Labsystems enzyme immunoassay for Chlamydia pneumoniae also detects Chlamydia psittaci infections.

In a recent article, Bas and collaborators compared different serological methods to detect chlamydial antibodies in patients with Chlamydia trachomatis infections and healthy blood donors (1). Using the Labsystems enzyme immunoassay (EIA) for C. pneumoniae, no cross-reaction between C. trachomatis and C. pneumoniae was found. However, Gnarpe and collaborators recently showed cross-reactions between C. trachomatis and C. pneumoniae, when the Labsystems EIA for C. pneumoniae was used on sera containing high titers of C. trachomatis antibodies documented by the microimmunofluorescence (MIF) test (2). A broader cross-reactivity between the different chlamydial species was suspected. The Labsystems EIA test has previously been shown to have high sensitivity and specificity in the diagnosis of acute infections caused by C. pneumoniae during an epidemic of C. pneumoniae (3). To examine the Labsystems EIA test for C. pneumoniae more extensively, we tested it on paired sera, taken from 43 patients for etiological diagnosis of pneumonia. The immunoglobulin G (IgG) and IgM antibody results for C. pneumoniae, C. psittaci, and C. trachomatis had previously been documented by the MIF tests, and IgG and IgM antibodies were now determined by the Labsystems EIA for C. pneumoniae. Of seven patients who were positive by the MIF test for C. pneumoniae, only three were positive by the EIA for C. pneumoniae, while five out of five patients who were positive by the MIF test for C. psittaci were positive by the EIA for C. pneumoniae. One patient with positive MIF test results for both C. pneumoniae and C. psittaci also tested positive for C. pneumoniae by the EIA. Sera from the remaining 30 patients, who showed no significant IgG or IgM antibody changes with the MIF test, also produced negative results with the EIA. Because infections caused by C. pneumoniae and C. psittaci have important epidemiological differences, it is important to make a correct etiological diagnosis. The study of Gnarpe and collaborators and the present study indicate that cross-reactions between the chlamydial species occur when the Labsystems EIA for C. pneumoniae is used. The EIA could be used for the purpose of screening for chlamydial infections, but in nonepidemic situations we suggest that a method that differentiates between the chlamydial species, such as the MIF test, should be used directly (or, alternatively, as a second step after a screen with the EIA).