is highly efficient, yielding antigen-specific antibody fragments. The recombinant approach is well suited to circumvent any limitations of polyclonal antibodies. This review presents comprehensive information on the generation, purification, modification and applications of polyclonal and monoclonal IgY antibodies.

Heterophilic antibody interference in commercial immunoassays; a screening study using paired native and pre-blocked sera

Heterophilic antibodies are still an important source of interference in immunoassays. We have conducted a screening study for interference in a panel of commercially available assays using two sera known to contain high titer Fc-reactive heterophilic antibodies.

The sera were distributed to laboratories participating in the Nordic External Quality Assessment cooperation (EQANord). Duplicate samples pre-blocked with aggregated murine monoclonal MAK33 were also supplied. Discrepancies (>50%) between the results for native and blocked samples were used to classify the tested assays as susceptible to interference. A total of 170 different assay kits covering 91 analytes were tested.

We found that 21 assays, covering 19 different analytes, were susceptible to interference from the heterophilic antibodies in the two sera. Many of these are clinically and commercially important assays. Some of the false results were grossly elevated and could have been detrimental to patient care in a clinical setting.

Heterophilic antibodies with Fc-reactivity remain a threat. A more widespread use of antibody fragments and aggregated immunoglobulin could potentially improve the heterophilic antibody resistance of assays intended for clinical use.

Evaluation of dysthyronemia in endocrinological patients

Dysthyronemia is the state of the thyroid gland in which the concentration of thyrotropin (TSH) in circulation is within the reference range, but the concentrations of free or total fractions of thyroid hormones (TH) are outside the reference range. Normal values of TSH and increased values of TH are referred to as hyperthyroxinemia or hypertriiodothyroninemia, while normal values of TSH and decreased values of TH are called hypothyroxinemia or hypotriiodothyroninemia.

Thyroid diagnostic tests were carried out at the Institute of Endocrinology, Prague, Czech Republic, in 1999–2009 using the immunoanalytical systems of Roche Diagnostics GmbH, Germany (Elecsys 2010, Modular E170).

Hyperthyroxinemia was found in 6.74% of all parallel sets of TSH and free thyroxine (FT4, n=259,590) values. Hypotriiodothyroninemia was observed in 8.48% of all parallel sets of TSH and total triiodothyronine (TT3, n=73,143). The occurrence of hyperthyroxinemia (TSH-FT4) and hypotriiodothyroninemia (TSH-TT3) was >3 times higher than the occurrence of dysthyronemia for the combinations TSH-FT4 (hypothyroxinemia), TSH-TT4 (total thyroxine, n=1996), TSH-FT3 (free triiodothyronine, n=94,090), and TSH-TT3 (hypertrijodthyroninemia), and >5 times higher in comparison with the combinations TSH-FT4-FT3 (n=93,683), TSH-FT4-TT3 (n=72,373), TSH-FT3-TT3 (n=2466), TSH-TT4-TT3 (n=1779), TSH-FT4-TT4 (n=1571), and TSH-FT3-TT4 (n=1466).

In light of our patient types, we are assuming that the observed hypotriiodothyroninemia (TSH-TT3, 8.48%) is due to a decreased concentration of thyroid binding globulin in postmenopausal women and that hyperthyroxinemia (TSH-FT4, 6.74%) is caused mainly by endogenous autoantibodies against thyroxine in patients with thyroid autoimmune diseases.

Simultaneous detection of Human Immunodeficiency Virus 1 and Hepatitis B virus infections using a dual-label time-resolved fluorometric assay

A highly specific and novel dual-label time-resolved immunofluorometric assay was developed exploiting the unique emission wavelengths of the intrinsically fluorescent terbium (Tb³⁺) and europium (Eu³⁺) tracers for the simultaneous detection of human immunodeficiency virus 1 (HIV-1) and hepatitis B virus (HBV) infections, respectively. HIV-1 infection was detected using a double antigen sandwich format wherein anti-HIV-1 antibodies were captured using an in vivo biotinylated version of a chimeric HIV-1 antigen and revealed using the same antigen labeled with Tb³⁺ chelate. Hepatitis B surface antigen (HBsAg), which served as the marker of HBV infection, was detected in a double antibody sandwich using two monoclonal antibodies (mAbs), one chemically biotinylated to capture, and the other labeled with Eu³⁺ nanoparticles, to reveal. The performance of the assay was evaluated using a collection (n = 60) of in-house and commercially available human sera panels. This evaluation showed the dual-label assay to possess high degrees of specificity and sensitivity, comparable to those of commercially available, single analyte-specific kits for the detection of HBsAg antigen and anti-HIV antibodies. This work demonstrates the feasibility of developing a potentially time- and resource-saving multiplex assay for screening serum samples for multiple infections in a blood bank setting.

Present and Future Biochemical Markers for Detection of Acute Coronary Syndrome

The use of biochemical markers in the diagnosis and management of patients with acute coronary syndrome has increased continually in recent decades. The development of highly sensitive and cardiac-specific troponin assays has changed the view on diagnosis of myocardial infarction and also extended the role of biochemical markers of necrosis into risk stratification and guidance for treatment. The consensus definition of myocardial infarction places increased emphasis on cardiac marker testing, with cardiac troponin replacing creatine kinase MB as the "gold standard" for diagnosis of myocardial infarction. Along with advances in the use of more cardiac-specific markers of myocardial necrosis, biochemical markers that are involved in the progression of atherosclerotic plaques to the vulnerable state or that signal the presence of vulnerable plaques have recently been identified. These markers have variable abilities to predict the risk of an individual for acute coronary syndrome. The aim of this review is to provide an overview of the well-established markers of myocardial necrosis, with a special focus on cardiac troponin I, together with a summary of some of the potential future markers of inflammation, plaque instability, and ischemia.

Intact free prostate-specific antigen and free and total human glandular kallikrein 2. Elimination of assay interference by enzymatic digestion of antibodies to F(ab')2 fragments

Various blood constituents can interfere with immunoassays, usually by binding the Fc portion of antibodies. Our previously developed assays for intact free prostate-specific antigen (PSA), free human kallikrein 2 (hK2), and total hK2 frequently yielded falsely high results despite including an excess of scavenger antibodies. We investigated whether this interference could be eliminated by replacing monoclonal capture or tracer antibodies with F(ab')2 or recombinant Fab fragments. Female heparin plasma samples (n = 1092), which should have negligible PSA and hK2, and male samples (n = 957) were analyzed to identify samples manifesting interference, which then were used to optimize protocols for the immunoassays. We compared original assays (monoclonal antibodies) versus optimized assays (F(ab')2 fragments: denatured mouse IgG added as scavenger) using another set of EDTA plasma (n = 113), heparin plasma (n = 160), and serum samples (n = 171). With the original assays, the frequency of falsely elevated hK2 and intact free PSA was 15 and 13%, respectively. The optimized assays eliminated 70-85% of these falsely elevated results and importantly reduced the magnitude in the remainder. F(ab')2 fragmentation was the most important factor in reducing interference. The optimized intact free PSA, free hK2, and total hK2 assays manifested high accuracy close to the lower limit of detection.

A new efficient method for eliminating the interference effect of human serum and increasing the sensitivity and recovery rate of enzyme immunoassay

A new, very simple method for increasing the sensitivity and recovery rate of enzyme-linked immunosorbent assay (ELISA) for the precise quantification of antigen in human serum is described. The assay design uses CATNF6A4c IgG2a monoclonal antibody and biotinylated anti-human tumor necrosis factor-alpha (hTNF-alpha) polyclonal mouse IgG as the capture and tracer antibodies, respectively. The assay is completed within 4 hours at room temperature and is capable of detecting both recombinant and native human TNF-alpha. The assay incorporates the use of saturated ammonium sulfate (SAS) as a component of the dilution buffer to amplify the resultant signal from antigen containing human serum and eliminating the endogenous interference of native human serum. SAS worked optimally at the final concentrations, ranging from 1.2% to 11%. The addition of SAS to the dilution buffer resulted in a dramatic increase in both sensitivity and recovery rate of the ELISA. The results demonstrated that 50 microL of dilution buffer, containing SAS, enabled the precise quantification of human TNF-alpha in 100 microL of human serum samples and eliminated the interference of native serum, which seemed to be related to complement proteins. Therefore, dilution buffer containing SAS, at a defined concentration, seemed to be a potential candidate for resolving sensitivity and recovery problems usually encountered in immunoassays when measurement was performed with native serum samples. The proposed technique provides an easy, practical, and consistent method for ELISA when using human native serum.

Heterophile antibodies produce spuriously elevated concentrations of cardiac Troponin I in patients with Legionella pneumophila

We found an unusually high positive rate for cTnI in patients recently infected with Legionella pneumophila. The aim of this study was to examine the possible origin of increased cTnI levels and to test if it could be associated with the immune response to legionellosis. The cTnI was above the cut point in 46.7% of patients infected with legionellosis when measured with reagent lot number RF421A. A strong correlation between high cTnI measurements and positive serologic values for legionellosis was found. With a revised formulation of cTnI reagent, lot number RF421C, the positive rate decreased by over 10-fold to 3.3%. We conclude that the revised lot of cTnI reagent minimized interference by heterophilic antibodies produced in response to legionellosis.

Use of an In Vivo Biotinylated Single-Chain Antibody as Capture Reagent in an Immunometric Assay to Decrease the Incidence of Interference from Heterophilic Antibodies

Background: Heterophilic antibodies are a common source of interference in immunometric assays. We tested the hypothesis that the incidence of such interference could be decreased by use of a recombinant in vivo-biotinylated single-chain antibody (scFv) as the capture reagent.

Methods: We established three assays for carcinoembryonic antigen (CEA) with the capture antibody either chemically biotinylated whole monoclonal T84.66 immunoglobulin, a corresponding F(ab′)2 fragment, or a site-specifically biotinylated T84.66-derived single-chain antibody (scFv). Antibodies were attached to streptavidin-coated microplates. A common europium-labeled anti-CEA tracer monoclonal antibody was used. The F(ab′)2 assay used a buffer that contained bovine immunoglobulin and aggregated irrelevant monoclonal antibody MAK33 as blocking agents. The whole T84.66 immunoglobulin and scFv assays were performed without addition of blocking agents. From a previous study of 11 261 sera, we tested 390 samples that had displayed heterophilic antibody interference and 179 samples that had not.

Results: After correction for bias and analytical variation [2.56 × SD (from the precision profile)], 383 samples displayed significantly different values (&gt;1 μg/L) in the whole T84.66-based assay and the F(ab′)2 assay. In contrast, only nine samples showed falsely high CEA concentrations in the scFv assay. After blocking agents were added to the assay buffer, eight of the nine samples displayed results equivalent to those of the F(ab′)2 assay, and sample dilution produced equivalent results for the remaining sample.

Conclusion: Their ability to be site-specifically biotinylated and their relative resistance to heterophilic antibody interference indicate that single-chain antibodies may be useful solid-phase reagents in immunometric assays.

Negative Interference in Cardiac Troponin I Immunoassays by Circulating Troponin Autoantibodies

Background: There are numerous potential sources of interference in immunoassays. Our aim was to identify the blood component that causes negative interference in cardiac troponin I (cTnI) immunoassays based on antibodies against the central part of cTnI.

Methods: We isolated an interfering factor (IF) from a sample with low recovery of added cTnI, using several consecutive purification steps: caprylic acid precipitation, ammonium sulfate precipitation, and purification on Cibacron Blue gel and protein G columns. Purified IF was identified by gel electrophoresis and mass spectrometric analysis of protein bands. For the direct detection of human antibodies to cardiac troponin in serum samples, we developed immunoassays using three different anti-human immunoglobulin antibodies and measured troponin antibodies in samples with low and normal cTnI recovery.

Results: Treatment with caprylic acid did not precipitate IF, but IF precipitated at 40% ammonium sulfate saturation. IF bound to a Cibacron Blue gel column, from which it was eluted with a linear salt gradient; it also bound to protein G. Gel electrophoresis of purified IF showed two major bands with molecular masses corresponding to the heavy (∼50 kDa) and light chains (∼25 kDa) of immunoglobulin, and their identities were confirmed by mass spectrometry. The presence of troponin-specific autoantibodies was confirmed in samples with low recoveries of cTnI by three different immunoassays. The median signals were significantly higher in 10 samples with low recovery than in 10 samples with normal recovery of cTnI (P ≤ 0.007).

Conclusions: Circulating autoantibodies to cTnI or other proteins of the troponin complex can be a source of negative interference in cTnI immunoassays.

Advances in the clinical laboratory detection of gestational trophoblastic disease

BACKGROUND

Gestational trophoblastic disease (GTD) consists of a spectrum of disorders that are characterized by an abnormal proliferation of trophoblastic tissue. Gestational trophoblastic neoplasia (GTN) refers to a subset of GTD with a persistently elevated serum hCG in the absence of a normal pregnancy and with a history of normal or abnormal pregnancy. Although previously a lethal disease, GTN is considered today the most curable gynecologic cancer. However, a delay in the diagnosis may increase the patient's risk of developing malignant GTN, and therefore the prompt identification of GTN is important.

SERUM MARKERS

hCG test is essential for detection of GTN. It has emerged that there are problems with hCG tests. In addition to regular hCG, at least five major variants of hCG are present in serum samples. False-positive hCG (phantom hCG) can occur in the absence of GTN. Low-level real hCG may occasionally persist in the absence of clinical evidence of pregnancy or GTD. Alternatively, low-level real hCG may be due to pituitary hCG. Other placental hormones, human placental lactogen (hPL), inhibin and activin, and progesterone have also been evaluated as tumor markers for GTD.

CONCLUSION

hCG has high diagnostic sensitivity, approaching 100% sensitivity, for managing the treatment of GTN and for detecting recurrences of disease. It is recommended to use hCG test that recognizes all forms of the hCG molecule. In cases where low-level hCG persists, it must be differentiated whether it is real or false. Real-hCG may be due to quiescent gestational trophoblastic disease or pituitary hCG. It has not yet been established whether measurement of markers other than hCG (hPL, inhibin, activin, and progesterone) is useful in the detection and follow-up of GTD.

Common causes of troponin elevations in the absence of acute myocardial infarction: incidence and clinical significance

Cardiac troponin is a preferred biomarker of acute myocardial infarction (MI). Unfortunately, elevation of troponin can be detected in a variety of conditions other than acute MI. This review focuses on the incidence and clinical significance of increased troponin in conditions commonly associated with troponin elevations, particularly in those that may resemble acute MI.

Negative interference in cardiac troponin I immunoassays from a frequently occurring serum and plasma component

BACKGROUND

Cardiac troponin I (cTnI) is a sensitive marker of cardiac injury, but cTnI assays, like other immunoassays, are susceptible to interferences. We evaluated the presence of interfering substances by measuring the recovery of cTnI added to samples from volunteers and from patients with acute coronary syndromes (ACS).

METHODS

We added a ternary complex of human cardiac troponin (30-500 microg/L) or cTnI from serum to samples from healthy volunteers and ACS patients. We measured cTnI with a two-site sandwich time-resolved immunofluorometric assay using two antibodies against epitopes in the central stable part of cTnI. We also analyzed 108 heparin-plasma samples from 16 ACS patients with this assay, with an assay based on four antibodies, and with two commercial cTnI assays, AxSYM and ACS:180.

RESULTS

In samples from both healthy persons and ACS patients, recoveries for our assay were 1-167% (range). Recoveries were increased by addition of an antibody with an epitope in the N-terminal region of cTnI to the solid phase and an antibody with an epitope in the C-terminal region as a second detection antibody. In 2 of 16 patients with ACS, normal cTnI concentrations found when measured with the original assay demonstrated clinically abnormal (up to 10-fold higher) results with the additional N- and C-terminal antibodies in the early phase of infarction. Both commercial cTnI assays also demonstrated clinically misleading, falsely low cTnI concentrations.

CONCLUSIONS

Some yet unidentified, variable component, present in the blood from healthy volunteers and ACS patients, interferes with the binding of antibodies against epitopes in the central part of cTnI used in two commercial assays. Our approach to supplement the mid-fragment cTnI antibodies with antibodies in the N- and C-terminal parts of the molecule in an experimental assay represents a step in resolving this interferent.

Towards a better understanding of heterophile (and the like) antibody interference with modern immunoassays

BACKGROUND

Heterophile antibodies interfere with immunoassays. Understanding the nature and characteristics of these antibodies provides a format for better identifying and removing them. Growing evidence suggests many of these antibodies are natural antibodies. Very large number of tests are being performed with automated analyzers and there has been a problem with misdiagnosis due to interference. New commercial agents for blocking heterophile antibodies have been developed.

METHODS

Review of the immunology and methodological literature with critical interpretation of the findings.

CONCLUSIONS

Heterophile antibodies consist of natural antibodies and autoantibodies. Both types are usually weak antibodies that interfere by noncompetitive mechanisms. Based on very strong circumstantial evidence, we propose that natural antibodies account for most interference with automated immunoassays. In terms of false positive results, the interference rate is very low, about 99.95% accuracy. Specific blocking agents have some theoretical advantage over nonspecific blocking agents, but in actual practice, the very low false positive frequency makes it difficult if not impossible to statistically compare blocking agents or other assay modifications with adequate statistical power. In the absence of a technique that lends itself to automation for removing all immunoglobulins, it appears that infrequent heterophile interference cannot be avoided.

Performance of a Revised Cardiac Troponin Method That Minimizes Interferences from Heterophilic Antibodies

Background: Recent guidelines for use of cardiac troponin to detect cardiac damage and for cardiovascular risk stratification have made increasingly sensitive troponin assays important. Troponin assays continue to be plagued by interferences caused by heterophilic antibodies (HAs). We evaluated the performance of a revised cardiac troponin I (cTnI) assay designed to have increased analytical sensitivity and to minimize the effect of HAs.

Methods: The revised Dade Behring Dimension® cTnI assay was evaluated according to NCCLS EP5-A at five institutions. Plasma samples from 14 309 patients were assayed by the original Dimension cTnI assay. To identify samples that may have interfering HAs, samples with values &gt;1.4 μg/L were reanalyzed on the Dade Behring Stratus® CS cTnI assay. Samples with possible interfering antibodies were also analyzed before and after selective absorbance studies on the revised Dade Behring Dimension cTnI assay.

Results: The limit of quantification in the revised method was 0.1 μg/L with imprecision (CV) of 11–17% at 0.1 μg/L. Values correlated well with the Stratus CS cTnI method: revised = 1.06(original) + 0.01; r = 0.98, Sy|x = 0.25 μg/L). Falsely increased results consistent with myocardial infarction by the original Dimension cTnI assay and presumably attributable to HAs were identified in 0.17% of all patients with samples submitted for cTnI analysis. The revised Dimension cTnI assay eliminated the interference in 17 of 25 samples identified and greatly decreased the interference in the other 8.

Conclusions: The revised Dimension cTnI method greatly minimizes the effect of interfering HAs. It also exhibits analytical performance characteristics consistent with recent guidelines for use of this assay to detect cardiac damage.

Immunometric Assay Interference

Background: The primary aim of the study was to reduce interference in an in-house two-site, two-step immunometric assay.

Methods: In the running laboratory routine, 11 261 samples were tested with a carcinoembryonic antigen (CEA) assay with bovine immunoglobulin but no murine immunoglobulins in the buffer, in parallel to our routine CEA assay, using 15 mg/L heat-treated nonspecific murine immunoglobulin (MAK33) in the buffer and with the Fc fragments removed from the capture antibody.

Results: The frequency of interference was estimated to be 4.0% (95% confidence interval, 3.3–4.7%). The addition of 15 mg/L native MAK33 had little effect (frequency, 3.9%; 95% confidence interval, 3.2–4.6%), whereas adding 15 mg/L heat-treated MAK33 reduced interference to 0.86% (0.61–1.12%), and adding 50 mg/L reduced it further to 0.06% (0–0.13%). Removing the Fc fragments by itself reduced interference to 0.10% (0.02–0.19%). There were no statistically significant differences for age (P &lt;0.23) or gender (P &lt;0.40) between patients with interference (n = 210) and a randomly selected interference-negative control group (n = 186). Interference was not constant in patients: 15 of 25 individuals positive for interference and with four or more samples screened for interference had an interference-negative sample either before or after the peak of interference.

Conclusions: In a two-site, two-step immunometric assay using mouse monoclonal antibodies, use of heat-treated nonspecific murine immunoglobulin in the buffer or removal of the Fc fragment from the capture antibody could improve performance.

Matrix-dependent bias in total thyroxine measurement on the Beckman Access

BACKGROUND

Total thyroxine assays continue to be an integral part of thyroid status testing. We experienced a significant number of elevated total thyroxine values in patients with normal thyroid stimulating hormone and thyroxine binding globulin concentrations using the Beckman Access and hypothesized that these were due to a sample matrix effect.

METHODS

We compared the total thyroxine assays on the Beckman Synchron, Beckman Access, and Dade Dimension using individual patient specimens and two uniform matrices: a serum pool and an albumin-based matrix that were both supplemented with L-thyroxine to span the linear assay range.

RESULTS

Access total thyroxine values in individual patient specimens exhibited sporadic positive bias as high as 77 nmol/l (6 microg/dl) when compared to the Synchron and Dimension. The use of uniform matrices had little effect on the Synchron in comparison to the Dimension but significantly improved the agreement between the Access and the Dimension or Synchron as indicated by a statistically significant improvement in correlation coefficients.

CONCLUSIONS

The Access total thyroxine assay is prone to a variable and clinically significant positive bias that is mediated by a component of the sample matrix.

False serum calcitonin high levels using a non-competitive two-site IRMA

Dual site antibody-base immunoassays are commonly used in clinical laboratories to quantify the CT serum concentrations as a specific and sensitive marker of medullary thyroid carcinoma (MTC). Heterophile antibodies can interfere with these assays, however, and cause erroneous results. In order to avoid this interference, immobilized and conjugated antibodies from two different animal species or immunoreactive antibody fragments, as well as the addition of non-immune globulins, are generally included among the assay reagents. We describe the case of a 73-year-old man affected by a multinodular goiter, who showed high basal CT plasma levels as measured by a monoclonal antibody based IRMA. The finding of negative results for the presence of MTC at fine needle aspiration (FNA) and the mild increase observed in plasma CT during a pentagastrin (Pg) stimulation test, suggested that the high CT levels might depend on a cross-reaction with heterophilic antibodies. In fact, after the addition of the heterophilic blocking tube (HBT) to each specimen, the CT levels markedly decreased by more than 80% (average decrease+/-SE= 87.6+/-2.668%). Such a decrease strongly suggests that in our case the routinely used F(ab')2 fragments were unable to eliminate all of the interference and that the elevated serum CT levels might have been caused by human heterophilic antibodies. In conclusion, these results indicate a novel cause of CT false positivity, suggesting that high serum CT levels, when combined with a slight increase during Pg stimulation, should be critically interpreted in view of the possible presence of heterophilic antibodies in the specimens.

Multiplex analysis of heterophil antibodies in patients with indeterminate HIV immunoassay results

We hypothesized that heterophil antibodies reactive with animal proteins used in blot preparation caused nonspecific staining (NSS) on HIV Western blot (WB) studies, causing indeterminate results. We analyzed samples showing NSS on HIV WB using a multiplexed immunoassay to simultaneously measure IgG antibodies to animal IgG (bovine, goat, sheep, mouse) and bovine serum albumin. Heterophil antibodies reactive with IgG from several animal species were detected in 23 (49%) of 47 samples showing NSS on HIV WB; 15 positive samples demonstrated antibodies to all 5 antigens. Similar IgG heterophil antibodies were detected in only 2 (8%) of 24 control samples. Of the HIV WB samples with a positive HIV-1 enzyme-linked immunosorbent assay (ELISA) result at the time of WB testing (11/47), heterophil antibodies were found in 8 (73%) of 11. Preabsorption with bovine, goat, and sheep IgG removed heterophil antibodies detected by the multiplexed assay and, in some cases, eliminated reactivity in ELISA and WB testing. Heterophil antibodies are associated with indeterminate HIV immunoassay results and are an important cause of false-positive HIV ELISA results. Multiplexed immunoassays provide a powerful tool for screening patients for heterophil antibodies and resolving possible false-positive results.

An IgM λ Antibody to Escherichia coli Produces False-Positive Results in Multiple Immunometric Assays

Background: Interferences in immunometric assays as a result of human anti-immunoglobulin antibodies frequently have been described in the literature. The etiology of these interfering antibodies is usually not known but has been associated with rheumatoid factors in some assays. It is known that microorganisms in experimental settings can induce anti-immunoglobulin antibodies.

Methods: Following Escherichia coli septicemia, a 56-year-old male patient had increased immunoassay results for cardiac troponin I, thyrotropin, human chorionic gonadotropin, α-fetoprotein, and CA-125 that were consistent with myocardial infarction, hyperthyroidism, and pregnancy, and suggestive of an occult neoplasm such as hepatic or ovarian cancer. None of these diagnoses were consistent with the rest of his medical exam. In addition, the patient had a restricted IgM λ paraprotein by immunofixation. Plasma from the patient was incubated with Sepharose-conjugated protein A, irrelevant murine monoclonal antibodies, and formalin-killed E. coli organisms from his infection to determine whether these immunoassay values were falsely increased.

Results: Incubation of the patient’s plasma with irrelevant murine monoclonal antibodies or the E. coli organism produced normal immunoassay values and removed the IgM λ paraprotein.

Conclusions: The patient produced a very restricted IgM λ antibody response to the E. coli infection that had anti-immunoglobulin activity and caused falsely increased values in numerous immunometric assays. Microorganism-induced anti-immunoglobulin antibodies are discussed in the context of this patient.

Dual-Label Time-resolved Immunofluorometric Assay of Free and Total Prostate-specific Antigen Based on Recombinant Fab Fragments

Background: Recombinant Fab fragments are attractive as reagents for novel sandwich immunoassays, but no such assays have been described. We developed a dual-label two-site immunoassay based entirely on recombinant Fab fragments and compared it to the same assay with intact monoclonal antibodies.

Methods: The capture Fab fragment, which binds free prostate-specific antigen (PSA) and PSA in complex with α1-antichymotrypsin on an equimolar basis, is site-specifically biotinylated and attached to the solid phase in streptavidin-coated microtitration wells. The Fab fragment that detects only free PSA is site-specifically labeled with a fluorescent europium chelate, and the Fab fragment that detects both free and serpin-complexed PSA in an equimolar fashion is labeled with a fluorescent terbium chelate. Time-resolved fluorescence is used to measure both europium and terbium signals in one well.

Results: The detection limits of the assay (mean + 3 SD of zero calibrator) were 0.043 and 0.28 μg/L, respectively, for free and total PSA. The within-run and day-to-day CVs were 2–11% and 4–10%, respectively. Mean recoveries were 93% and 98% in female and male sera, respectively. Compared with the commercial ProStatus PSA Free/Total Assay, the intercepts of the regression equations (r &gt;0.99) were not significantly different from zero, and the slopes were 0.95–1.01. In one female serum sample, PSA was falsely increased with the monoclonal assay but was undetectable with the recombinant assay.

Conclusions: The performance of this novel assay based on recombinant components is comparable to a conventional assay based on monoclonal antibodies. The more complete control of the essential characteristics of site-specifically derivatized recombinant Fab fragments will be valuable for the design of miniaturized and multianalyte assay concepts where correct antibody orientation, density, and capacity as well as uncompromised binding affinity are required.

Human anti-murine antibodies interfere with CPR assays performed with commercial kits

Quantitation of C-peptide is important for the assessment of insulin secretion, in particular in patients receiving insulin therapy. Since the CPR levels become much higher than the concentration of C-peptide for several reasons, such as the high concentration of proinsulin, CPR values sometimes need to be assessed carefully. We have had two diabetic patients whose CPR values were abnormally high when determined with a Daiichi C-peptide kit III (method 1). CPR values determined by other methods were from two to ten times lower, indicating considerable interference when method 1 was used. Since method 1 uses mouse monoclonal antibodies (mmab) for detection antibodies, we suspected that human anti-murine antibodies (HAMA) were responsible for the interference. HAMA were detected in serum from both patients (45 and 460 ng/ml in case 1 and case 2 (at peak), respectively). Removal of HAMA from serum eliminated the interference. Modification of method 1 to exclude mmab from the assay system removed all interference. HAMA were, therefore, considered to be the cause of the interference. In case 2, the peak concentration of HAMA was recorded 16 months earlier than the maximum of interference. Further analysis revealed that HAMA with high affinities were responsible for the interference.

The new era of automated immunoassay

Use of immunoassays and other ligand-binding assays in clinical diagnosis has increased dramatically during the last several years. Despite impressive technical advances, "mass production" of these assays in a routine laboratory still presents many difficulties. This review of ligand-binding assay technology highlights some recent developments, emphasizing challenges and possible solutions for cost-effective patient care.

Transient nature of interference effects from heterophile antibodies: examples of interference with cardiac marker measurements

Two-site immunoassay methods have become the standard technique for measurement of a wide variety of drugs, hormones, and cell proteins. One limitation of these methods is their susceptibility to interference from heterophilic antibodies present in the sera of some patients. Human anti-murine antibodies represent a common heterophile antibody that can bind to mouse immunoglobulin and as well as to immunoglobulin from other species. While the mechanism of human anti-murine antibody interference has been well characterized, the time course over which this interference occurs and the susceptibility of different immunoassay procedures to human anti-murine antibody interference from patients with human anti-murine antibody have not been as well described. We report on the time course of interference in assays for cardiac markers for two patients with human anti-murine antibodies. We measured creatine kinase MB isoenzyme (CKMB) and troponins I and T using three different vendors' immunoassay procedures. Our results demonstrate that assay interference due to human anti-murine antibody interference is a transient phenomenon. In one of our patients, human anti-murine antibody interference appeared suddenly, peaked approximately 9 days following its appearance, and gradually resolved over the next 3 weeks. In addition, we found that immunoassay methods from different vendors can show highly variable interference effects when human anti-murine antibody-containing specimens are analyzed.

Human Anti-Animal Antibody Interferences in Immunological Assays

Purpose: The scope and significance of human anti-animal antibody interference in immunological assays is reviewed with an emphasis on human anti-animal immunoglobulins, particularly human anti-mouse antibodies (HAMAs).

Issues: Anti-animal antibodies (IgG, IgA, IgM, IgE class, anti-isotype, and anti-idiotype specificity) arise as a result of iatrogenic and noniatrogenic causes and include human anti-mouse, -rabbit, -goat, -sheep, -cow, -pig, -rat, and -horse antibodies and antibodies with mixed specificity. Circulating antibodies can reach gram per liter concentrations and may persist for years. Prevalence estimates for anti-animal antibodies in the general population vary widely and range from &lt;1% to 80%. Human anti-animal antibodies cause interferences in immunological assays. The most common human anti-animal antibody interferent is HAMA, which causes both positive and negative interferences in two-site mouse monoclonal antibody-based assays. Strategies to prevent the development of human anti-animal antibody responses include immunosuppressant therapy and the use of humanized, polyethylene glycolylated, or Fab fragments of antibody agents. Sample pretreatment or assay redesign can eliminate immunoassay interferences caused by anti-animal antibodies. Enzyme immunoassays, immunoradiometric assays, immunofluorescence, and HPLC assays have been designed to detect HAMA and other anti-animal antibodies, but intermethod comparability is complicated by differences in assay specificity and lack of standardization.

Conclusions: Human anti-animal antibodies often go unnoticed, to the detriment of patient care. A heightened awareness on the part of laboratory staff and clinicians of the problems caused by this type of interference in routine immunoassay tests is desirable. Efforts should be directed at improving methods for identifying and eliminating this type of analytical interference.

When Is a Heterophile Antibody Not a Heterophile Antibody? When It Is an Antibody against a Specific Immunogen

Heterophile antibodies are antibodies produced against poorly defined antigens. These are generally weak antibodies with multispecific activities. Human anti-animal antibodies that develop as a result of treatments with animal immunoglobulins are antibodies with strong avidities, produced against well-defined antigens. Although heterophile antibodies and human anti-animal antibodies interfere with immunological assays by similar mechanisms, modes for identifying the sources of the antibodies and for circumventing or retarding the interference may differ. Unfortunately, there has not been a well-organized attempt to encourage correct definition of these antibodies. This problem of inexact definition is highlighted by recent articles in this Journal. In the present discussion, we examine the history leading to this problem and discuss the origins and the reasons that the nature of the antibody is important for rectifying the problem. We propose a simple nomenclature for general usage that should appropriately characterize these antibodies in most cases.

Automated Homogeneous Immunoassay for Gentamicin on the Dimension Clinical Chemistry System

Background: Monitoring of the concentration of gentamicin in serum and plasma during therapy is widely recommended and practiced in hospitals. Our aim was to develop a homogeneous immunoassay based on particle-enhanced turbidimetric inhibition immunoassay technology to quantify gentamicin on the Dimension® clinical chemistry system.

Methods: Assay performance was assessed on each of the Dimension models in a 15-instrument interlaboratory comparison study. A split-sample comparison (n = 1171) was also performed between the gentamicin methods on the Dimension system and the Abbott®TDx® analyzer, using multiple reagent and calibrator lots on multiple instruments.

Results: The Dimension method was linear to 25.1 μmol/L (12.0 μg/mL) with a detection limit of 0.63 μmol/L (0.3 μg/mL). Calibration was stable for 30 days. The within-run imprecision (CV) was &lt;1.3%, and total imprecision ranged from 1.8% to 3.2% between 4.2 μmol/L (2.0 μg/mL) and 16.7 μmol/L (8.0 μg/mL) gentamicin. Linear regression analysis of the results on the Dimension method (DM) vs the Abbott TDx yielded the following equation: DM = 0.98TDx − 0.42; r = 0.987. Minimal interference was observed from structurally related compounds such as sagamicin, netilmicin, and sisomicin.

Conclusion: The monoclonal antibody used in this method has similar reactivities toward the individual gentamicin subspecies C1, C1a, and C2, thus providing analytical recovery not significantly dependent on relative subspecies concentrations.

Antibody interference in thyroid assays: a potential for clinical misinformation

Measurements of thyrotropin and of total and free thyroxine and triiodothyronine are widely used diagnostic methods for thyroid function evaluation. However, some serum samples will demonstrate a nonspecific binding with assay reagents that can interfere with the measurement of these hormones. Several recent case reports have described the presence of such interferences resulting in reported abnormal concentrations of thyroid hormones inconsistent with the patient’s thyroid state. Circulating thyroid hormone autoantibodies, described in thyroid and nonthyroid disorders, are an important class of interference factor and can bind to hormone tracers used in various immunoassays. Two additional categories of interfering antibodies may particularly interfere within two-site immunoassays for thyrotropin. These include heterophile antibodies, especially human anti-mouse antibodies, and rheumatoid factors, which can cause interferences by immunoglobulin aggregation and (or) cross-linking of both capture and signal antibodies. Here we review the nature of these disturbances; their occurrence, prevalence, and detection; and the clinical consequences of the failure to recognize such interference.

Reducing interference from heterophilic antibodies in a two-site immunoassay for carcinoembryonic antigen (CEA) by using a human/mouse chimeric antibody to CEA as the tracer

To reduce heterophilic antibody interference in a two-site immunoassay for carcinoembryonic antigen (CEA), we utilized a human/mouse chimeric antibody to CEA as the tracer. One mouse monoclonal antibody (MAb), F82-61, which reacts with an epitope present on the domain N of CEA, was immobilized on 96-well polystyrene microtiter plates. A human/mouse chimeric antibody (Ch F11-39), which recognizes an epitope present on the domain B3 of CEA, was biotinylated for the tracer (Ch F11-39 system). Another MAb F11-39, the parental MAb of Ch F11-39, was also biotinylated and used as the control tracer (F11-39 system). For a fair comparison, the same 503 serum samples from healthy individuals were simultaneously assayed in the present study. When a tentative common reference limit of 5 ng/ml was used, the false positive rate with the Ch F11-39 system was only 2.8% (14/503) and that with the F11-39 system was 29.0% (146/503). Adding normal mouse serum (NMS; 1%) or a mixture of purified mouse IgG subclasses (heterophilic blocking reagent (HBR, 15 micrograms/test)) to the F11-39 system reduced the false positive rate from 29.0% to 6.2% (31/503) or 4.8% (24/503), respectively, suggesting that heterophilic antibodies reactive with mouse IgG gave rise to the high positive rate in normal populations with the F11-39 system. On the other hand, the false positive rate with the Ch F11-39 system was only slightly reduced from 2.8% to 2.6% (13/503) or to 2.0% (10/503) by adding NMS or HBR to the Ch F11-39 system. The false positive rates with two commercially available assay systems, CEA Roche EIA.DM or Abbott IMx CEA, were 5.4% (27/503) and 5.8% (29/503), respectively, which both corresponded roughly to that with the F11-39 system including NMS or HBR. These results indicate that the application of human/mouse chimeric antibodies in two-site immunoassays is more effective for reducing interference from heterophilic antibodies than the adding of NMS or purified mouse IgG in the assay using conventional MAbs.

Monoclonal antibody based enzyme-linked and chemiluminescent assays for the human interleukin-1 receptor antagonist. Application to measure hIL-1ra levels in monocyte cultures and synovial fluids

Interleukin-1 receptor antagonist (IL-1ra) has the potential to counteract at least part of the biological effects of interleukin-1. The outcome of an inflammatory reaction may therefore be determined by the balance between IL-1 and IL-1ra, rather than by IL-1 alone. We have developed an immunoassay to address this issue as well as to assess the effects of anti-inflammatory agents on the expression of IL-1 and IL-1ra in vitro or in body fluids. Recombinant human IL-1ra was expressed in an E. coli system, purified to homogeneity, and used to derive monoclonal antibodies in mice as well as polyclonal antibodies in rabbits. A sandwich ELISA was constructed with F(ab')2 fragments of a high affinity monoclonal antibody and the rabbit serum as a source of secondary antibody. The assay required no sample treatment to avoid interference by rheumatoid factor. The measuring range was 0.020-2 ng/ml. By labelling a second monoclonal antibody with an acridinium ester, a chemiluminescence assay with a wider measuring range (0.050-15 ng/ml) was generated. In accord with published data, we found that IL-1ra was secreted by human monocytes stimulated with LPS, Zymosan, IL-1 alpha, or human IgG. After an induction phase of ca. 4 hours and depending on the stimulus, IL-1ra accumulated linearly for periods up to 96 h. IL-1ra levels in synovial fluids of 19 patients suffering from various inflammatory joint diseases were compared with the cytokine levels of IL-1 beta, IL-6, IL-8, and TNF-alpha. Highest positive correlations were found with IL-8 and IL-1 beta. In normal blood donors IL-1ra serum levels were 150-800 pg/ml (Median: 387 pg/ml). Owing to its sensitivity and large measuring range the newly developed assays appear to be suitable for measuring IL-1ra in cell cultures as well as in biological fluids.