Double-conjugate enzyme-linked immunosorbent assay for immunoglobulins G and M against Treponema pallidum

An unexpected result in an evaluation of a serological test to detect syphilis

Traditional tests for detection of syphilis are labour intensive and costly. Enzyme immunoassays (EIAs) are readily automated and cost effective if large numbers of tests are performed. Four experiments were devised to evaluate a syphilis EIA test kit where resources are limited: (1) testing antenatal patients; (2) testing refugees; (3) testing a high prevalence population; and (4) testing "problem sera" (containing autoantibodies or antibodies to other infective agents). Forty-one available syphilitic sera from antenatal patients were tested to evaluate sensitivity. Specificity was determined through testing sera determined to be nonreactive with rapid plasma reagin and Treponema pallidum hemagglutination tests, calculating the sample size (456) on the confidence interval (CI) required. Two runs were performed on antenatal sera, giving sensitivities of 32% (95% CI: 20%, 47%) and 37% (95% CI: 24%, 52%) and specificities of 92% (95% CI: 89%, 94%) and 90% (95% CI 87%, 92%), respectively. We present a method to evaluate a serological test where resources are limited. Unexpectedly, the test kit performed poorly as a screening test. New serological tests need to be evaluated in-house prior to adoption.

Reactivity of recombinant Treponema pallidum (r-Tp) antigens with anti-Tp antibodies in human syphilitic sera evaluated by ELISA

We evaluated the immunoreactivity of recombinant Treponema pallidum (r-Tp) antigens with human sera by indirect enzyme-linked immunoabsorbant assay (ELISA). We expressed antigens with a molecular weight (MW) of 17KDa, 15KDa, 47KDa, and 42KDa, which are believed to be major immunoreactive membrane proteins of Tp cells. The expressed proteins were described by adding the prefix M, S, and G to the corresponding Tp antigens, namely, mature antigens, signal sequence containing antigens, and glutathione s-transferase (GST)-fused antigen in this report. A rather high expression occurred for M47 and S42 proteins in the Escherichia coli system, whereas for M15 and M17 proteins, a poor expression was observed. However, a fairly high expression occurred for G15 and G17. Thus expressed proteins were purified by means of chromatographies to a level of > 95%, and the purified proteins were found to be reactive with TPHA positive serum by Western blotting (WB). An ELISA performed with a serum of 1/1000 dilution using these purified antigens for coating on the solid phase showed that G17 antigen was more effective in detecting syphilis antibodies in human serum than M47, S42, and G15. There was a good consistency between ELISA and TPHA, whereby the cutoff indexes (CI) on ELISA showed a correlation coefficient of 0.7276 in logarithmic TPHA titers.

Recombinant antigen-based enzyme immunoassay for screening of Treponema pallidum antibodies in blood bank routine

This work reports a comparison of an enzyme immunoassay (EIA) using two major Treponema pallidum recombinant antigens with a T. pallidum hemagglutination (TPHA) assay and a nontreponemal Venereal Disease Reference Laboratory (VDRL) test. A total of 1,822 normal donor serum samples was tested for cardiolipin and T. pallidum antibodies, respectively, by the VDRL assay and EIA. Among these samples, 440 were further tested by TPHA technology. Four samples were found positive by EIA, while all were reported to be negative by both TPHA and VDRL routine assays. Subsequent testing of EIA-positive samples confirmed 100% (four of four samples) and 25% (one of four samples) positive results, respectively, by immunofluorescence assay and a Western blot (immunoblot) syphilis kit. The sensitivity of the recombinant EIA was estimated at virtually 100% with a reference panel of 50 syphilitic samples. According to this study, the newly developed EIA kit shows 100% sensitivity combined to a specificity greater than 99.8% for detecting treponemal immunoglobulin G antibodies in blood bank syphilis screening.

Evaluation of a Treponema pallidum enzyme immunoassay as a screening test for syphilis

The CAPTIA Syphilis-G enzyme immunoassay for the detection of antibodies to Treponema pallidum was evaluated as a screening test for syphilis in comparison with the standard rapid plasma reagin (RPR) test. One thousand samples were tested, and the standard fluorescent treponemal antibody absorption test and the standard microhemmaglutination test were used to confirm the presence of treponemal antibodies. Diagnosis of syphilis was based on traditional standard serology results. Clinical data used in the diagnosis of patients whose samples yielded conflicting results were provided by physicians. Initially, 7 patients whose samples were reactive in the RPR test and 14 patients whose samples yielded positive or equivocal results in the CAPTIA Syphilis-G test were diagnosed as not being infected. After discrepancies due to technical problems were reconciled, samples from six patients remained reactive in the RPR test and that from one patient remained positive in the CAPTIA Syphilis-G test. In addition, seven patients whose samples were nonreactive in the RPR test and two patients whose samples were negative in the CAPTIA Syphilis-G test were diagnosed as having untreated syphilis. After discrepancies were reconciled, samples from five patients remained nonreactive in the RPR test and none remained negative in the CAPTIA Syphilis-G test. Final results indicate that the specificities are 99.4 and 99.9%, respectively. In addition to the improved sensitivity and specificity of the CAPTIA Syphilis-G screen, other potential benefits of this assay lead us to believe that this method could serve as a better screening tool than the RPR test.

Enzyme immunoassay for anti-treponemal IgG: screening or confirmatory test?

AIMS

To review the performance of the Venereal Diseases Research Laboratory (VDRL) test and the Treponema pallidum haemagglutination assay (TPHA) as a combined screen for syphilis to provide a baseline for assessing screening by anti-treponemal IGG EIA.

METHODS

Between 1980 and 1987 all serum samples were screened by both VDRL and TPHA tests. The FTA-ABS test was also used in suspected early primary syphilis, or when one of the other tests was positive. A positive result in a screening test was confirmed by quantitative testing. From 1988 all specimens were screened with an enzyme immunoassay (Captia Syph G) as a single screening test.

RESULTS

Of the 44 primary, 47 secondary, and 38 early latent cases of syphilis, the VDRL and TPHA detected 32 (73%) and 31 (71%) of the primary cases; the combination detected 37 (84%). All 85 cases of cases of secondary and early latent infection were reactive in the TPHA test, whereas the VDRL was reactive in only 68 (80%). EIA had a reported sensitivity of 82% for primary infection.

CONCLUSIONS

EIA can be used as a single screening test for detecting early syphilis because its results are comparable with those of the combined VDRL and TPHA tests. The conventional VDRL test should not be used as a single screening test.

Evaluation of the Captia enzyme immunoassays for detection of immunoglobulins G and M to Treponema pallidum in syphilis

Two new enzyme-linked immunosorbent assays (ELISA), one for the measurement of immunoglobulin G (IgG) (Captia Syphilis-G) and one for the measurement of IgM (Captia Syphilis-M), were evaluated for detecting antibodies to Treponema pallidum. Serum samples from 169 patients, 96 with various stages of untreated syphilis, 63 with treated syphilis, and 10 who were noninfected, were investigated. All sera were also examined by traditional treponemal and cardiolipin tests and by the fluorescent treponemal antibody absorption (FTA-ABS) test for 19S(IgM). The overall sensitivity of Captia Syphilis-G was 98.3%. The IgG ELISA was very sensitive (100%) in all stages of untreated syphilis, except in primary syphilis (82%). In all diagnostic groups of syphilis, the reactivity of Captia Syphilis-M was similar to that of the 19S(IgM) FTA-ABS test, except in reinfections, in which the IgM capture ELISA was less sensitive. False-positive IgM capture ELISA results were not found in the 10 neonates born to mothers adequately treated for syphilis. However, of six serum samples containing rheumatoid factor, two were reactive in the Captia Syphilis-M test but not in the 19S(IgM) FTA-ABS test. This indicated that the specificity of the IgM capture ELISA was not absolute. All serum samples from treated patients were reactive in the IgG ELISA, but only 15 samples were reactive in the IgM capture ELISA, which appeared to be as effective as the 19S(IgM) FTA-ABS test in monitoring the effect of treatment. Simultaneous measurement of IgG and IgM antibodies for T. pallidum by the Captia immunoassays appears to be an efficient and simple method for confirming the diagnosis of syphilis as well as for indicating whether active disease is present.

Evaluation of immunoglobulin M western blot analysis in the diagnosis of congenital syphilis

Western immunoblots of solubilized Treponema pallidum antigens were reacted with sera and cerebrospinal fluid (CSF) and developed with enzyme-conjugated antibodies to immunoglobulin M (IgM). A blot was considered positive if reactions included bands at the 47-, 17-, and 15.5-kilodalton positions along with a variable pattern at other low-molecular-weight positions. Sera from 23 of 25 symptomatic infants diagnosed with congenital syphilis yielded positive reactions. Of 80 asymptomatic infants considered at risk for developing symptomatic infection, 16 exhibited IgM patterns consistent with those seen in congenital syphilis, although 5 of these 16 gave reactions that were equivocal. To exclude false-positive reactions due to IgM rheumatoid factor, sera were fractionated and the IgM fractions were retested. Only the five initially equivocal sera gave nonreactive blots with the IgM fractions, whereas all others gave more prominent reactions that were qualitatively similar to those seen in serum samples. Sera from 18 normal infants failed to show any IgM reactivity to T. pallidum antigens on Western blots. The IgM Western blot was both more sensitive and more specific than the fluorescent treponemal antibody-absorbed (IgM) test using fractionated serum. Of the 17 CSF samples from infants with symptomatic congenital syphilis, 14 showed IgM reactivity in Western blots, whereas only 12 had a reactive CSF in the Venereal Disease Research Laboratory test. Our results indicate that this technique can be used to identify both symptomatic and asymptomatic infection in infants with T. pallidum, in some cases before standard serologic studies can confirm the diagnosis.

Screening for treponemal infection by a new enzyme immunoassay

A new enzyme immunoassay (EIA, Captia Syphilis-G) for detecting IgG antibodies against Treponema pallidum was evaluated as a screening test for syphilis. When serum samples were tested at a dilution of 1 in 20 (EIA20), the overall agreement between the IgG EIA and serological status based on the T pallidum haemagglutination assay (TPHA) and the fluorescent treponemal antibody absorption (FTA-ABS) test was 99.2% (1310/1321). The sensitivity of the EIA20 was 98.4% (60/61) and the specificity 99.3% (1251/1260). Discrimination between patients with and without treponemal infection was good: the mean EIA20 absorbance ratios (patient/mean low titre positive control results) were 0.49 for antibody negative patients, 3.30 for patients with positive Venereal Diseases Research Laboratory (VDRL) test and TPHA results, and 1.77 for patients with negative VDRL but positive TPHA results. The cut off point for excluding treponemal infection was taken as 0.9. Specimens with ratios of more than 0.9 should be confirmed by the FTA-ABS test and evaluated for specific IgM antibodies to treponemes. When serum samples were tested at a 1 in 50 dilution (EIA50) the sensitivity was lower (80.3%) but the specificity was absolute. The reduction in sensitivity correlated with low absorbance ratios in the patients who were VDRL negative and TPHA positive. The screening performance of the IgG EIA20 is thus comparable with that provided by a combination of the VDRL test and TPHA. The potential for automation makes the EIA an attractive alternative, particularly in larger centres. Alternatively, the test can be performed at a 1 in 50 dilution (EIA50), at which level it is ideally suited for confirming the treponemal status of antibodies in serum samples preselected by positive cardiolipin antigen screening test results.

Ig class and IgG subclass responses to Treponema pallidum in patients with syphilis

The Ig class and IgG subclasses of anti-Treponema pallidum antibodies in human serum were quantified using solid-phase enzyme-linked immunosorbent assays. Development of these assays with monoclonal antibodies, each specific for a human immunoglobulin class or IgG subclass, provided quantitative data concerning the major antibody specificities. In patients with primary syphilis, anti-T. pallidum activity was limited almost exclusively to IgG1 and IgM. Coordinate, restricted expression of IgG1 and IgG3 responses in T. pallidum-specific assays was observed with sera from patients with active secondary syphilis. IgG1 and IgG3 accounted for roughly 53 and 43% of the total anti-treponemal IgG antibody activity, respectively. While IgM antibody levels were elevated in the patients with secondary syphilis, IgG2 and IgG4 levels, if present at all, represented less than 10 and 2% of the total IgG activity, respectively. Ig in sera from patients who had been treated adequately for secondary syphilis were restricted almost entirely to IgG3 and IgG1. Considering the low level of IgG3 in serum, disproportionately high percentages of antitreponemal antibodies were found in this subclass during and after treatment for secondary syphilis. The restricted, coexpression of the IgG1 and IgG3 isotypes may reflect the close genetic linkage of the gamma 1 and gamma 3 genes and possibly the impact of immunoregulatory mechanisms in response to the induction and expression of autoantibodies which arise during the course of secondary syphilis.

Detection of immunoglobulin M antibodies to Treponema pallidum in a modified enzyme-linked immunosorbent assay

The indirect enzyme-linked immunosorbent assay (ELISA) for detection of immunoglobulin M (IgM) antibodies to Treponema pallidum in sera of syphilitic patients is complicated by false positive reactions due to the interference of IgM rheumatoid factor (IgM-RF) activity and the presence of treponemal IgG antibodies. Another source of error producing false negative results is the competition between treponemal IgG and IgM antibodies for the binding sites on the antigen. To avoid these complications in the indirect Treponema pallidum-specific IgM-ELISA, total IgG was immunoprecipitated from sera of syphilitic patients prior to the assay. The IgM-RF from non-precipitated sera reacted in an IgM-RF-ELISA and in the Treponema pallidum-IgM-specific ELISA with identical titers. After precipitation of total IgG no reaction of the IgM-RF in the assay could be demonstrated. Competition between IgG and IgM antibodies can be prevented almost completely by the precipitation procedure. The sensitivity and specificity of the Treponema pallidum-specific IgM-ELISA after immunoprecipitation of total serum IgG were shown to be higher than 97 percent.

Detection of immunoglobulin M in cerebrospinal fluid from syphilis patients by enzyme-linked immunosorbent assay

Cerebrospinal fluid (CSF) samples were evaluated in an immunoglobulin M enzyme-linked immunosorbent assay (IgM ELISA) for syphilis with sonic extracts of Treponema pallidum coated on polystyrene plates. The ELISA procedure was reproducible, and T. pallidum antigens were stable., A total of 15 CSF samples from patients with neurosyphilis, 18 CSF samples from patients with syphilis, 12 CSF samples from patients treated for syphilis, and 494 CSF samples from patients with neurologic or other systemic diseases were tested. The IgM ELISA gave reactive results in all of six symptomatic and congenital neurosyphilitic patients and none of nine asymptomatic neurosyphilitic patients. Of 524 CSF samples from nonneurosyphilitic individuals, 513 were nonreactive, resulting in 98% test specificity. The IgM ELISA in CSF should prove to be useful for confirmation of symptomatic neurosyphilis.

Treponemal antibody-absorbent enzyme immunoassay for syphilis

An enzyme immunoassay for the diagnosis of syphilis (ELISA-SY) was developed with solid-phase extracts of Treponema pallidum, specimen diluent containing Reiter treponeme absorbent, and three 30-min incubations. The ELISA-SY results were determined in comparison with a standardized positive control and reported as a percentage of strong positive control. In tests with 1,005 serum samples from a venereal disease clinic and other sources, 98.2% agreement was found with fluorescent treponemal antibody-absorption (FTA-ABS) results, and 98.3% agreement was found with T. pallidum passive hemagglutination (PHA) findings. Only 1 of 29 sera originally considered to be biologically false-positive by ELISA-SY; the latter specimen was also positive by PHA and FTA-ABS tests performed in our laboratories. Serum samples from clinically diagnosed syphilitics (16 primary-stage isolates, 7 secondary-stage isolates, and 3-latent-stage isolates) were all positive by ELISA-SY, FTA-ABS, and PHA. Serum samples from 51 newborns suspected of having syphilis on the basis of positive cardiolipin flocculation tests showed 98% agreement of ELISA-SY results with FTA-ABS and PHA findings. Sera from all 61 patients with a variety of autoimmune and other diseases known to be associated with biologically false-positive reactions for syphilis were negative by this ELISA-SY. The specificity of the ELISA procedure for T. pallidum antibody was also confirmed immunologically by blocking experiments.

Rheumatoid factor in syphilis

Immunoglobulin M (IgM) antibodies directed against IgG antibodies (rheumatoid factor [RF]) are known to occur often in patients with syphilis and to interfere with serological tests measuring specific antibodies of the IgM class. In this study we examined the occurrence and specificity of the RF and demonstrated a simple method to detect and eliminate the RF for a specific Treponema pallidum IgM enzyme-linked immunosorbent assay. We measured the occurrence of the RF with a sensitive enzyme-linked immunosorbent assay and found that it increased with the duration of syphilitic disease: 1 of 13 primary syphilis serum specimens, 3 of 13 secondary syphilis serum specimens, and 10 of 27 latent syphilis serum specimens were reactive in this RF test. Those sera containing IgM RF were immunoprecipitated with anti-human gamma chain antibodies and 2% polyethylene glycol until the RF was removed. One serum specimen from a patient in the secondary stage of syphilis and eight serum specimens from patients with latent disease still presented the RF after immunoprecipitation. Removal of the IgG antibodies also improved the sensitivity of the treponemal IgM test, indicating competition of these antibodies for binding sites of the antigen. The enzyme-linked immunosorbent assays for detection of RF and antitreponemal IgM antibodies are performed on the same plate. Theoretically, only sera positive for both tests have to be immunoprecipitated. But our findings indicated an increase in sensitivity of the IgM enzyme-linked immunosorbent assay after removal of IgG antibodies responsible for competition at the binding sites.