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Journal of Clinical Microbiology, November 2004, p. 4996-5000, Vol. 42, No. 11
0095-1137/04/$08.00+0 DOI: 10.1128/JCM.42.11.4996-5000.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Department of Pathology and Immunology, Division of Laboratory Medicine, Washington University School of Medicine, St. Louis, Missouri
Received 16 March 2004/ Returned for modification 22 May 2004/ Accepted 2 July 2004
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The diagnosis of acute EBV infection can be challenging since the symptoms are typically protean, resembling those of other viral infections. Although the presence of atypical lymphocytes on a peripheral smear supports the diagnosis of infectious mononucleosis, these are not universally detectable. Furthermore, because of the impracticality of EBV culture and the high prevalence of EBV in the population, specific viral detection by culture or PCR is not frequently used. Alternatively, the detection of heterophile antibodies and specific antibodies against EBV antigens has proven useful in the diagnosis of acute EBV infection, especially in cases with a high index of suspicion and a lack of atypical lymphocytes.
There are four basic categories of EBV status into which patients can be divided by serology. These include patients with no history of infection, patients who are acutely infected, patients with evidence of a prior or remote infection, and patients with reactivation of latent virus (14). Anti-EBV serology, in combination with heterophile antibody testing, has been used to categorize patients in this manner, and the classification of patients based on the pattern of antibody responses has been reviewed elsewhere (9, 13, 14). Briefly, acute disease is suggested when immunoglobulin M (IgM) antibodies to the viral capsid antigen (VCA) are detected. The presence of heterophile antibodies and/or IgG antibodies to the D component of the EBV early antigen (EA-D) further supports the diagnosis of acute infection. In patients with a prior or remote infection, anti-VCA IgG and IgG antibodies to the EBV nuclear antigen (NA) are typically observed. The anti-VCA IgG antibodies slowly rise during acute infection but persist for life. In contrast, the anti-VCA IgM and anti-EA-D IgG antibodies rapidly decline as the patient recovers and are typically undetectable after 12 months (8, 13, 14). Patients who have reactivated latent virus will likely have evidence of past infection, e.g., anti-VCA IgG and anti-NA IgG, but also one or more markers of acute infection such as anti-VCA IgM. Because patients recovering from a recent acute infection might have antibody profiles similar to those with reactivated virus, it is sometimes difficult to distinguish these two groups of patients based on serology alone. In addition to patients who present with a possible acute infection, EBV serology is often performed on patients being evaluated for solid-organ or bone marrow transplant (10).
At present, the humoral response to EBV infection is commonly evaluated using indirect immunofluorescence assay (IFA) or enzyme-linked immunosorbent assay (ELISA)-based formats. According to a 2003 survey of laboratories accredited by the College of American Pathologists, ELISA-based methods are now the most widely used (2). Furthermore, ELISA methods are well characterized and are considered to be the reference laboratory methods (3, 5, 7, 14, 15). Recently, Bio-Rad Laboratories (Hercules, Calif.) introduced a new method for the detection of antibodies to EBV (BioPlex 2200). The instrument combines Lab-Map multi-analyte profiling technology (Luminex, Austin, Tex.) (6) with unique antigen-coated fluoromagnetic bead chemistry and versatile software. Briefly, dyed (fluorescent) bead sets, each of which is coated with a different, specific EBV antigen, are mixed with diluted patient sample. After incubation and a wash cycle, an anti-human IgG or IgM antibody conjugated with phycoerythrin (PE) is added. The instrument can then identify beads based on the fluorescence of the dyes and quantitate antibody on each bead based on the fluorescence of the PE. This is a robust system that can assess the response to multiple, independent antigens in a single incubation (multiplex). It has a number of potential advantages over conventional microtiter plate serologic assays, including a fully automated random access platform with minimal user interaction. Furthermore, common test groupings, such as the assays performed to evaluate EBV serologic status or ToRCH testing, can be performed in one tube. This strategy can potentially lower cost, minimize aliquot errors, and reduce turnaround time and sample requirements (5 µl).
In the present study, the EBV assays on the BioPlex 2200 system were evaluated by comparing them to a conventional rapid card assay for the detection of heterophile antibodies and ELISA-based assays for the detection of anti-VCA IgG and IgM, anti-NA IgG, and anti-EA-D IgG antibodies. In addition, the agreement between the two systems in categorizing patients by EBV status was assessed.
(These findings were presented in part at the American Association of Clinical Chemistry Annual Meeting in July 2004.)
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EBV testing.
Captia (Trinity Biotech, Jamestown, N.Y.) EBV anti-VCA IgG and Wampole (Wampole Laboratories, Cranbury, N.J.) EBV anti-VCA IgM, anti-EA IgG, and anti-NA IgG analyses were performed as described in the manufacturers' package inserts on the Dynex DSX (Dynex Laboratories, Chantilly, Va.) automated platform. Briefly, diluted patient serum was incubated (30 min for VCA-IgM and 20 min for EA-IgG, NA-IgG, and VCA-IgG) with antigen-bound microtiter plates. Antigen-antibody complexes bound to the plate were then incubated with horseradish peroxidase-conjugated goat anti-human IgG or IgM (30 min for IgM and 20 min for other antibodies) and developed (10 min) with tetramethylbenzidine. The reactions were stopped with H2SO4, and color was spectrophotometrically detected at 450 nm. Samples used for IgM detection were treated to remove excess IgG with goat or sheep anti-human IgG. The antigen for the IgM anti-VCA kit is affinity-purified VCA (gp125). The antigen for IgG anti-VCA is purified recombinant VCA (47-kDa fusion protein including 53 amino acids from the C terminus of p18). EA-D antigen is a recombinant protein derived from the carboxy terminus of the EA-D gene, representing 200 codons. NA-1 antigen is a recombinant protein derived from the carboxy terminus of the NA-1 gene, also representing
200 codons.
The Status Mono (LifeSign LLC, Somerset, N.J.) heterophile antibody assay was performed as described in the manufacturer's package insert. Briefly, patient serum was placed on the test card by using the supplied capillary tube. Two to three drops of developer solution (phosphate-buffered saline with 0.2% sodium azide) were then added to the same well, and the cartridge was allowed to develop for 8 min. The device contains a membrane strip coated with bovine erythrocyte extract (test lane) and a pad impregnated with the monoclonal mouse anti-human IgM antibody-dye conjugate in a protein matrix. A positive result was reported if a line appeared in the test lane and in the internal control lane. A single line in the control lane only was considered negative.
The five assays were also performed by using the Bio-Rad BioPlex 2200 system. These assays were carried out by the manufacturer. At that time, the assays were experimental and not yet available commercially. Market introduction after U.S. Food and Drug Administration clearance is expected in late 2004. The manufacturer was blinded to the ELISA and heterophile test results. Briefly, two sets of incubations were performed for each sample: one for the IgG assays and one for the IgM assays. Colored beads coated with the Escherichia coli-derived recombinant VCA (40-kDa), EA-D (28-kDa), or NA-1 (28- and 45-kDa) antigens were mixed together, along with the patient sample and diluent and then allowed to incubate at 37°C for 20 min. After a wash cycle, anti-human IgG antibody conjugated to PE was added to the dyed beads and allowed to incubate at 37°C for 10 min. After removal of excess conjugate, the bead mixture was passed through the detector that identifies the beads based on the fluorescence of the dyes. The amount of antibody bound to the bead was determined by the fluorescence of PE. Raw data were initially measured as the relative fluorescence intensity and then converted to the fluorescence ratio by using a predyed internal standard bead. A series of calibrators were analyzed with the patient samples to convert fluorescence ratio to international units per milliliter. The heterophile and VCA-IgM assays were performed in a separate incubation. In these IgM assays, IgG antibodies were absorbed from the sample during mixing with the beads. After this step, the subsequent procedure was the same as that for the IgG incubation, except for the use of anti-human IgM instead of anti-IgG. The two beads used in this incubation were coated with an E. coli-derived recombinant VCA antigen (40 kDa) or horse erythrocyte stromal extract (heterophile antigen) (4, 11). Two additional control beads were also included in all IgG and IgM incubations. A serum verification bead and a blank bead were added to verify the addition of serum to the reaction vessel and the absence of significant nonspecific bead binding, respectively.
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The BioPlex 2200 system was evaluated by assessing concordance to standard ELISA and heterophile antibody assays. As seen in Table 1, there is strong concordance between the two systems for each test evaluated. The anti-EA IgG assay has the lowest concordance at 91%, whereas the anti-VCA-IgG and IgM assays both have concordances of 92%. The comparison methods agree 97% of the time on the heterophile antibody and anti-NA IgG assays.
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TABLE 1. Correlation of anti-EBV serology between the BioPlex 2200 and conventional assay systems
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TABLE 2. Patterns of anti-EBV antibody production associated with EBV disease stagea
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TABLE 3. Comparison of serological pattern interpretations in determining acute EBV infection between the BioPlex 2200 and conventional assay systems
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TABLE 4. Comparison of clinical diagnosis based on serologic pattern from EBV testing on the BioPlex 2200 and conventional assay systems
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As seen in Table 1, the agreement between the BioPlex 2200 and conventional assays for all five EBV assays was excellent (91 to 97%). In fact, the agreement is higher than that observed in another study comparing different ELISA and IFA methods (5). This was especially evident with anti-EA IgG, since the previous study compared ELISA to IFA methods, whereas the present study compares two solid-phase immunoassays.
Although each test has good concordance, each individual test disagreement could potentially lead to the assignment of a different EBV disease status. As such, the concordance between the two systems in assigning one of four specific EBV diagnoses was not as high (82%) as was observed for each individual test. Nevertheless, the agreement between these two systems in assigning a diagnosis is superior to that reported between IFA and ELISA systems in two similar studies (3, 5). As derived from Table 4, most of the disagreement in assigning EBV status occurs when one is determining whether a patient was previously infected or is in a reactivation or recovery stage. This was due to the slightly lower concordance in anti-VCA IgM and anti-EA-D IgG in this group of patients. This group of disconcordant patients was examined closely; we found no pattern with respect to the clinical presentation, to the results of the other EBV serologic tests, or to the results of additional serologic testing (such as with cytomegalovirus or herpes simplex virus).
The most common use of EBV serology is to help distinguish acute versus nonacute EBV disease, suggesting that this parameter is the most important in comparisons of the two assay systems. When the BioPlex 2200 system and conventional assays are compared on this basis (Table 3), they show excellent concordance (92%). Furthermore, the disagreements in Table 3 are primarily in distinguishing nonacute patients from those with unresolved serologic patterns. There is very little disagreement in distinguishing acute from either nonacute or unresolved patients. In fact, there was only one patient classified by either method as having an acute infection that was not classified as such by both methods. This patient had a bilateral lung transplant 1 year prior to testing and was on immunosuppressive therapy. She did not have classic infectious mononucleosis-like symptoms, nor did she have atypical lymphocytes on the peripheral smear. However, the EBV anti-VCA IgG and IgM and anti-EA-D IgG were positive by the BioPlex 2200 testing, but only the anti-VCA IgG and EA-D were positive by ELISA. There was no clear clinical evidence of infectious mononucleosis in this case, nor is there a gold standard that could be utilized to accurately diagnose EBV-related disease. As such, it was not possible to determine which of the two methods was correct with respect to the anti-VCA IgM results.
Most laboratories routinely offer only three of the five available EBV teststhe heterophile antibody test and the anti-VCA IgM and IgG assayssince most patients can be properly diagnosed by using these three antibodies. The assumptions typically made are that patients who are anti-VCA-IgM or heterophile positive have acute disease and those who are positive for IgG and negative for IgM and heterophile had a past infection but do not currently have disease. One problem with these assumptions is evident in Table 2. With only the three tests, patients who are VCA IgG and IgM positive and heterophile negative would be diagnosed with acute-phase illness. However, these patients could potentially be classified differently if all five tests were performed. For example, if a patient with the VCA and heterophile pattern described above were negative for both anti-NA-1 and anti-EA-D, then they would be considered to be in a recovery stage of the illness (last row of Table 2). This change in classification can have implications on long-term patient management, especially in neonatal and immunocompromised populations. Furthermore, if the anti-NA-1 was positive, then the illness would be more likely to be secondary to reactivation of latent virus, which could be useful information for physicians monitoring transplant patients (10). Ten patients within our population fit into one of these two scenarios. Since the practical issues limiting most centers to the three tests do not apply to the BioPlex 2200 system, all five tests can be routinely performed on each patient without an increase in labor, reagents, or sample requirements.
The BioPlex 2200 has been developed for high-throughput analysis of autoimmune analytes, as well as for common serological markers. The advantages of this system are that it offers full integration of sampling, antibody extraction, incubations, washing, and detection. The processing rate is 100 specimens/h (i.e., 300 EBV IgG test results or 200 EBV IgM/heterophile test result/h). The disadvantages are that it is a closed system, which limits its flexibility, and given the current menu, it is best suited for only larger laboratories.
This report provides a comparison between the Bio-Rad BioPlex 2200 and traditional ELISA-based assays. The two systems show excellent test-by-test concordance and also excellent agreement in the diagnosis of acute EBV infection. However, the BioPlex 2200 system offers practical advantages that allow for rapid evaluation of all five EBV antibodies. This can potentially yield laboratory benefits, as well as advantages in the medical management of patients that may have EBV infections.
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