18.2 Viral Serodiagnosis: Hepatitis, EBV, HIV, CMV, Rubella, Measles

18.2 Viral Serodiagnosis: Hepatitis, EBV, HIV, CMV, Rubella, Measles

Quick Answer: Read panels, not single markers. Acute HAV is anti-HAV IgM. HBV is HBsAg (current infection), anti-HBs (immunity), anti-HBc IgM/total (exposure, not vaccine), and HBeAg (higher infectivity). HCV antibody is a screen; RNA confirms current infection. Acute EBV is VCA IgM/IgG positive and EBNA negative; past EBV is VCA IgG plus EBNA. HIV is Ag/Ab combo, then differentiation, then NAAT if needed. CMV IgM is limited; IgG avidity times primary infection; viral load is better for transplant disease. Rubella and measles: IgM for acute disease, IgG for immunity.

Outline III.E.3 (guideline 2025-09-25) tests serodiagnosis, clinical significance, and epidemiology of hepatitis A/B/C, EBV, HIV, CMV, rubella, and measles. Virus biology and plasma viral-load matrices were Chapter 15. This section is what the antibodies and antigens mean when they come back as a panel. West Nile and other arbovirus encephalitis serology is SM-only (III.E.4) and is not M-tested.

Hepatitis A, B, and C

Hepatitis A is fecal–oral and never chronic. Anti-HAV IgM diagnoses acute infection. Anti-HAV IgG (or total anti-HAV after IgM has gone) marks immunity from infection or vaccine. Do not look for a chronic HAV carrier state.

Hepatitis B is the panel exam. Markers are not interchangeable.

Classic patterns to memorize cold:

• Vaccinated: anti-HBs only. HBsAg negative, anti-HBc negative.
• Acute HBV: HBsAg positive, IgM anti-HBc positive, total anti-HBc positive, anti-HBs negative.
• Window: HBsAg negative, anti-HBs still negative, IgM anti-HBc positive. This is why isolated IgM anti-HBc is not “nothing.”
• Chronic HBV: HBsAg positive for more than 6 months, total anti-HBc positive, IgM anti-HBc negative or low, anti-HBs negative. HBeAg versus anti-HBe then splits high versus lower infectivity.
• Recovered: HBsAg negative, anti-HBc positive, anti-HBs positive.
• Isolated anti-HBc: false-positive core antibody, window, occult HBV, or remote infection with waning anti-HBs. Do not call it vaccine immunity.

Hepatitis C screening is HCV antibody. Antibody does not prove viremia: it remains after spontaneous clearance and after cure. HCV RNA (NAAT) confirms current infection and is the test that changes management. Antibody-positive, RNA-negative means resolved or treated infection or a false-positive antibody—not a license to skip RNA. There is no HCV vaccine, so anti-HCV is never a “vaccinated” pattern.

Epidemiology still matters. HAV is food, water, and close contact. HBV is blood, sexual contact, and perinatal. HCV is blood, with injection-drug use the dominant current risk. Do not assign fecal–oral transmission to HBV or HCV on a serology item.

EBV: VCA, EBNA, and heterophile

Epstein–Barr virus causes infectious mononucleosis in adolescents and young adults and then persists for life. Serology times acute versus past infection; it does not replace plasma EBV PCR when a transplant patient is being monitored for PTLD (that viral-load use was Chapter 15).

• VCA IgM: acute infection. Appears early and usually fades over weeks to a few months.
• VCA IgG: appears in acute infection and lasts for life.
• EBNA IgG: nuclear antigen antibody appears later (often 1–6 months). EBNA is negative in true acute IM and positive in past infection.
• Heterophile antibody (Monospot / Paul–Bunnell): IgM that agglutinates sheep or horse red cells. It is not EBV-specific. It supports acute IM in older children and adults but is often false-negative in young children and can be false-positive in other illnesses.

Reactivation and EA (early antigen) patterns appear in immunocompromised hosts; M still wants you to separate acute IM (EBNA negative) from past infection (EBNA positive) before you invent a fourth pattern.

HIV: combo, differentiation, NAAT

Adult screening uses a fourth-generation HIV-1/2 antigen/antibody combination immunoassay that detects IgM/IgG plus p24 antigen. p24 shortens the window relative to antibody-only tests. A reactive combo is not a final diagnosis.

CDC laboratory algorithm:

1. Reactive Ag/Ab combo.
2. HIV-1/HIV-2 antibody differentiation immunoassay.
3. If differentiation is negative or indeterminate, HIV-1 NAAT (RNA).

Acute (Fiebig) infection can be combo-reactive because of p24 while differentiation is still negative and NAAT is positive—that is true early HIV, not a random false positive. HIV-2 is uncommon in the United States but the differentiation step exists so you do not treat or counsel the wrong virus. Third-generation antibody-only EIAs miss the p24-only window. Do not diagnose HIV on a single rapid antibody device without the algorithm when the stem is a laboratory algorithm question. Plasma viral load monitors known infection; it is not the population screening test.

CMV, rubella, and measles

CMV IgM is a weak stand-alone test. It can be falsely positive (RF, heterologous IgM), can persist, and can reappear with reactivation. CMV IgG documents past infection and is the donor/recipient serostatus used to risk-stratify transplant patients (D+/R− is the highest primary-infection risk). IgG avidity times primary infection: low avidity supports recent primary infection (typically within about 3–4 months); high avidity supports past infection. In transplant recipients with fever, leukopenia, pneumonitis, or colitis, quantitative plasma CMV viral load is the disease and preemptive-therapy marker—not a rising IgM. Congenital CMV is diagnosed with neonatal urine or saliva PCR (and blood PCR), not with maternal IgG, which only proves the mother was ever infected.

Rubella IgM diagnoses acute infection in a compatible rash illness. Rubella IgG documents immunity, which is why prenatal panels order it. Congenital rubella syndrome uses infant IgM (fetal isotype) and persistent IgG; neonatal IgG alone may be maternal. Nonimmune pregnant patients need counseling and follow-up, not a measles-style outbreak PCR as the first immunity test.

Measles IgM, collected at or after rash onset (sensitivity rises after the first couple of days), supports acute measles together with NAAT on throat or NP and urine. Measles IgG documents immunity from vaccine or past disease. During an outbreak, IgM plus PCR and public-health notification matter more than a lone heterophile. Do not use EBV VCA to diagnose measles, and do not use measles IgG as an acute diagnostic in a rash that started yesterday.

Across this list the exam trick is the same: one marker is a fragment. HBsAg without anti-HBc cannot tell vaccine from infection. HCV antibody without RNA cannot tell cured from viremic. VCA IgG without EBNA cannot tell acute from past. HIV combo without differentiation/NAAT is a screen. CMV IgM without viral load does not diagnose transplant disease.