False positive results with an ELISA for detection of chlamydia antigen

Inappropriate use of nonculture tests for the detection of Chlamydia trachomatis in suspected victims of child sexual abuse: A continuing problem

The introduction of nonculture tests for detection of Chlamydia trachomatis has revolutionized the management of chlamydial infections in sexually active adolescents and adults. However, these tests are insufficiently specific for use in genital and rectal sites in children; false-positive tests can be frequent. We report here 4 cases involving inappropriate use of nonculture tests in children in New York City during 1998. Two cases involved the use of enzyme immunoassays with vaginal specimens. In both cases the initial results were positive, however, cultures for C trachomatis performed later were negative. In the third case, the DNA probe test for C trachomatis was used. The fourth child was being evaluated for sexually transmitted diseases after rape. Although the pediatrician sent a rectal culture to a large commercial laboratory it was later determined that the laboratory was using an enzyme immunoassay for culture confirmation leading to a false-positive result. At the least the use of these inappropriate tests resulted in unnecessary retesting and at the worst, unnecessary hospitalization, erroneous reports of sexual abuse and possibly unjustified prosecution and incarceration. Because of the social and legal implications it is important that practitioners be aware of these recommendations and require that commercial laboratories adhere to approved C trachomatis culture methods.

False-positive enzyme immunoassay test results for Chlamydia trachomatis because of contact of the collection swab with agar

BACKGROUND

We noted an increased incidence of false-positive Chlamydia trachomatis enzyme immunoassay results using the Abbott IMx SELECT C. trachomatis EIA test when a single swab was used for urethral sampling for both gonococcal culture inoculation and chlamydial detection.

GOALS

To evaluate if contact of the enzyme immunoassay collection swab with an agar produces false-positive chlamydial enzyme immunoassay results.

STUDY DESIGN

Samples containing agar-based culture media were tested by two enzyme immunoassays and a ligase chain reaction technique.

RESULTS

We report false-positive chlamydial enzyme immunoassay results using the Abbott IMx SELECT C. trachomatis EIA test if the collection swabs are in contact with gonococcal culture media (Modified New York City agar, chocolate, Thayer-Martin, or GC-lect) before insertion of the swab in the transport media of the enzyme immunoassay. The other assay results were negative.

CONCLUSIONS

Using a single collection swab to screen for genital infections with gonococcal cultures and chlamydial enzyme immunoassay is inappropriate because it may lead to false-positive chlamydial enzyme immunoassay results, at least with the Abbott IMx SELECT C. trachomatis EIA test, incurring public health and financial consequences.

Mandatory use of confirmation stage with Chlamydiazyme during urinary sediment analysis

AIMS

To assess whether false positive results found when the first stage Chlamydiazyme test is performed on urinary sediment could be reduced by using the more specific second stage blocking assay.

METHODS

Sediment from 173 urine samples from patients with suspected urinary tract infection caused by Gram negative bacteria and 23 control urine samples were tested using the Chlamydiazyme assay system, which included a blocking assay.

RESULTS

A reaction result with the first stage Chlamydiazyme assay test was seen in 102 (58.9%) of the test urine samples. First stage reactivity was not blocked by the Chlamydiazyme confirmatory assay performed on repeat testing. All were correctly identified as true negative (first test false positive) results.

CONCLUSIONS

Use of a second (specific) blocking assay for the analysis of urinary sediment using Chlamydiazyme eliminates false positive results in Gram negative urinary tract infections.

Evaluation of chlamydiazyme enzyme immunoassay for detection of Chlamydia trachomatis in urine specimens from men

Paired first-voided urine and urethral swab specimens were collected from 540 men attending sexually transmitted disease clinics in three geographic locations. Urine specimens were tested for the presence of Chlamydia trachomatis by commercial enzyme immunoassay (Chlamydiazyme), and the results were compared with those of urethral swab cultures. Overall prevalence of urethral C. trachomatis by culture was 14%, and the Chlamydiazyme assay had an overall sensitivity of 83%, a specificity of 96%, a positive predictive value of 76%, and a negative predictive value of 97%. Sensitivity was greater (94%) in those culture-positive samples with a high antigen load (> or = 20 inclusion-forming units per coverslip) than those with a lower antigen load (68%). Assay of urine specimens from men attending sexually transmitted disease clinics by Chlamydiazyme appears to be a reliable, noninvasive method of detection of C. trachomatis infection, and further evaluation of its performance in asymptomatic and low-prevalence populations is indicated.

Value of confirmation of Chlamydiazyme enzyme immunoassay results in the detection of Chlamydia trachomatis

The specificity of an EIA (Chlamydiazyme, Abbott) for detection of Chlamydia trachomatis was evaluated by means of a monoclonal antibody blocking reagent (Abbott) and a direct fluorescent antibody test (DFA) (Micro Trak, Syva). Of the 12,864 tested specimens from the urethra and cervix, 666 (5.2%) were positive. When retested with the same technique 625 (4.9%) were again positive. The result in 514 (4.0%) specimens was verified by means of the blocking reagent. Of specimens with absorbance values just below the established cut-off limit (0.07-0.114), 31 of 79 (39.2%) were specifically inhibited, increasing the rate of positive results by 0.25%. The results confirmed with the blocking reagent were in agreement with those obtained by DFA. The findings show that verification of positive Chlamydiazyme results is strongly advisable, in particular in specimens from the cervix.

Laboratory techniques for the diagnosis of chlamydial infections

Yolk-sac inoculation of embryonated eggs was superseded 25 years ago by the use of cell cultures (often McCoy) for the isolation of Chlamydia trachomatis. Centrifugation of specimens onto the cell monolayers was shown to increase sensitivity, but little of late has further improved sensitivity which is at least ten-fold greater than that of eggs. However, culture is slow and labour intensive so that non-cultural techniques without these drawbacks have come to dominate. Direct fluorescent antibody (DFA) tests are rapid and have sensitivities that range from 70% to 100% for men and 68% to 100% for women, and specificities that range from 87% to 99% for men and 82% to 100% for women; if the tests are read by competent observers the values are at the top end of the ranges. The detection rate may be enhanced even further by relatively low-speed centrifugation of specimens before staining. Skilled reading is not a feature of enzyme immunoassays (EIAs) which according to the literature have sensitivities that range from 62% to 97% for men and 64% to 100% for women, and specificities that range from 92% to 100% for men and 89% to 100% for women. However, comparison against poor reference tests is responsible for most of the higher values and the clinician should not be misled into believing that EIAs have excellent sensitivity; the lower values in the ranges are closer to reality. Furthermore, EIAs that are being designed for use by general practitioners should be regarded with the greatest caution since lack of sensitivity means that chlamydia-positive patients will go undetected. The polymerase chain reaction (PCR) is not bedevilled by insensitivity but it is no more sensitive than the most sensitive cell culture or DFA tests. PCR is unsuitable for routine diagnosis but has a place as a research tool. For men, examination of "first-catch" urine samples by the best of the non-cultural procedures provides an acceptable non-invasive approach to diagnosis; for women, the value of examining urine may be less, but needs to be thoroughly tested. However, there is little doubt that a Cytobrush used to obtain cervical specimens holds no practical advantage over a swab. Serological tests are reliant on the provision of paired sera for making a diagnosis; high antibody titres in single sera may be suggestive of an aetiological association in deep-seated chlamydial infections (epididymitis, arthritis, salpingitis, etc), but unequivocal interpretation is unusual, particularly in an individual case, since the distinction between a current and past infection is problematical.