Comparison of a polymer conjugate-enhanced enzyme immunoassay to ligase chain reaction for diagnosis of Chlamydia trachomatis in endocervical swabs

Is lipopolysaccharide a factor in infectivity of Chlamydia trachomatis?

Lipopolysaccharide (LPS) is a major surface component of Chlamydia trachomatis, as with all Gram-negative bacteria. The effect of C. trachomatis LPS on C. trachomatis infectivity of human epithelial cells was investigated. C. trachomatis LPS and C. trachomatis LPS antibody significantly reduced infectivity, mostly in a dose-dependent manner. As the structure of LPS in C. trachomatis is simple and consists only of lipid A and 3-deoxy-D-manno-octulosonic acid (Kdo), we investigated whether lipid A or Kdo was inhibitory to chlamydial infectivity. Polymyxin B, as a lipid A inhibitor, and Kdo considerably reduced C. trachomatis infectivity. With all the LPS inhibitors used, there was greater inhibition against serovar E than serovar LGV. These results suggest a role for LPS in chlamydial infectivity. Elucidation of how LPS acts in infectivity and identification of host-cell receptors would help in understanding pathogenicity.

[Modern diagnosis of Chlamydia trachomatis infections]

Chlamydia trachomatis is the most frequent sexually transmitted bacterial infection. Tests to diagnose C. trachomatis infections include growth on cell culture and assays detecting bacterial antigens or nucleic acids. Until recently, culture was considered the gold standard of testing because of its high specificity. However, the sensitivity of culture is limited. Nucleic acid amplification tests (NATs) have the highest sensitivity and are almost as specific as culture. They are best suited for screening. Chlamydial antigen EIAs are considerably less sensitive and specific, resulting in low positive predictive values, especially when low-prevalence populations are studied. Thus, positive results of chlamydia antigen EIAs should be confirmed by another test.

The superiority of polymerase chain reaction over an amplified enzyme immunoassay for the detection of genital chlamydial infections

BACKGROUND/OBJECTIVES

The polymer conjugate enhanced enzyme immunoassay (IDEIA) and Cobas Amplicor polymerase chain reaction Chlamydia trachomatis (CT) (Amplicor PCR) are two commonly used assays for the diagnosis of CT infection. The performance of these assays was compared for the diagnosis of genital CT infection among 1000 consecutive patients attending a genitourinary medicine (GUM) clinic. Confirmation of positive results and the clinical significance of the absence of cryptic plasmid in chlamydia on the diagnosis of infection by Amplicor PCR were also investigated.

METHODS

IDEIA, Amplicor PCR, and two nested in-house PCR assays targeting cryptic plasmid and omp1 gene were performed on all samples. DNA from Amplicor PCR negative samples was pooled for in-house PCR assays. Each pool contained DNA from seven Amplicor PCR negative samples.

RESULTS

Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and efficiency of IDEIA in the diagnosis of genital CT infection were 80%, 97%, 80%, 97%, and 95%, respectively. Sensitivity, specificity, PPV, NPV and efficiency of Amplicor PCR were 99%, 98%, 89%, 100%, and 98%, respectively. 16 (11%) of 144 Amplicor PCR positive results were identified as false positive by in-house PCR assays. No isolate of plasmid free CT was detected among the study population.

CONCLUSIONS

IDEIA should not be used for the diagnosis of CT infection because of its poor sensitivity. Although the analytic specificity of Amplicor PCR was 98%, because of the adverse medical, social, and psychological impact of false positive results for patients, confirmation of Amplicor PCR positive results by a different assay with comparable sensitivity is essential. Amplification assays targeting cryptic plasmid are appropriate for the diagnosis of genital CT infections.

Enhanced enzyme immunoassay with negative-gray-zone testing compared to a single nucleic Acid amplification technique for community-based chlamydial screening of men

We evaluated a low-cost diagnostic strategy for detecting Chlamydia trachomatis in a low-prevalence population. We used an amplified enzyme immunoassay (EIA) with a reduced-cutoff "negative gray zone" to identify reactive specimens for confirmation by a nucleic acid amplification test. As part of the Chlamydia Screening Studies project, men provided a first-pass urine specimen, which they returned by post for testing. We tested 1,003 specimens by IDEIA PCE EIA (Dako) and Cobas PCR (Roche). There were 32 (3.2%) true positive specimens according to a combined standard using an algorithm requiring concordant results from at least two independent tests. All of these were positive by Cobas PCR and 24 were confirmed to be positive by PCE EIA, including 2 that gave results in the negative gray zone. There were 971 true negative specimens, 2 of which were positive by Cobas PCR and 19 of which were initially inhibitory for PCR. The relative sensitivity, specificity, positive predictive value, and negative predictive value of PCE EIA with PCR confirmation were 75.0% (95% confidence interval [CI], 56.6 to 88.5%), 100% (95% CI, 99.7 to 100%), 100% (95% CI, 88.3 to 100%), and 99.2% (95% CI, 98.4 to 99.6%), respectively. The corresponding values for Cobas PCR were 100% (95% CI, 89.1 to 100%), 99.8% (95% CI, 99.3 to 100%), 94.1% (95% CI, 76.9 to 98.2%), and 100% (95% CI, 99.6 to 100%), respectively, with 1.9% (19/1003) of the samples being initially indeterminate. When the prevalence of C. trachomatis is low, the use of an amplified EIA on urine specimens, with confirmation of results in the negative gray zone by use of a nucleic acid amplification technique, is not suitable for screening asymptomatic men. In addition, positive nucleic acid amplification test results should be confirmed and an inhibition control should be used.

Diagnosis and assessment of trachoma

Trachoma is caused by Chlamydia trachomatis. Clinical grading with the WHO simplified system can be highly repeatable provided graders are adequately trained and standardized. At the community level, rapid assessments are useful for confirming the absence of trachoma but do not determine the magnitude of the problem in communities where trachoma is present. New rapid assessment protocols incorporating techniques for obtaining representative population samples (without census preparation) may give better estimates of the prevalence of clinical trachoma. Clinical findings do not necessarily indicate the presence or absence of C. trachomatis infection, particularly as disease prevalence falls. The prevalence of ocular C. trachomatis infection (at the community level) is important because it is infection that is targeted when antibiotics are distributed in trachoma control campaigns. Methods to estimate infection prevalence are required. While culture is a sensitive test for the presence of viable organisms and nucleic acid amplification tests are sensitive and specific tools for the presence of chlamydial nucleic acids, the commercial assays presently available are all too expensive, too complex, or too unreliable for use in national programs. There is an urgent need for a rapid, reliable test for C. trachomatis to assist in measuring progress towards the elimination of trachoma.

Chlamydia trachomatis diagnostics

Nucleic acid amplification (NAA) assays for the diagnosis of Chlamydia trachomatis infections started to appear in the peer reviewed literature about 12 years ago and during that period we have seen an incredible effort put into the development and evaluation of commercially developed NAA kits to diagnose and treat infections.

Moving to nucleic acid-based detection of genital Chlamydia trachomatis

Laboratory diagnosis of the bacterium Chlamydia trachomatis has gone through a complete phase of evolution since it was first identified as a significant cause of sexually transmitted infection. As a fragile, obligatory intracellular organism, it was initially only grown in eggs. Subsequently, diagnosis relied on culture in continuous cell lines. To address the limitations of culture, immunological methods were developed and direct antigen detection using enzyme immunoassay and immunofluorescence flourished. With the advent of molecular technologies, nucleic acid-based amplification techniques became the methods of choice, offering improved standard of care for diagnosis and opening up the possibility of screening using noninvasive, patient-acceptable specimens. In this article, the various currently available molecular methods are examined, some of the existing problems discussed and a view on what we think might happen in the next 5 years to the technology and requirement in diagnosis and screening is given.