Editor,—Genital infection with Chlamydia trachomatis is highly prevalent and recognised as a major threat to public health.

There is now a wealth of evidence to demonstrate the superiority of DNA amplification techniques over antigen detection and culture.¹ Only one large study has directly compared ligase chain reaction (LCR) with enzyme immunoassay (EIA) on identical clinical material² and no studies have analysed the health economic impact of LCR in a genitourinary medicine (GUM) clinic population.

We studied the diagnostic effectiveness and cost of LCR compared with EIA.

All GUM attendees undergoing sexual health screening were offered the opportunity to participate. Men presenting with dysuria or urethral discharge were defined as symptomatic. Swabs were collected in a pre-randomised order from the cervix in female patients and 4–5 cm proximal to the urethral meatus in male patients. Urethral specimens in male patients were evaluated for evidence of urethritis (defined by ≥4 polymorphs per high powered field).

EIA was performed using a standard immunoassay technique (Organon Chlamydia-Tek),¹ with confirmation of reactive tests by microdot DIF.³ LCR (LCX system, Abbott Laboratories) was also performed on every specimen.⁴ Specimens testing positive by LCR alone were retested by an alternative PCR assay for DNA sequences coding for the major outer membrane protein (MOMP) of Chlamydia trachomatis.

A total of 148 male and 153 female patients were tested; 23/148 (16%) swabs from male patients and 10/153(7%) from female patients were positive for Chlamydia trachomatis by LCR (see fig 1).

The sensitivity, specificity, negative and positive predictive values, and cost/test of LCR and EIA, respectively, were 100%, 100%, 100%, 100%, £5.64 and 58%, 100%, 95%, 100%, £4.05.

Of 33 cases of chlamydial infection, 15 cases (12 (52.2%) in men and two (20.0%) in women) would have remained undetected if EIA had been used alone.

Although EIA tests cost less than LCR, the inferior detection rate for EIA (17 patients need to be screened per case detected) compared with LCR (nine patients screened per case detected) was also included in analysis of the results. The cost per case of chlamydial infection detected using EIA in this population was £65, compared with £50 for LCR.

In a hypothetical cohort of 100 GUM attendees, with an 11% prevalence of chlamydial infection (as in the present study), testing with EIA would cost £405 and would detect 6.4 of the 11 cases. Testing the cohort with LCR would cost £564 and detect all 11 cases. The additional cost of LCR is thus £159. The additional benefit is 4.6 additional cases detected. The additional cost of LCR per additional case detected is £34.

The clinic in which the study was conducted sees 6000 new attendees annually. Had EIA been used alone, 276 cases of chlamydial infection would have been missed in a one year period, at an estimated cost of over £82 000. A full economic evaluation would require that these long term health and resource costs be more thoroughly quantified and compared with other uses of NHS resources

In summary, this study demonstrates that the overall sensitivity of LCR was double that of EIA, the previous standard diagnostic test used. Because of its improved sensitivity and increased case detection rate, the cost of LCR per case detected is equivalent to that of EIA in an urban UK GUM clinic population. Use of LCR as the diagnostic test of choice for both screening and clinical diagnosis in this setting thus represents a cost effective strategy.

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Figure 1

Chlamydia detection by diagnostic test. LCR = ligase chain reaction; EIA = enzyme immunoassay.