Should a Pap smear swab or an STI swab be collected first?

Specimen Collection Order for Pap Smears and STI Testing

STI swabs should be collected before Pap smear specimens to optimize test accuracy and specimen adequacy. 1

Rationale for Collection Order

The 2021 CDC Sexually Transmitted Infections Treatment Guidelines specifically address this question, stating that "during a pelvic examination, endocervical specimens for STI testing can be collected first" 1. This recommendation is based on several important considerations:

1. Test Performance: While the sequence of cytology testing in relation to collection of other endocervical specimens does not significantly influence Pap test results or their interpretation 1, collecting STI specimens first ensures optimal pathogen yield before any potential disruption of the cervical surface.

2. Specimen Adequacy: The 2021 CDC guidelines note that "typically, vaginal specimens are preferred for chlamydia and gonorrhea screening" 1, and collecting these first prevents potential contamination or dilution from lubricants or solutions used during the Pap smear collection.

3. Clinical Workflow: This order (STI swabs first, then Pap) represents a logical progression from less to more invasive sampling techniques.

Evidence Supporting This Approach

The 1998 CDC guidelines previously stated that "in an STD clinic setting or when other cultures or specimens are collected for STD diagnoses, the Pap smear may be obtained last" 1. This recommendation has been maintained and clarified in subsequent guidelines.

A prospective study by Smith et al. (2006) specifically evaluated the effect of specimen order on test performance and found that neither Pap specimen adequacy nor test performance of nucleic acid amplification tests (NAATs) for chlamydia and gonorrhea were significantly affected by swab order 2. However, the researchers noted that point estimates for sensitivity were higher in the early swabs compared to late swabs for several STI tests, suggesting a potential clinical advantage to collecting STI specimens first.

Practical Collection Protocol

1. First: Collect endocervical specimens for STI testing

   • For chlamydia and gonorrhea screening, vaginal specimens are typically preferred 1
   • Use appropriate collection devices as specified by your laboratory

2. Second: Perform Pap smear collection

   • Use the extended tip spatula for the ectocervix, followed by the endocervical brush 3
   • The combination of Cytobrush (endocervix) and spatula (ectocervix) is superior for quality Pap smears 4

Important Considerations

• Mucopurulent Discharge: If present, it should be carefully removed with a saline-soaked cotton swab before proceeding with either test 1, 3
• Lubricants: Should be avoided as they can interfere with cytology 3
• Menstruation: If possible, conventional cytology tests should be scheduled 10-20 days after the first day of menses, while liquid-based cytology can be performed at any time during the menstrual cycle 1

Pitfalls to Avoid

• Inadequate Sampling: Proper visualization of the cervix and use of appropriate collection devices are essential to prevent inadequate sampling of the transformation zone 3
• Delayed Fixation: Specimens should be fixed immediately to prevent air drying, which can distort cellular morphology 3
• Inappropriate Devices: Using only a broom-type device is associated with higher rates of absent endocervical cells and unsatisfactory results 4

By following this protocol of collecting STI specimens first followed by the Pap smear, you can optimize both test accuracy and specimen adequacy while maintaining an efficient clinical workflow.