Heterophilic antibodies produce spuriously elevated concentrations of the MB isoenzyme of creatine kinase in a selected patient population

The interference of monoclonal antibodies with laboratory diagnosis: clinical and diagnostic implications

Diagnostic test interference is due to the presence of material that falsely changes an analytic test result. The development of monoclonal antibodies is discussed with focus on their extensive use as both therapeutic and diagnostic agents. In this review the interference of monoclonal antibodies with laboratory test methods and the potential impact on clinical care is addressed. Recognition of the types of interference, endogenous and exogenous, and the varied mechanisms by which monoclonal antibodies may cause interference are discussed in this report. Review of the literature identifies cases which exemplify the issues facing laboratorians and clinicians and describe the impact on patients. Approaches to reducing and eliminating sources of interference are also addressed. Education of ordering clinicians concerning the possibility of interference in at-risk patients is key in limiting the impact on care. Laboratorians and medical practitioners should be cognizant of the risk of interference to avoid incorrect management of patients.

Identifying and reducing potentially wrong immunoassay results even when plausible and "not-unreasonable"

The primary role of the clinical laboratory is to report accurate results for diagnosis of disease and management of illnesses. This goal has, to a large extent been achieved for routine biochemical tests, but not for immunoassays which remained susceptible to interference from endogenous immunoglobulin antibodies, causing false, and clinically misleading results. Clinicians regard all abnormal results including false ones as "pathological" necessitating further investigations, or concluding iniquitous diagnosis. Even more seriously, "false-negative" results may wrongly exclude pathology, thus denying patients' necessary treatment. Analytical error rate in immunoassays is relatively high, ranging from 0.4% to 4.0%. Because analytical interference from endogenous antibodies is confined to individuals' sera, it can be inconspicuous, pernicious, sporadic, and insidious because it cannot be detected by internal or external quality assessment procedures. An approach based on Bayesian reasoning can enhance the robustness of clinical validation in highlighting potentially erroneous immunoassay results. When this rational clinical/statistical approach is followed by analytical affirmative follow-up tests, it can help identifying inaccurate and clinically misleading immunoassay data even when they appear plausible and "not-unreasonable." This chapter is largely based on peer reviewed articles associated with and related to this approach. The first section underlines (without mathematical equations) the dominance and misuse of conventional statistics and the underuse of Bayesian paradigm and shows that laboratorians are intuitively (albeit unwittingly) practicing Bayesians. Secondly, because interference from endogenous antibodies is method's dependent (with numerous formats and different reagents), it is almost impossible to accurately assess its incidence in all differently formulated immunoassays and for each analytes/biomarkers. However, reiterating the basic concepts underpinning interference from endogenous antibodies can highlight why interference will remain analytically pernicious, sporadic, and an inveterate problem. The following section discuses various stratagems to reduce this source of inaccuracy in current immunoassay results including the role of Bayesian reasoning. Finally, the role of three commonly used follow-up affirmative tests and their interpretation in confirming analytical interference is discussed.

Spurious hypercalcitoninemia in patients with nodular thyroid disease induced by heterophilic antibodies

BACKGROUND

Serum calcitonin is the most useful tumor marker for the diagnosis and follow-up of medullary thyroid carcinoma (MTC). Spurious hypercalcitoninemia caused by heterophilic antibody interference (HAI) is rarely found in patients without MTC.

METHODS

We studied 2 patients with hypercalcitoninemia and thyroid nodules, but no evidence of MTC on fine-needle aspiration cytology. We performed calcium stimulation tests, measured serum calcitonin with another calcitonin kit, performed dilution tests, and remeasured serum calcitonin after applying heterophilic blocking tubes.

RESULTS

In a 31-year-old woman with no response to the calcium stimulation test, serum calcitonin was <5 pg/mL using another kit. After we applied heterophilic blocking tubes, the serum calcitonin level decreased to normal range. We concluded that patient had spurious hypercalcitoninemia. In a 63-year-old woman, all tests revealed that the patient had true hypercalcitoninemia. The patient underwent total thyroidectomy that revealed MTC.

CONCLUSIONS

We suggest that patients suspected for spurious hypercalcitoninemia should undergo further investigation due to HAI.

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.

Interferences in hormone immunoassays

Despite the numerous potential interferences that were discussed in this article, immunoassays, in general, are robust measurement systems. There is no practical way to identify specimens a priori that are likely to have immunoassay interference. Therefore, laboratories must rely on communication from clinicians to identify suspicious test values that may be caused by assay interference. After laboratories are alerted, multiple investigations can be undertaken. The common causes of immunoassay interferences (see Box I) are dependent on the analytic format. Competitive assays are most likely to have problems with cross-reactivity, whereas immunometric assays most often have problems with heterophile antibodies. The simple steps to test for dilutional linearity, spiked recovery, heterophile antibody blocking,and testing with alternate technology can help in the evaluations of cases that are suspected to have assay interference.

Do glycoprotein IIb/IIIa inhibitors interact with assays for myocardial necrosis?

OBJECTIVES

To determine whether glycoprotein (GP) IIB/IIIA inhibitors interact with cardiac marker assays: total creatine kinase (CK), CK-MB, and troponin-I (TnI).

DESIGN AND METHODS

Two blood samples were collected during coronary angiography or angioplasty and analyzed for cardiac markers: one after arterial sheath insertion and the second 10 mins later. In the first phase (n=101), heparin and GP IIb/IIIa inhibitors were given between samples, in the second phase (n=45) only heparin was given, and in the third phase (n=54) neither drug was given.

RESULTS

In each phase, there was a significant decline in total CK over the 10 min period. There were no consistent significant changes in CK-MB or TnI.

CONCLUSIONS

Neither GP IIb/IIIa inhibitors nor i.v. heparin interact with cardiac marker assays. There is a small but significant decline in total CK levels within 10 mins of arterial puncture that is unrelated to administration of these drugs.

The measurement of cardiac markers: where should we focus?

Cardiac markers are presently a hot topic, with active debate on their use. They now have a major role for cost-effective management of acute chest pain and suspected acute coronary syndrome. The laboratory has a pivotal role in proper selection and interpretation of available markers, depending on the creation of evidence-based knowledge about test utilization and sources of variation. This article reviews this knowledge in the field of biomarkers determination and summarizes the major analytic and clinical issues, with reference to various recent recommendations of laboratory medicine and cardiology expert groups.

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.

Recent approaches in standardization of cardiac markers

The development of commercial assays for the determination of new cardiac proteins has been one of the most important innovations in the field of cardiovascular diagnostics in the last decade. This significant and sudden advancement has however led to some analytical and interpretative problems. There are problems in test standardization, imprecision, interference and preanalytical variability. We also need to standardize utilization of biomarkers in diagnosis and management of acute cardiac syndromes and clearly define decision thresholds. Powerful tests, such as cardiac markers, on which critical decisions will rest, need highly reliable methods. The feeling is that some assays are inadequately appraised prior to their introduction in clinical use. More studies are needed to implement new devices in the laboratory routine, and only well documented assays should be used in hospital-based laboratories. The technology to address many analytic problems is at hand, but commitment on the part of manufacturers and their customers in the laboratory community is essential.

Identification of human serum interferants in the recombinant P-selectin glycoprotein ligand-1 clinical ELISA using MALDI MS and RP-HPLC

A colorimetric enzyme-linked immunosorbent assay (ELISA) was developed to detect circulating levels of rPSGL to permit pharmacokinetic analysis of clinical samples. The ELISA is an asymmetric sandwich utilizing a monoclonal antibody pair. Initial validation studies indicated that 57% of normal individuals scored above the limit of detection of the assay. Specificity experiments indicated that the signal was not due to circulating endogenous P-selectin glycoprotein ligand-1 (PSGL-1). Using matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) and sampling within the individual microplate wells, the interferant was detected in the vicinity of 6.6 kDa in lipemic and normal human sera, but not delipidized sera. These results were consistent with the ELISA data where 97.5% of known lipemic, 57% of normal, and 0% of delipidized sera scored above detectable limits in the ELISA. Preparative isolations of the 6.6 kDa species were performed using reversed-phase high performance liquid chromatography (RP-HPLC) with UV and MS detection. Edman N-terminal sequencing identified the 6.6 kDa unknown as Apolipoprotein C-I. Additional apolipoproteins were found by MALDI and RP-HPLC. Digestion of sera with liposome lipase and extraction of sera with anti-apolipoprotein C-I, C-II, and C-III antibody beads significantly reduced the ELISA interference. These experiments combined with the MALDI detection of phosphatidylcholine-type lipids from NHS eluate suggested that lipoprotein particles or remnants were causing the interference. A method combining Triton-X 100 with sonication was developed to overcome this interference without altering rPSGL recovery in the ELISA.

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.

Present issues in the determination of troponins and other markers of cardiac damage

OBJECTIVE

To review some of the recently proposed improvements and the corresponding apparent issues in the field of biochemical markers of cardiac damage.

CONCLUSIONS

The continuous development of new analytical tools for the biochemical evaluation of patients with suspected myocardial injury brings without doubt new challenges of careful technological evaluation, implementation, and standardization but it may also provide a unique opportunity to markedly enhance our diagnostic performance in the clinical setting of acute coronary syndrome.

Performance of the enhanced Abbott AxSYM cardiac troponin I reagent in patients with heterophilic antibodies

The presence of heterophilic antibodies in the serum of a small subpopulation of individuals continues to cause false results for modern-day immunoassays. In order to determine the frequency of heterophilic antibody (HA)-related false positives within our population of positive cardiac troponin I (cTnI) patients, we assayed 200 samples using the original in-house cTnI assay (Abbott AxSYM) and the Bayer ACS:180 cTnI, which we had previously observed to be more effective at blocking HA interference. Four samples were identified as false positives based on discordant results between the two assays, as well as the correction of the false positives by treatment of the samples with heterophilic antibody blocking reagent (HBR). An 'enhanced' version of the AxSYM cTnI reagent was designed to greatly reduce or eliminate HA interference, and has now replaced the original reagents. The present study shows that the enhanced reagent significantly reduced or eliminated much of the HA interference. Comparative studies between the enhanced cTnI reagent and the original Abbott AxSYM cTnI reagent showed excellent correlation and equivalent diagnostic concordance, when HA samples were excluded from the analysis.

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.

Heterophile antibodies to bovine and caprine proteins causing false-positive human immunodeficiency virus type 1 and other enzyme-linked immunosorbent assay results

Heterophile antibodies are a well-recognized cause of erroneous results in immunoassays. We describe here a 22-month-old child with heterophile antibodies reactive with bovine serum albumin and caprine proteins causing false-positive results to human immunodeficiency virus type 1 and other infectious serology testing.

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.

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.