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Clinical and Diagnostic Laboratory Immunology, September 2004, p. 924-929, Vol. 11, No. 5
1071-412X/04/$08.00+0 DOI: 10.1128/CDLI.11.5.924-929.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Medical Diagnostic Laboratories, Mt. Laurel, New Jersey
Received 13 May 2004/ Returned for modification 25 June 2004/ Accepted 19 July 2004
| ABSTRACT |
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| INTRODUCTION |
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There have been no reports on the performance of confirmatory serological tests for LD since 1998. Tilton recently reviewed new serological tests for LD (10), and although substantial progress toward the sensitive and specific detection of immunoglobulin G (IgG) and IgM antibodies to B. burgdorferi has been made, there is presently no definitive evidence that any of these tests are capable of being stand-alone tests with no confirmatory or supplemental WB. Some of these tests, such as the C6 Lyme antibody test (8) and the VlsE antibody test (6), show promise because of their high specificity and acceptable sensitivity in all stages of the disease.
As of early 2004, three Lyme WB products were available in the United States: the Marblot (MarDx, Carlsbad, Calif.), the Boston Biomedica (BBI; West Bridgewater, Mass.) WB test kit, and the Virablot (Viramed, Steinkirchen, Germany). Two other products, QualiCode (Immunetics, Cambridge, Mass.) and a WB kit from Focus Technologies (Cypress, Calif.) were unavailable because of reformatting and withdrawal from the market, respectively. Of the three available products, two (Marblot and the BBI kit) are FDA approved. The Virablot kit is for research purposes only pending FDA review and approval.
This study used two performance panels containing a total of 57 characterized serum and plasma specimens. One panel is from the CDC and consists of 42 samples, and the other is from BBI and includes 15 samples. These performance panels were used to evaluate the sensitivities, specificities, and operating characteristics of the Marblot, the BBI WB, the Virablot, and the C6 Lyme antibody test.
| MATERIALS AND METHODS |
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The BBI mixed-titer performance panel, catalog no. PTL202, is also available commercially. The purpose of the panel is to enable manufacturers and diagnostic laboratories to validate their kits by using well-characterized serum samples. The panel consists of 11 positive plasma specimens, 3 positive serum specimens, and 1 negative plasma specimen. The panel provided clinical information confirming the diagnosis of LD for seven of the panel members. The remaining seven positive specimens and the single negative plasma sample had no accompanying clinical information. None of the panel members had information on the time of specimen collection relative to the onset of symptoms.
The differences between the two panels were that there was less clinical information with the BBI panel and, unlike the CDC panel, the BBI panel provided test results using 11 commercially available LD ELISA kits, 3 WB kits (Marblot, an in-house WB, and a GenBio kit), and results from internationally known reference laboratories using research-level testing. The GenBio dot blot contains four Lyme-specific antigens: OspC, p39, flagellin, and a high-molecular-mass protein (83 to 93 kDa). The specimens from both sources were undiluted, and no preservatives were added. The BBI panel members were collected between 1994 and 1995, and aliquots have been frozen at 80°C since collection. The Marblot and Virablot WB kits were purchased from the manufacturers, and the BBI WB kits were provided to Medical Diagnostic Laboratories by BBI for evaluation.
BBI Lyme WB test kit. The BBI WB kit uses sodium dodecyl sulfate-solubilized B. burgdorferi strain 2591 proteins that are separated by gel electrophoresis and transferred to a nitrocellulose membrane. Strain 2591 is one of the three B. burgdorferi strains recommended by the CDC-ASTPHLD report (2). The membranes are processed according to the product insert. Positive and negative controls are included. Strips are developed, and the reaction is terminated after 10 to 12 min of incubation. Bands are identified using an IgG and IgM blot reading guide consisting of a previously developed and mounted strip from the same strip lot by BBI. One band on the reading guide is identified as the minimum intensity (threshold) band. Only bands of intensity equal to or greater than this minimum are scored. Blots are interpreted using the CDC criteria (2) (Table 1).
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Viramed Borrelia Virablot test kit. The Virablot WB test kit uses proteins from two European isolates: B. burgdorferi sensu stricto, which is analogous to the American isolate B31, and Borrelia afzelii, a component of the B. burgdorferi sensu lato complex. According to the product insert, this antigen combination guarantees optimum sensitivity and specificity of the assay. Each test strip has an integrated conjugate reactivity control which indicates whether the test is for IgG or IgM as well as a functional control which, when positive, indicates that all reagents were added. Each kit includes a developed positive control strip that is used as a band locator. The Virablot also uses IgG and IgM cutoff controls. Incubation is stopped when the IgG 41-kDa band or the IgM 23-kDa band becomes clearly visible. Bands are scored as weak (intensity < cutoff), clear (intensity = cutoff), strong (intensity > cutoff), or extremely strong (color of the 41-kDa band is violet to black). For use in the United States, the band patterns are interpreted using the CDC criteria and reported as positive or negative (2) (Table 1).
Procedure. Each serum sample from the BBI mixed-titer performance panel was repeated on two separate occasions for each WB kit evaluated. The CDC samples were not tested in duplicate due to limitations imposed by the provided sample volumes. All tests were performed according to instructions provided by the respective manufacturers, with the exception that an automated WB processor (Autoblot 2000; MedTec Inc., Chapel Hill, N.C.) was used for all runs. Each strip was read blindly by two technologists, and the results were independently recorded before review by a senior scientist. Quality control procedures included those specified by the manufacturer plus the inclusion of both positive and negative controls in each WB run. Bands were scored as present or absent using the intensity standard described in the product insert from each manufacturer.
C6 B. burgdorferi (Lyme) ELISA kit.
The C6 ELISA was performed in duplicate on all members of both panels according to the manufacturer's (Immunetics) instructions. Results were scored as negative (ELISA index score,
0.90), equivocal (0.91 to 1.09), or positive (
1.1).
| RESULTS |
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Composite results of the three WB kits tested against the 57 combined BBI and CDC specimens showed that the IgG sensitivity was 85% for both Marblot and Virablot and 81% for the BBI WB. The IgG specificity was 97% for both Marblot and the BBI WB and 90% for Virablot. The overall agreement with the 57 characterized specimens was 88 to 91%. The IgM sensitivites were 62% (Marblot), 85% (Virablot), and 77% (BBI WB), while the IgM specificities were 97% (Marblot), 77% (Virablot), and 100% (BBI WB). The overall agreement for IgM WB was 81% for Marblot and Virablot and 89% for the BBI WB. Tables 4 and 5 also show the results of the C6 ELISA compared to the consensus results of both the CDC and the BBI panels. The reference IgG and IgM results were combined since the C6 test detects both IgG and IgM antibodies. The sensitivity was 100%, the specificity was 73%, and the overall agreement was 93%.
There were four false-positive C6 ELISA results. All WB results for these four specimens, including the reference Marblot and the WBs being evaluated, were interpreted as negative with the exception of 91-1354-CDC, which was Virablot IgM positive. Clinical information on all four of these patient samples, 92-1682-CDC, 91-1846-CDC, 91-1354-CDC, and 91-1352-CDC, revealed physician-diagnosed EM, cultures positive for B. burgdorferi, and patients who were cured after a month or less of antibiotic therapy.
To test the possibility that specimen deterioration as a result of protracted storage affected results, similar data generated in 1997 with the same CDC panel and two similar WB products (BBI and Marblot) were compared. The BBI WB showed an IgG sensitivity of 74% in 1997 compared to 81% in 2004. Marblot was less sensitive in 1997 (47%) than in 2004 (85%). The IgG specificity of both products was 100% in 1997 and 97% in 2004. These data suggest that specimen quality may not be a factor.
| DISCUSSION |
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The IgG specificity of the Marblot and the BBI WB was 97%, while that of the Virablot was lower at 90%. The Virablot recorded three false-positive results compared with one each for the Marblot and the BBI WB. Sample 91-1222-CDC was reported as positive by all three WBs. The Wisconsin patient from whom this sample was drawn presented with EM that was culture positive for B. burgdorferi. The specimen was drawn 33 days after the onset of symptoms. It is likely that this false-positive result is a true positive and that the reference WB was falsely negative (4 of 10 significant bands present). The two other samples reported as positive by the Virablot and negative by the Marblot and the BBI WB as well as the original reference WB were from patients with physician-diagnosed EM that was reported as being culture positive. The specimens were drawn 43 and 29 days after onset, respectively. The Virablot may be more sensitive for these two specimens than the Marblot, the BBI WB, or the reference Marblot.
Some of the IgM WB products did not perform as well as those for IgG. The sensitivity ranged from 62% (Marblot) to 85% (Virablot), with that of the BBI WB in between at 77%. Of the 57 total CDC and BBI panel specimens, there were between 4 and 10 false-negative results, depending on the WB product. There were two samples (91-0544-CDC and 202-02-BBI) that were missed by all three WB products. Sample 202-02-BBI was drawn from a patient with laboratory and clinically confirmed LD and exhibited no bands with any of the WBs tested. Sample 91-0544-CDC had a 23-kDa band, but no 39- or 41-kDa bands. The Marblot exhibited the worst sensitivity (62%), consistently missing the 39- and 41-kDa bands while detecting the 23-kDa band. The CDC interpretive criteria require that two of these three bands be present for a positive result (Table 1). Sample 91-0544-CDC was from a Massachusetts patient with inflammatory arthritis of the right knee. The IgG and IgM reference WBs were strongly positive at the time. The three WB kits failed to detect either the 39- or the 41-kDa band for specimens 202-02-BBI and 91-0544-CDC.
The BBI IgM WB was 100% specific, while the Marblot and Virablot had one and seven false-positive results, respectively. However, of these seven putatively negative samples detected by the Virablot, six were in the CDC panel and were from patients with culture-positive LD. The CDC reference results were negative in all six samples because either the 23- or the 41-kDa band was not detectable. The seventh specimen, 202-15-BBI, was IgG WB positive and IgM WB negative by the reference tests; no clinical information was available.
There are minor technical differences in the three WBs. The Virablot is a smaller strip, approximately one-half the width of the other two. The bands were more distinct than those of the Marblot and easier to read, even though the strip was smaller. The BBI WB strip was the same size as that of the Marblot and usually produced crisp, easily read WBs. Occasionally, the BBI WB strips had a dark speckling pattern that was not observed on the other strips. The strip development time was more difficult to control with the Marblot and the Virablot than with the BBI WB, which was robust and easily reproducible with a development time of 10 to 12 min independent of the necessity to score a control as a function of the length of incubation time. A disadvantage of the BBI WB is the total incubation time, which is 205 min compared to 165 min for the other two products.
It is possible that the C6 ELISA is a more sensitive antibody test than a WB. Liang et al. (8) and Lawrenz et al. (6) reported that the early C6 antibody response produces both IgG and IgM isotypes and may appear very soon after a tick bite. Magnarelli et al. (9) compared 11 recombinant antigens in an ELISA format and determined that VlsE was the most suitable antigen for the laboratory diagnosis of LD. While VlsE and C6 ELISAs are not identical, they vary primarily in the nature of the capture antigen; VlsE is a recombinant protein (6), and C6 is a synthetic peptide which is a component of the VlsE envelope antigen (8). There are several advantages to the use of C6 antibody tests, including no interference in patients who have been vaccinated with the 31-kDa antigen (3), detection of antibodies to the European strains of B. burgdorferi (7), high specificity, and a shorter reaction time (approximately 1 h). Patients with over 12 different diseases such as systemic lupus erythematosus, rheumatoid arthritis, and other spirochetal diseases were uniformly negative by the C6 ELISA (6, 8). The high sensitivity and specificity of the C6 test makes it an attractive candidate for a primary test for LD, which may obviate the need for WB confirmation.
The results can be summarized as follows: (i) the IgG WB sensitivity and overall agreement among the three kits were similar, (ii) the BBI WB and the Marblot were more specific for IgG, (iii) the IgM sensitivity was highest for the Virablot (84%) and lowest for the Marblot (62%), (iv) the IgM WB specificity was 100% for the BBI WB and 77% for the Marblot and the Virablot, and (v) the C6 antibody test was highly sensitive. Although there were four false-positive results with the C6 antibody test, all four patients had clinically and microbiologically confirmed LD with negative traditional immunological tests.
The performance characteristics of WB tests must be assessed in the context of how these serological tests are used. If the WB is used solely for confirmation of indeterminate or positive ELISA results, then specificity may be more desirable than sensitivity. However, a specific but insensitive WB may invalidate a sensitive and specific ELISA (11). For an effective two-tiered diagnostic system, a highly sensitive test and a highly specific test are desirable. Of major significance is the fact that despite the CDC recommendations, many physicians use the WB as a primary serological test or order both ELISA and WB. Unless a WB is both highly specific and sensitive, it cannot fulfill the expectations of physicians attempting to diagnose both acute and chronic or persistent LD. The answer to this enigma may be the availability of a highly sensitive and specific single test, such as the C6 antibody test.
The performance characteristics of either the WB or the C6 antibody test may be different when used on a patient population whose disease is not well documented. Particularly with LD, there is much disagreement not only on the definition of chronic disease, but also on the existence of LD in nonendemic areas of the United States. Performance panels with documented reference results are useful for test comparisons when the only "gold standard" for laboratory diagnosis is culture, a test which is technically difficult, insensitive, and not readily available.
| FOOTNOTES |
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