Guidelines for Interpreting Indeterminate HSV‑1/2 IgG Results

Interpretation of an Indeterminate Index

An index value of 0.98 falls within the “gray zone” (0.9–1.1) and cannot definitively confirm or exclude HSV‑2 infection. 1
Because the value is very close to the assay cut‑off, it cannot reliably separate true infection from laboratory variability or antibody cross‑reactivity. 1
The reflex HSV‑2 inhibition test is automatically triggered by borderline reactivity, yet the final index of 0.98 remains below the positive threshold. 1

For index values 1.1–2.9, false‑positive rates can reach ≈60 %. 1
Even at high index values (≥3.0), assay specificity is only 78.6 %, meaning more than 1 in 5 positive results may be false. 1
Individuals with prior HSV‑1 infection are markedly more prone to false‑positive HSV‑2 results, especially when the HSV‑2 index is low, due to antibody cross‑reactivity. 1
HSV‑1 antibody detection itself has a sensitivity of ≈70 %, leading to frequent false‑negative outcomes. 1

Do not assume HSV‑2 infection based solely on an indeterminate index. 1
If a possible exposure occurred within the past 12 weeks, repeat serologic testing after the 12‑week window because antibodies may still be maturing. 1, 2
When exposure happened more than 12 weeks ago, a repeat test 4–6 weeks later can help determine whether the index value shifts toward a definitive result. 1
Avoid immediate retesting after a recent exposure; allow at least 12 weeks for adequate antibody development. 1, 2

Polymerase chain reaction (PCR) or viral culture of active lesions is the diagnostic gold standard; PCR shows 96–98 % sensitivity and 95–99 % specificity for HSV detection from lesions. 3
Serology alone should not be used when lesions are available for direct testing. 1, 2
Western blot/immunoblot testing is regarded as the serologic gold standard for HSV. 1, 2
Employing a second assay that targets a different glycoprotein G antigen raises specificity from 93.2 % to 98.7 %. 1

Approximately 70 % of adults worldwide possess HSV‑1 antibodies acquired in childhood. 4
The presence of HSV‑1 antibodies increases the likelihood of false‑positive HSV‑2 results on commercial IgG assays. 1

Do not diagnose HSV‑2 infection based on an indeterminate serologic result, as this can cause unnecessary psychological distress, relationship issues, and unwarranted treatment. 1, 2
Do not rely on HSV‑1 serology to diagnose genital herpes; HSV‑1 is commonly acquired non‑sexually in childhood and cannot differentiate oral from genital infection. 1, 4