Herpes Simplex Virus 1 (HSV-1) Antibody, IgM

The Herpes Simplex Virus 1 (HSV-1) Antibody, IgM assay is ordered when a recent primary HSV-1 infection is suspected, including individuals presenting with an apparent first episode of genital herpes. It complements type-specific HSV-1 and HSV-2 IgG testing. When interpreted with paired type-specific IgG results, HSV-1 IgM (herpes simplex virus type 1 IgM antibodies) can help distinguish primary infection from reactivation and provide context for counseling regarding timing of acquisition and potential transmission.

Herpes simplex viruses are double-stranded DNA viruses that establish lifelong latency. HSV-1 is classically linked to orofacial disease and HSV-2 to genital disease, though either type can involve either site. After a primary infection, HSV-specific IgM typically becomes detectable about 9–10 days after symptom onset, persists for approximately 1–2 weeks, and then declines; in uncommon cases it may remain detectable for up to 6 weeks. IgG develops by 3–4 weeks, persists lifelong, and may increase with recurrences. During reactivation, an HSV-1 IgM response can occur but is usually modest and does not reach the titers seen in primary infection. Type-specific serologic assays that separately measure IgM and IgG to HSV-1 and HSV-2 improve interpretation in first-episode genital herpes and help infer timing relative to exposure. Serology does not identify the anatomic portal of entry; both orofacial and genital primary infections can elicit HSV-1 IgM. Results are interpreted alongside the clinical presentation and, when appropriate, direct detection methods such as PCR.