Heterophilic Antibody Interference Research Articles

Point-of-care testing: false elevation of cardiac troponin I assayed in the emergency department

Abstract In October 2007, an 18-year-old woman with no cardiac history was admitted to the emergency department for a major epigastric pain. The chest pain led to assay cardiac troponin I (cTnI) with the emergency department point-of-care testing analyzer (Stratus CS), which disclosed a value of 0.70 ng/mL, discordant with the atypical clinical presentation and the noncontributive electrocardiography. The biologist contacted for biological advice controlled cTnI with an Access II, which disclosed a value less than 0.04 ng/mL. These findings were confirmed with the discordant results of a new sample assayed on both analyzers. The interference of heterophilic antibodies (HAs) was suspected because these antianimal antibodies may lead to analytical errors in sandwich immunoassays using animal sources of immunoglobulins and can cause false-positive results. In this case, the HAs bind to the capture antibody and the conjugate antibody, simulating cTnI. The diagnosis of HA involvement was confirmed using Heterophilic Blocking Tube, a device that contains a blocking reagent composed of specific binders that attach HA. After treatment in Heterophilic Blocking Tube, the cTnI concentration measured by the Stratus CS decreased from 0.62 to 0.05 ng/mL. Finally, potentially invasive test or this patient's unnecessary hospitalization in cardiology was avoided. Our experience supports that collaboration between staffs of laboratories and medical departments owning point-of-care testing analyzers is essential to avoid mistaken diagnosis linked to analytical interferences and to ensure quality of results assayed outside the laboratory.

Heterophile Antibody Interference with Thyroid Assay

Immunoassays are one of the main methods of measuring thyroid hormones. They are prone to interference. Interference of the immunoassays has been increasingly recognized as a problem because it may lead to misinterpretation of results resulting in unnecessary investigation and wrong course of treatment. We describe a patient who had interference in her thyroid function measurements (free T4, free T3 and TSH) during the course of treatment for Graves' disease.

Heterophilic antibody interference in a tryptase immunoassay

Objective Investigation of the susceptibility of a tryptase immunoassay to interference by heterophilic antibodies. Methods The effect of preincubation with a blocking agent was investigated on the levels of tryptase, human anti-mouse antibodies and IgM rheumatoid factor in sera with elevated IgM rheumatoid factor levels. Results In 5 of 30 sera with IgM rheumatoid factor, tryptase levels were reduced at least twofold after pre-incubation with blocking reagent. A significant association was observed between the presence of IgM rheumatoid factor in the sera and the interference of tryptase immunoassay. There was no quantitative correlation found between the reduction in serum tryptase level by treatment with a blocking agent, and the amount of IgM rheumatoid factor was present. However, this reduction in serum tryptase was significantly correlated with the amount of human anti-mouse antibodies in the sera. After incubation with blocking agent, there was no change in IgM Rheumatoid factor level, but a significant decrease in human anti-mouse antibodies. Conclusion The Phadia tryptase assay method, in its present form, is sensitive to interference by heterophilic antibodies.

Very High Inhibin A Concentration Attributed to Heterophilic Antibody Interference

Maternal serum screening for Down syndrome is commonly performed in the 2nd trimester using α fetoprotein (AFP), unconjugated estriol (uE3), human chorionic gonadotropin (hCG), and inhibin A. Concentrations of each marker are combined with maternal age to calculate a patient-specific risk of fetal Down syndrome. In cases of Down syndrome, inhibin A concentration is, on average, approximately twice as high as in unaffected singleton pregnancies (1). Second trimester maternal serum inhibin A is also increased in twin pregnancies [1.99 multiples of the median (MoM) (1)] and in Turner syndrome with hydrops (3.91 MoM; (2)). Markedly increased inhibin A has been observed in pregnancies with complete hydatidiform mole [4–7 MoM; (3)]. Increased inhibin A may also be seen in nonpregnant women with ovarian cancer (4). We describe a woman having 2nd trimester serum inhibin …

Heterophile antibody interference in a solid phase sandwich immunoassay for detection of equine growth hormone in plasma

Heterophile antibodies (HAs) present in serum recognize animal immunoglobulins and are one of the most unpredictable causes of false results in immunoassays. However, no study has yet reported their interference on the diagnostic reliability of immunochemical analyses on horse plasma. Recently, we developed a sandwich ELISA for detection of equine growth hormone (eGH) in plasma. In a pilot study to measure basal eGH levels (blood samples were drawn from 13 horses every 10 min for 1 h), we noted one horse with abnormally high eGH (>100 ng/mL). We demonstrate here that this plasma eGH level was falsely elevated due to interference from HAs. The interfering antibodies were polyspecific immunoglobulins, with fairly broad species-specificity, which affected the eGH immunoassay by bridging the mouse IgG capture antibody and the rabbit IgG conjugate. This produced artificial sandwiches which led to overestimation of the eGH plasma concentration. Spiking horse plasma with pure mouse and rabbit immunoglobulins or whole plasma of several species significantly reduced but did not totally eliminate the HAs interference. Immunoglobulins and whole plasma differed in their ability to block the interference, suggesting that HAs may recognize other proteins beside immunoglobulins in animal sera. To investigate whether HAs have any implications in equine clinical practice, we decided to seek information on the incidence of HAs interference in normal animals. We collected single plasma samples from another 114 horses and we found that 5 of these had plasma HAs. Therefore, in total 6 out of the 127 horses examined (4.7%) had plasma HAs generating falsely elevated eGH measures. In conclusion, this study provides the first evidence of HAs in horse plasma interfering with an immunoassay and indicates that veterinary surgeons and diagnostic laboratory staff should be aware of this potential for interference in tests on horse plasma using monoclonal or polyclonal antibody reagents.

Performance Evaluation and Subsequent Clinical Experience with the Abbott Automated Architect STAT Troponin-I Assay

Cardiac troponins are specific biochemical markers of myocardial injury used in the diagnosis of acute myocardial disease and cardiac risk stratification. To avoid misclassification of patients, troponin assays must demonstrate precision at the low end of the measuring range. We report our evaluation of the Architect STAT Troponin-I assay (Abbott Diagnostics), comparison of low-positive results with 2 other assays, and occurrence of heterophile antibody interference in the assay. We assessed analytical performance on the ci8200 according to CLSI protocols, using quality-control and patient samples. Our healthy reference population included 480 blood donors. For correlation studies against the AxSYM first-generation cTnI (Abbott Diagnostics) and Access second-generation AccuTnI (Beckman Coulter) assays, we used 339 samples from hospital patients. The CV of the Architect STAT Troponin-I assay was 10% near the 99th percentile for the reference population (0.03 microg/L). Comparison with the AxSYM first-generation cTnI assay showed good correlation at higher concentrations, but better sensitivity of the Architect cTnI assay at low concentrations, which were clinically relevant as shown by review of patient histories. Correlation was good at the low end of the measuring range with the Access second-generation AccuTnI. Over the last 12 months we have identified 6 patients with heterophile antibodies causing positive interference. The Architect STAT Troponin-I assay provides highly sensitive measurement of cTnI with a CV of 10% near the upper limit of a reference population; however, heterophile antibodies can interfere with this assay.

Heterophile antibodies may cause falsely lowered serum cortisol values

We describe a case where interference by heterophile antibodies in a cortisol immunoassay led to a falsely lowered serum cortisol level. As part of a diagnostic workup for fatigue, a 42-yr-old woman was found to have a low 9.00 h serum cortisol and an abnormal Synacthen stimulation test consistent with adrenal insufficiency. On insulin hypoglycemia testing, markedly discrepant cortisol levels were obtained from paired samples tested by two different immunoassays. Further tests, including plasma ACTH and overnight metyrapone test, confirmed normal hypothalamic-pituitary-adrenal axis function. Negative interference on the initial immunoassay from a heterophile antibody in the patient's serum was detected, thereby explaining the abnormal results. This report demonstrates that potentially significant consequences can arise from the failure to consider heterophile antibody interference. Clinicians who interpret test results need to be aware of the potential for interference in immunoassays by heterophile antibodies.

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

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. 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. After correction for bias and analytical variation [2.56 x SD (from the precision profile)], 383 samples displayed significantly different values (>1 microg/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. 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.

Human heterophilic antibodies display specificity for murine IgG subclasses

Objectives: The study investigated heterophilic antibodies: the human immunoglobulin classes involved and their specificity for different murine IgG subclasses. Design and methods: Using immunofluorometric assays for human IgA, IgM and IgG binding murine IgG1, we analyzed 173 samples displaying positive interference and 97 negative control samples from a previous study. We also set up assays for heterophilic antibody interference using Mabs from different murine IgG subclasses. Three Mabs each of murine IgG1, IgG2a and IgG2b subclasses, one murine IgG3 Mab and one rat Mab were used. Results: Elevated levels of human murine IgG1-binding immunoglobulins of IgM class only were found in 40% of interference-positive samples, human IgG only in 1.7%, and human IgA only in 2.3% of the samples. Both elevated human IgG and IgM classes were found in 3.5% of the samples, IgA and IgM in 4.0%, and finally, all three immunoglobulin classes in 1.7% of the samples. Eighty percent of interference positive samples showed heterophilic assay interference for at least one murine IgG1 Mab, 35% for IgG2a, 66% for IgG2b, 52% for IgG3a and 17% for the rat Mab. Conclusions: Heterophilic antibody interference is mainly caused by IgM class human antibodies with a marked murine IgG subclass specificity. Combining assay antibodies from different murine IgG subclasses may reduce interference in immunoassays.

Techniques for Identifying Heterophile Antibody Interference Are Assay Specific: Study of Seven Analytes on Two Automated Immunoassay Analyzers

Interference by human anti-animal immunoglobulins, commonly referred to as heterophile antibodies, in immunologic assays is known to be an important consideration for medical testing laboratories (1)(2)(3). Although automated immunometric assays are formulated to reduce these effects, it is unlikely that complete elimination occurs (2), and artifactual results attributable to heterophile antibodies have been reported for some assays (4)(5). Such results often are identified by addition of blocking agents to the samples before assay. A simple sample pretreatment method uses a commercially available blocking tube, HBT. An alternative technique uses polyethylene glycol (PEG) to precipitate immunoglobulin-sized molecules before assay. For both of these techniques, a difference between values for the treated and untreated specimens is interpreted as evidence for heterophile antibody interference. It is not clear that either of these methods is appropriate for every immunoassay. It is important to know the effect of sample pretreatment on the results of each assay. This has been done recently for HBT and a thyroglobulin assay (4). We examined seven automated analyzer hormone assays, using pretreatments with both HBT and PEG of samples from healthy adults, with the aim of determining the expected change in results post treatment for both techniques for each assay. We investigated luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin (PRL), and growth hormone (GH) on the Access2 analyzer and insulin, parathyroid hormone (PTH), and cross-linked C-terminal telopeptide of type I collagen (β-CTx; β CrossLaps) on the Elecsys analyzer. Analyzer reagents and consumables were obtained from Beckman-Coulter and Roche Diagnostics, respectively. Plasma (EDTA) samples were collected from healthy individuals in a study approved by the …

The War on Heterophilic Antibody Interference

Human heterophilic antibodies may bind the animal antibodies used in an immunoassay and thus produce erroneous results. Such interference has been increasingly recognized as a diagnostic problem (1)(2). In this issue of Clinical Chemistry , Adel Ismail (3) discusses the two main issues of heterophilic antibody interference in immunometric assays: how to detect it and how to avoid it. This is an important discussion to which all clinical chemists should feel invited. This field requires not only a profound understanding of heterophilic antibody interference but also hands-on experience with immunoassays. To detect heterophilic antibody interference in the sample, the first critical step is to suspect it. The starting point is often a clinician contacting the laboratory about a mismatch between the clinical information and laboratory results. Once there is suspicion of such interference, a classic approach has been to make serial dilutions of the sample. As Ismail (3) points out, most samples with heterophilic antibody interference will then display nonlinearity. The author does not explain this nonlinearity but draws attention to the polyclonality of interfering antibodies, with affinities ranging from high to low, and he points out that with human antibodies more than one functional binding site is often involved. Human antibodies with high affinities for animal antibodies are specific for one or more animal species and do not bind human immunoglobulins. High-affinity human anti-human immunoglobulins would instantly form larger complexes, either to be removed from circulation or to cause serious disease. In our experience before the surge in therapeutic use of murine or humanized murine monoclonal antibodies, we found little evidence of interfering immunoglobulins from the immunoglobulin classes that normally exhibit higher affinities, i.e., IgA or IgG (personal observation). Low-affinity human antibodies often bind to both animal and human immunoglobulins, making heterophilic antibodies (human anti-animal) and rheumatoid …

Prevalence of Heterophilic Antibody Interference in Eight Automated Tumor Marker Immunoassays

Heterophilic antibodies are human antibodies that can bind animal antibodies. They can cause problems in immunoassays, particularly immunometric assays, where they can form a bridge between the capture and detection antibodies, leading to a false-positive result in the absence of analyte or, if analyte is also present, to a false increase in measured concentrations. Very rarely, heterophilic antibodies can also lead to false-negative or falsely low results. By adding blocking reagents, assay manufacturers have reduced the incidence of heterophile interferences from the 2–5% observed in unblocked assays but have been unable to completely eliminate the problem (1)(2). During the last 10 years, tumor marker assays for human chorionic gonadotropin (hCG), prostate-specific antigen (PSA), cancer antigen 125 (CA 125), carcinoembryonic antigen, and calcitonin have all been reported to suffer from this problem, frequently with undesirable clinical outcomes (3)(4)(5)(6)(7). In our own laboratory we noted that our thyroglobulin assay displayed a higher than acceptable rate of heterophile interference (8). Although we were able to rectify this problem through pretreatment of samples in heterophile blocking tubes (HBT) (8), we became concerned that some of our other tumor marker assays may suffer from similar problems. We therefore decided to systematically evaluate all of our tumor marker assays for evidence of interference from heterophilic antibodies. Our laboratory performs the following tumor marker assays: calcitonin on the Nichols Advantage, gastrin on the Immulite2000, CA 125 and cancer antigen 15-3 (CA 15-3) on the Vitros ECi, and α-fetoprotein (AFP), hCG, total PSA, and free PSA on the Beckman Access. All of these assays use at least one mouse monoclonal antibody (Table 1⇓ ). We designated the maximum tolerable heterophile interference rate for any of these assays as 0.5% and started collecting samples prospectively. Power calculations …

A method for routine detection of heterophile antibody interference in the Abbott AxSYM hCG assay

A method for routine detection of heterophile antibody interference in the Abbott AxSYM hCG assay

Heterophile Antibody Interference in a Multiplexed Fluorescent Microsphere Immunoassay for Quantitation of Cytokines in Human Serum

While modern immunoassays provide sensitive and specific means for the quantitation of cytokines in biological fluids, heterophile antibodies are still a well-recognized cause of interference in the measurement of cytokines in these assays. We have developed a multiplexed fluorescent microsphere immunoassay for the simultaneous quantification of 10 cytokines in only 75 microl of serum. During the development of this multiplexed assay, the amount of assay interference due to heterophile antibodies was also determined, and methods for detecting heterophile interference and minimizing its effect were incorporated into the assay. Heterophile antibodies resulted in significantly elevated cytokine values compared to those of normal blood bank samples. These falsely elevated values, and thus the components of the assay the heterophile antibodies were binding to, were identified through the use of internal controls. This information was then used to design assay-specific blockers and absorbents that were shown to significantly reduce falsely elevated cytokine values while not affecting the standard and control values. The fluorescent multiplexed microsphere-based immunoassay can be used to quantitate multiple cytokines from a single sample and should be a useful tool in furthering our understanding of the role of cytokines in disease processes.

Phantoms in the assay tube: heterophile antibody interferences in serum thyroglobulin assays.

Serum thyroglobulin (Tg) measurement is a major means of detecting thyroid cancer recurrence. Unlike anti-Tg autoantibody interferences, heterophile antibody (HAB) immunoassay interferences are not well recognized by laboratorians or clinicians as a Tg assay problem. When HAB interferences occur, they usually result in false positive test results. With the current trend to treat some thyroid cancer patients with radioiodine on the basis of an elevated serum Tg result alone, this has the potential to result in unwarranted therapy. We evaluated the prevalence of HAB interference in a commonly used automated immunoassay in 1106 consecutive specimens with Tg values greater than 1 ng/ml. All Tg measurements were repeated after sample incubation in heterophile-blocking tubes (HBT). Results, which showed a more than 3 SD percentage difference from the original result, were considered to suffer from HAB interference. All possible interferences were confirmed by dilution testing. After HBT treatment, Tg levels dropped to less than 1 ng/ml in 32 specimens (P < 0.0000001), 20 of which fell to less than 0.1 ng/ml (P < 0.00002). Of these 20, 17 were anti-Tg autoantibody negative, and all 32 showed a fall of greater than 3 SD percentage (>56.91%) compared with the original result. There were also two samples that showed a significant increase of greater than 56.91% after HBT treatment. HAB interference is relatively prevalent (1.5-3%) in a commonly used automated Tg assay and can lead to clinically significant artifacts. It is currently unknown, but possible, that other immunometric Tg assays suffer from similar problems. Unless a Tg assay is confirmed to be free of HAB interference or uses additional blocking steps, as ours now does, HAB interference should be suspected if Tg results do not fit the clinical picture.

Rules-based detection of discrepancies between TSH and free T 4 results

Background: Analytical errors in clinical laboratory testing are unavoidable. Recent reports have suggested the idea of “physiological profiling” which uses several results from a given patient to identify clinically unlikely results. The objective of this study was to establish rules-based criteria for identifying physiologically unlikely TSH and free T 4 (fT 4) results. Methods: For a 30-month period, all samples with fT 4 concentrations >2 ng/dl and TSH concentrations >0.1 μIU/ml were investigated. Results: Among 7918 plasma samples for which both TSH and fT 4 concentrations were measured, 18 (0.23%) had fT 4 and TSH exceeding the investigated limits. Of these, two were due to heterophile antibody interference (with the TSH assay), one was proven to be due to random error, four could be explained by the patients' conditions, three were from infants <1 week of age, and the remaining eight were unresolved, primarily due to insufficient sample, discarded samples, and inability to obtain patient histories. Conclusions: This study defined a rules-based alert system for clinically unlikely combinations of TSH and fT 4 results. This pilot study demonstrates that this system is capable of detecting at least two different types of laboratory errors that would have otherwise gone undetected.

SERUM HETEROPHILE ANTIBODIES INTERFERE WITH PROSTATE SPECIFIC ANTIGEN TEST AND RESULT IN OVER TREATMENT IN A PATIENT WITH PROSTATE CANCER

We evaluated how naturally occurring heterophile antibodies in patient serum interfered with prostate specific antigen (PSA) immunoassay, resulting in over treatment for prostate cancer. Serum samples were treated with heterophilic blocking reagent (Scantibodies Laboratory, Inc., Santee, California). Treated and untreated samples were tested by the Medics (Tosoh, Foster City, California ) Tandem-R (Beckman-Coulter Inc., Chaska, Minnesota) and Elecsys (Roche Molecular Biochemical, Indianapolis, Indiana) PSA assays. Heterophile antibodies were measured directly in treated and untreated samples by the human anti-mouse antibody immunoradiometric assay and heterophilic antibody identification enzyme immunoassay (Scantibodies Laboratories, Inc.). Human anti-mouse Ig heterophile antibodies in patient serum caused false-positive PSA test findings after radical prostatectomy, resulting in over treatment for presumed disease recurrence. If PSA is detectable after radical prostatectomy and the likelihood of incomplete resection or systemic disease is low, the presence of heterophile antibodies should be considered.

Effect of Rheumatoid Factor on Cardiac Troponin I Measurement Using Two Commercial Measurement Systems

The Abbott AxSYM cardiac troponin I (cTnI) immunoassay may generate false-positive results in the presence of specific human anti-animal antibodies (HAAAs) (1). We use the term HAAAs rather than heterophile antibodies, a term used for weak, multispecific antibodies against diverse antigens (2), either human or animal. Techniques are available to detect and correct the interference attributable to HAAAs. We have used the Heterophile Blocking Tube (HBT; Scantibodies Inc.) to identify interference from suspected HAAAs. Anecdotal reports and experience in our laboratory have also suggested that the Abbott AxSYM cTnI may also be susceptible to interference from rheumatoid factor (RF), a heterophile antibody. Abbott Laboratories has recently modified its cTnI reagent system to address the issue of HAAA and heterophile antibody interference. The objective of this study was to compare the Beckman Access …

Heterophilic antibodies remain a problem for the immunoassay laboratory.

The prevalence of heterophilic antibody interference in a modern immunochemiluminometric assay containing blocking agents was determined using thyrotropin as an illustrative example. Serum samples were obtained from 295 consecutive patients who underwent routine thyroid function testing. The following versions of the thyrotropin assay were used: protocol A (zero blocker), protocol B (routine blocker concentration), and protocol C (extra blocker). Ten patients (prevalence 3.4%) had significant levels of heterophilic antibodies (protocol A value greater than 9 SD from the protocol B value). The observed thyrotropin levels for protocols B and C were the same for all patients, consistent with the reagent blockers in routine assays adequately eliminating heterophilic antibody interference. However, seven more patients (0.03%) in series of 21,000 assessed by routine thyroid function testing had discordant results because of a concentration of heterophilic antibodies so high as to overwhelm the added blocking agents.

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 μg/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 MMS 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.