Diagnostic Confirmation for Chlamydia
Nucleic acid amplification tests (NAATs) are the recommended method for diagnostic confirmation of chlamydia, with superior sensitivity (86-100%) and specificity (>97%) compared to all other testing modalities. 1
Primary Diagnostic Testing
First-Line Testing Approach
NAATs should be used as the primary diagnostic method for detecting Chlamydia trachomatis infections due to their markedly superior sensitivity while maintaining exquisite specificity compared to culture or antigen-based tests. 1, 2
For women: Vaginal swabs (self-collected or clinician-collected) or first-void urine specimens are the recommended first-line specimen options. 1
For men: First-void urine or urethral swabs are the recommended first-line specimen options. 1
The same specimen can be used to test for both chlamydia and gonorrhea simultaneously, which is clinically efficient given frequent co-infection. 1
Specimen Collection for NAATs
NAATs offer the major advantage of not requiring viable organisms, eliminating the need for specific transport and storage conditions required by culture. 2
First-void urine specimens are acceptable for both males and females, allowing noninvasive collection that facilitates screening of asymptomatic individuals. 3
For extragenital sites (anorectal, oropharyngeal, ocular), NAATs should be performed on corresponding mucosal swabs, with AC2 (APTIMA Combo 2) demonstrating superior sensitivity (93-100%) compared to culture (27-44%) at these sites. 4
When Culture is Required
Medical-Legal Situations
Culture using fluorescein-conjugated C. trachomatis-specific antibody is the diagnostic method of choice when very high specificity is required, particularly in medical/legal situations. 5
Culture approaches 100% specificity and allows preservation of organisms for additional studies including immunotype determination and antimicrobial susceptibility testing. 5
Specific Clinical Scenarios Requiring Culture
Urethral specimens from asymptomatic men: Nonculture tests are not recommended for this population due to insufficient performance data. 5
Rectal specimens: Culture is the preferred method for detecting chlamydia from rectal sites, though recent data suggest NAATs perform better. 5
Vaginal specimens from prepubertal girls: Culture is recommended due to the need for highest specificity in this population. 5
Nasopharyngeal specimens from infants: Culture remains the standard for this specimen type. 5
Confirmation of Positive Screening Tests
When Confirmation is Needed
Clinicians should verify positive screening test results with a supplemental test if a false-positive result is likely to have adverse medical, social, or psychological consequences. 5
Verification should be routine in low-prevalence patient populations (prevalence <5%), where up to 20% of positive results may be false-positives even with 99% specificity. 5
In high-prevalence populations (≥15%), verification might be selective rather than routine, as the positive predictive value is substantially higher (88% vs 44%). 5
Methods for Confirming Positive Results
Three approaches are available, listed in order of preference:
Culture verification: Use culture with fluorescein-conjugated C. trachomatis-specific antibody on a second specimen. This is the most sensitive and specific method but requires a second specimen and is recommended only when very high specificity is required. 5
Second nonculture test with different target: Perform a second NAAT that identifies a different C. trachomatis antigen or nucleic acid sequence than the screening test. This approach has had limited field evaluation but theoretically provides adequate specificity. 5
Blocking antibody or competitive probe: Use an unlabeled "blocking" antibody that verifies a positive result by preventing attachment of the labeled antibody used in the standard assay. The Chlamydiazyme blocking antibody test appears to produce adequate results and does not require a second specimen. 5
Important Caveats About Confirmation
Recent evidence suggests that repeat testing of the same specimen by the same NAAT confirms 84-98% of positive results, and testing by different NAATs confirms 89-99% of positive results. 6
Failure to verify an initial positive test does not rule out chlamydial infection, as the sensitivity of verification tests is uncertain but less than 90%. 5
Some NAATs cannot be used to confirm other NAATs due to cross-reactivity or similar target sequences. 6
Doing more extensive testing confirms more positive results, with >90% of all positive NAATs ultimately confirmable. 6
Tests That Should NOT Be Used
Inadequate Diagnostic Methods
Enzyme immunoassays (EIAs) and rapid diagnostic tests (RDTs) for antigen detection are unsuitable due to insufficient sensitivity and specificity, though recent PCR-based RDTs may be non-inferior to standard NAATs. 2
Serum chlamydia serology has no value in diagnosing genital tract or prostatic infections, as it cannot distinguish current from past infection. 7
Gram stain lacks sufficient sensitivity for endocervical, pharyngeal, or rectal specimens and should not be relied upon. 1
Specific Test Limitations
Older EIA tests (particularly Chlamydiazyme) may yield false-positive results in urine from men with bladder infections due to cross-reactivity with lipopolysaccharide from other bacteria. 5, 7
The leukocyte esterase test (LET) has variable sensitivity (46-100%) and is insufficient for definitive diagnosis, though it can screen for urethritis in sexually active teenage males. 5
Common Pitfalls to Avoid
Mishandling of urine specimens can lead to false-negative NAAT results, though few clinicians are aware of this potential for incorrect results. 3
Relying on a single negative test to rule out infection is inappropriate, as neither nonculture tests nor culture are sufficiently sensitive (typically 70-90%) to exclude chlamydial infection based solely on a negative result. 5
Using nonculture tests in asymptomatic men without adequate validation may lead to missed infections due to limited performance data in this population. 5
Failing to consider the prevalence of infection in the tested population can result in misinterpretation of positive results, particularly in low-prevalence settings where false-positives are more common. 5