Neisseria Gonorrhoeae, Direct Detection

Gonorrhea is the most frequently reported bacterial infection in the United States. Most infections involve the lower genital tract through direct infection of the columnar epithelium of mucosal membranes. Asymptomatic infections occur in 1-5% of males and more frequently in women. Symptoms usually occur within 7 days of infection and include anterior urethritis with purulent exudate in men, and cervicitis in women. Untreated gonococcal infection may progress to pelvic inflammatory disease (PID) in 8-10% of women. Fitz-Hugh-Curtis (perihepatitis) syndrome may be associated with gonococcal PID. One to 3% of persons with gonorrhea develops disseminated gonococcal infection (DGI), which may be associated with a late-acting complement deficiency (C7-9). Clinical manifestations include skin lesions on the extensor surfaces of the hands and feet and septic arthritis. Gonococci may be isolated from synovial fluid cultures in approximately 50% of septic arthritis cases. Endocarditis and meningitis are rare complications.

Roche PCR is an amplified nucleic acid detection method for N. gonorrhea. The sensitivity is 98%, with a specificity of 99-100%. False negative tests can result from improper specimen collection, technical error, concurrent antibiotic therapy, or excessive mucus in endocervical samples. Grossly blood specimens may cause false positive results. Microorganisms that can cause similar infections, such a s Ureaplasma urealyticum, Mycoplasma hominis, Candida albicans, Chlamydia trachomatis, and Herpes simplex types 1 and 2 do not cause false positive reactions.

Although the sensitivity and specificity of PCR are high (98% and 99%), the positive predictive value (PPV) is significantly impacted by the prevalence of disease in the population being tested. Prevalence of NG in PCR testing performed by SLRL is 1 to 2%, which corresponds to a positive predictive value of only 60%, making clinical correlation essential.

Specificity of PCR tests for NG is affected by cross-reactivity with other non-gonococcal Neisseria species. These Neisseria species are not frequently recovered from urogenital sources, but do represent normal respiratory tract flora. Therefore, throat swabs are unsuitable for NG testing by PCR.

Although non-gonococcal Neisseria species generally have an equivocal signal, which is resulted as negative, occasionally a signal above the positive cut-off for the assay occurs, causing a false-positive result. Specificity can be improved by testing a second specimen, or by an alternative method such as NG culture.

A recent publication by the CDC recommends that all positive screening tests for CT and NG should be considered presumptive evidence of infection. Additional testing should be considered after a positive screening test when the PPV is <90%, or when there are significant adverse social or psychological consequences from a positive screening result (MMWR 2002; 51,No.RR-15). Laboratories might want to consider adding a comment to all positive results stating that false positive results should be considered when patients have an unexpected positive screening test for Chlamydia trachomatis or Neisseria gonorrhoeae, especially when clinical findings are not supportive.

Results are reported as Neisseria gonorrheae positive or not detected by PCR.

Acceptable specimens include endocervix, male urethra or urine. Swabs must be transported to the laboratory in M4 Transport Media and refrigerated. For female collections, an initial swab should be used to remove excess mucus from the cervical os and then discarded. The swab should be rotated for 10-30 seconds in the endocervical canal to ensure adequate sampling. This specimen cannot be used for culture. Urine specimens should consist of the first 10-50 mL of the urine stream.

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