STI Testing Beyond Syphilis: HIV Algorithms, GC/CT NAAT, and the Herpes Serology Dilemma

 

STI Testing Beyond Syphilis: HIV Algorithms, GC/CT NAAT, and the Herpes Serology Dilemma

The testing rules that have changed, the test you probably shouldn't be ordering, and the window periods that still trip everyone up.

The syphilis post covered one STI in depth. But the full STI panel has its own set of testing nuances, window periods, and pitfalls that new NPs need to master. HIV testing has evolved dramatically with 4th-generation assays, gonorrhea/chlamydia NAAT is site-specific, and herpes serology is the test that causes more unnecessary distress than almost any lab in primary care.

HIV Testing: The Modern Algorithm

4th-Generation Ag/Ab Combo Test (The Standard)

The current recommended initial screen is the 4th-generation HIV-1/2 antigen/antibody combination immunoassay. It detects both HIV antibodies AND p24 antigen, shortening the window period to approximately 2–4 weeks after exposure (compared to 3–12 weeks for older antibody-only tests).

The CDC Recommended Algorithm

1. 4th-gen Ag/Ab combo test (screen). If nonreactive → negative. Done.
2. If reactive → HIV-1/HIV-2 antibody differentiation immunoassay (confirmatory).
3. If differentiation assay is indeterminate or negative → HIV-1 RNA (NAT) to detect acute infection (p24 antigen-positive but antibody-negative = acute HIV).

Key Pitfalls

• Western blot is obsolete for HIV confirmation. The differentiation immunoassay has replaced it. If your lab is still using Western blot, they're outdated.
• The window period still exists: 4th-gen tests can miss very early acute infection (<2 weeks). If exposure was very recent and clinical suspicion is high (acute retroviral syndrome), order an HIV-1 RNA (viral load) directly.
• PrEP patients: HIV testing every 3 months per guidelines. Always use the 4th-gen combo, not a rapid antibody-only test, which has a longer window period.
• Universal screening: CDC recommends all adults 13–64 be tested at least once, regardless of risk factors. All pregnant patients should be tested at the first prenatal visit.

Gonorrhea and Chlamydia: NAAT Is King

Nucleic Acid Amplification Testing (NAAT)

NAAT is the gold standard for GC/CT detection—far more sensitive than culture. Key points:

• Site-specific testing: Urogenital NAAT does NOT detect pharyngeal or rectal infections. If the patient has risk factors for extragenital exposure, you must specifically order pharyngeal and rectal NAATs. This is routinely missed.
• Specimen type: First-void urine (for urethral/cervical) or swab (vaginal self-swab is FDA-approved and equally accurate). Urine testing in women is less sensitive than vaginal swab.
• Test of cure (TOC): For GC, repeat NAAT at 14 days post-treatment (not sooner—dead organisms can cause false-positive NAAT up to 14 days). For CT, TOC is recommended for pregnant patients and when adherence is uncertain; retest at 4 weeks.
• Reinfection screen: Retest ALL patients treated for GC or CT at 3 months (high reinfection rates). This is the most commonly skipped follow-up in STI management.

Pitfall

A negative urine GC/CT in a patient with pharyngeal symptoms or rectal complaints means nothing—you tested the wrong site. Always ask about sexual practices to determine which sites need testing. MSM should routinely be screened at all three sites (urogenital, pharyngeal, rectal).

Herpes Serology: The Test You Probably Shouldn't Order

The CDC Does Not Recommend Routine HSV Screening

This is the most controversial testing decision in STI medicine. The CDC explicitly recommends against routine serologic screening for HSV in asymptomatic individuals. Here's why:

• HSV-1 seroprevalence is ~50–70% in the US adult population. A positive HSV-1 IgG tells you almost nothing clinically useful—it could be oral herpes from childhood, genital HSV-1, or completely asymptomatic carriage.
• HSV-2 IgG has a significant false-positive rate at low index values (1.1–3.5). The test has poor positive predictive value in low-prevalence populations, causing unnecessary psychological harm, relationship disruption, and anxiety.
• A positive HSV IgG doesn't tell you the site of infection, when it was acquired, or whether the patient is or will be symptomatic.

When Herpes Testing IS Appropriate

• Active genital lesions: PCR swab of the lesion (not serology) is the test of choice. PCR is far more sensitive than viral culture.
• Recurrent genital symptoms suspicious for herpes but prior swab negative: Type-specific HSV IgG (HSV-1 and HSV-2 separately) can help confirm diagnosis.
• Partner of someone with known genital herpes: Type-specific serology can determine if the uninfected partner already has antibodies.
• HIV-positive patients: Serologic screening for HSV-2 is reasonable given increased risks.
• Pregnancy with unknown HSV status and partner with known genital herpes: Serology helps guide suppressive therapy decisions near delivery.

The Low-Index-Value Trap

An HSV-2 IgG index value of 1.1–3.5 has a high false-positive rate and should be confirmed with a supplemental assay (like the Biokit HSV-2 rapid test or the University of Washington Western blot, the gold standard). Do NOT diagnose a patient with genital herpes based on a low-positive HSV-2 IgG alone. The psychological and relational damage from a false diagnosis is real and lasting.

Trichomonas

NAAT is now the most sensitive test for Trichomonas vaginalis and is preferred over wet mount (which misses up to 50% of infections). Many GC/CT NAAT platforms now include trichomoniasis. Screen sexually active women with vaginal symptoms, and consider routine screening in high-prevalence populations (especially Black women, who have disproportionately high rates).

Screening Summary by Population

Population	What to Screen	How Often
All adults 13–64	HIV (4th-gen)	At least once
Sexually active women <25 (or ≥25 with risk factors)	GC/CT (urogenital NAAT)	Annually
MSM	HIV, syphilis, GC/CT (all 3 sites), HCV (if HIV+)	At least annually; every 3–6 months if high risk
Pregnant patients	HIV, syphilis (×3), HBV, GC/CT, HCV	First prenatal visit; repeat syphilis at 28 weeks and delivery
PrEP patients	HIV (4th-gen), syphilis, GC/CT (all sites), renal function	Every 3 months
Asymptomatic individuals	NOT: HSV serology, routine trichomoniasis	Do not screen routinely

Bottom Line

HIV testing has evolved—use the 4th-gen combo and stop relying on Western blot. GC/CT testing must be site-specific to be useful. Herpes serology should almost never be ordered routinely and causes more harm than good in asymptomatic patients. And the most commonly missed step in all of STI management: the 3-month retest after treating GC or CT.

Stay sharp out there.