What is the proper method for collecting and sending a swab for Treponema pallidum hemagglutination assay (TPHA) or dark‑field microscopy?

Specimen Collection for Darkfield Microscopy and TPHA Testing

For darkfield microscopy, cleanse the lesion with sterile gauze and saline, then swab the lesion base directly onto a glass slide and transport immediately to the laboratory at room temperature. 1

Darkfield Microscopy Specimen Collection

The proper technique for darkfield microscopy requires specific handling:

• Cleanse the lesion thoroughly with gauze and sterile saline to remove surface debris and contaminants 1, 2, 3
• Swab the lesion base directly onto a glass microscope slide 1
• Transport immediately to the laboratory at room temperature 1
• Critical timing: The specimen must reach the laboratory immediately—darkfield microscopy requires live, motile treponemes for visualization 1

Important Caveats for Darkfield Microscopy

Darkfield microscopy is not widely available and is typically performed only in public health laboratories 1. The test requires specialized equipment and skilled microscopists 2, 3. Even when available, sensitivity ranges from 73-85.9% in primary syphilis 4, 5, 6.

Direct Fluorescent Antibody (DFA) Testing - The Practical Alternative

If darkfield microscopy is unavailable, DFA testing for T. pallidum is the preferred alternative:

• Cleanse the lesion with gauze and saline 1
• Swab the lesion base and apply directly to a glass slide 1
• Allow the slide to dry completely before placing in holder and transporting to laboratory 1
• Transport at room temperature 1

DFA testing has comparable or superior sensitivity to darkfield (73-92.2%) and does not require live organisms 5, 6. The slide can be stored and transported more easily than darkfield specimens.

TPHA Testing - Serology, Not Direct Detection

TPHA (Treponema pallidum hemagglutination assay) is a serological test, not a direct detection method from lesions. This is a critical distinction:

• TPHA requires serum, not a swab from lesions 1
• Collect blood in a clot tube (red top tube without anticoagulant) 1
• Transport at room temperature within 2 hours 1
• TPHA is a treponemal confirmatory test used to verify positive screening results, not for direct pathogen detection 1

Understanding the Confusion

The question appears to conflate direct detection methods (darkfield/DFA from lesions) with serological testing (TPHA from blood). These serve different diagnostic purposes:

• Direct detection (darkfield/DFA): Identifies treponemes in lesion material, useful in primary syphilis when serology may be negative
• Serology (TPHA, RPR, VDRL): Detects antibodies in blood, becomes positive 1-4 weeks after chancre appears 2, 4

Modern Molecular Testing Options

Nucleic acid amplification tests (NAATs/PCR) are increasingly available and offer superior sensitivity, especially for healing or atypical lesions 1, 7:

• Swab the lesion base after cleansing 1
• Can use dry cotton swabs or commercial transport swabs (Eswab) 8, 9
• Room temperature storage is acceptable; specimens remain stable up to 28 days 8
• Sensitivity ranges from 75-95% for primary lesion exudate 7
• Does not require live organisms or immediate transport 8, 9

Practical Algorithm for Genital Ulcer Evaluation

1. Cleanse the lesion with sterile gauze and saline
2. If darkfield microscopy is available: Swab lesion base directly to slide, transport immediately
3. If DFA is available: Swab lesion base to slide, allow to dry, transport at room temperature
4. If NAAT/PCR is available: Swab lesion base with dry cotton or transport swab, can store at room temperature
5. Always obtain serology (RPR/VDRL and treponemal test like TPPA) from blood, even if direct detection is performed 2, 3, 2

Critical pitfall: In primary syphilis, serological tests may be negative in 22-38% of cases during the first weeks of infection 4. Direct detection methods are essential for early diagnosis when clinical suspicion is high but serology is negative 10.