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Review: The US Centers for Disease Control and Prevention has provided guidelines for selecting laboratory tests to screen men and women for C. trachomatis and N. gonorrhoeae infections.

Screening tests to detect Chlamydia trachomatis and Neisseria gonorrhoeae infections - 2002.
Centers for Disease Control and Prevention.
MMWR Morb Mortal Wkly Rep. 2002;51(No. RR-15): 1-38
 
Question:
What are the updated guidelines for selecting laboratory tests for C. trachomatis and N. gonorrhoeae, with an emphasis on screening men and women in the United States?

Data Sources:
The CDC staff identified pertinent concerns, compiled related literature, prepared tables of evidence, and drafted recommendations. Consultants, selected for their expertise or disciplinary and organizational affiliations, reviewed the draft recommendations. The final guidelines are the recommendations of CDC staff, who considered contributions from scientific consultants.

Study Selection Criteria:
Related literature published during 1990 or later was reviewed. These guidelines are intended for laboratorians, clinicians, and managers, who must choose among available tests, establish procedures, and interpret results.

Main Results:
This report describes the advantages and disadvantages of laboratory-based tests that are available for detection of C. trachomatis and N. gonorrhoeae, the performance of the tests when used for screening (testing persons that have no symptoms or signs of infection), the considerations for selecting a screening test, and specimen collection. Tests designed for batch testing include culture, nucleic acid amplification tests (NAATs), nucleic acid hybridization and transformation tests, enzyme immunoassays (EIAs), and direct fluorescent assays (DFAs). Point-of-care tests include Gram stain for N. gonorrhoeae, solid phase and optical immunoassays for C. trachomatis, and leukocyte esterase to screen for urinary tract inflammation.

Culture for C. trachomatis and N. gonorrhoeae has been the reference standard for other tests. However, culture methods are difficult to standardize and require technical expertise and careful transportation and storage to maintain viable organisms. Therefore, nonculture tests have been developed that do not require viable organisms, including tests that can be automated. NAATs, due to their increased sensitivity, provide testing of specimens that can be collected by non-invasive methods (urine) and at non-traditional venues, and detection of both organisms from the same sample.

Screening tests should be selected based on their sensitivity (low number of false negatives), specificity (low number of false positives), ease of specimen collection, ability to assess antimicrobial susceptibility (N. gonorrhoeae), cost, and the laboratory environment. Repeat testing of positive samples should be considered, especially when the prevalence is low, in this order: 1) testing a second specimen with a different test that detects a different target. Use only another NAAT to confirm a positive NAAT. 2) Retest the original sample with a different test. 3) Retest the original sample with the same test but include a blocking antibody or probe. 4) Repeat the test on the same sample. Due to the high cost of NAATs, alternate testing strategies may also be considered. These include: 1) gray-zone testing, in which samples are screened with a nonNAAT test for which the cut-off value for a positive result has been lowered. Perform a NAAT on those samples with a result greater than the lowered cut-off value. 2) Pool samples for analysis by NAAT. 3) Test urine samples with a leukocyte esterase test, then analyze LE positive samples using a more specific assay.

Conclusions
Tests recommended for screening are summarized in the table.
 

For correspondence: This report is available at www.cdc.gov/std/labguidelines. Copies can be ordered at this website.

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