Infectious Disease Serology
Viral Infectious Disease Serology
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Viral serology is time dependent. An antibody result reflects the host response, while an antigen or nucleic-acid result detects a viral target. Correct interpretation combines the specimen date, symptom or exposure date, vaccination and infection history, immune competence, and any direct detection result.
Reading viral markers
The meaning of each IgM, IgG, antigen, and nucleic-acid result depends on the virus, specimen, assay, and collection time. Neutralizing antibody binds extracellular virus and can block attachment or entry. Secretory IgA helps protect mucosal surfaces. Type I interferons help create an antiviral state, and natural killer cells can remove infected cells before a virus-specific antibody response is fully developed. Once a cell is infected, CD8-positive T lymphocytes can recognize viral peptides presented by HLA class I and kill the infected cell. Mutation, latency, reactivation, and altered antigen presentation can change which viral targets remain detectable. Serology alone may have limited sensitivity during early infection or in an immunocompromised patient.
| Result type | What it usually establishes | Main interpretation limit |
|---|---|---|
| Virus-specific IgM | A recent immune response for selected viruses | Persistence, reactivation, vaccination, cross-reactivity, and weak responses can change its meaning |
| Virus-specific IgG | Past exposure or a maturing response | Protective immunity depends on the virus, assay, and clinical context; a single result often leaves the time of infection unresolved |
| Seroconversion | A change from antibody negative to positive in paired specimens | Both specimens need suitable timing and a comparable validated method |
| Significant titer rise | A recent response when organism-specific guidance defines the required rise | The rule, collection interval, and calculation are method specific |
| Viral antigen | Presence of the measured viral protein | The detectable interval and relation to infectivity vary by virus and assay |
| Nucleic-acid amplification test (NAAT) | Presence of the target viral DNA or RNA | The result may represent active replication, latent genome, or low-level detection according to the virus and specimen |
Maternal IgG crosses the placenta. Virus-specific IgG in a newborn can therefore reflect maternal antibody, and congenital infection requires an organism-specific approach. Immunocompromised patients may form antibody late, weakly, or incompletely. A direct test can answer the question sooner when antibody is absent during the window period.
Hepatitis virus patterns
Hepatitis A virus (HAV), hepatitis B virus (HBV), hepatitis C virus (HCV), hepatitis D virus (HDV), and hepatitis E virus (HEV) can produce similar increases in alanine aminotransferase and bilirubin. Those chemistry findings show liver injury and cholestasis, while the viral marker pattern identifies the cause and stage.
| Virus | Main exposure route | Chronic infection | Laboratory focus |
|---|---|---|---|
| HAV | Fecal-oral | None | IgM anti-HAV for recent infection; IgG or total anti-HAV for prior infection or vaccination1 |
| HEV | Commonly fecal-oral; zoonotic food exposure also occurs | Uncommon, mainly in immunocompromised people | Anti-HEV IgM and HEV RNA for recent or current infection2 |
| HBV | Blood and body fluids; sexual and perinatal transmission | Yes, with the greatest risk after perinatal or early-childhood acquisition | A marker panel separates susceptibility, vaccination, acute infection, chronic infection, and recovery3 |
| HDV | Blood and body fluids in a person with HBV infection | Yes after selected superinfections | Total anti-HDV followed by HDV RNA identifies exposure and current viremia4 |
| HCV | Primarily blood exposure | Common | Anti-HCV with reflex HCV RNA separates exposure from current infection5 |
Hepatitis A and E
HAV. IgM anti-HAV is commonly detectable shortly before symptoms and can remain detectable for about 6 months. A reactive IgM result supports current or recent HAV infection in a patient with a compatible illness or exposure. Recent vaccination and cross-reactivity can also produce reactivity, so population screening with IgM creates misleading results. IgG anti-HAV persists after infection or vaccination. Total anti-HAV contains IgM and IgG; a reactive total result with nonreactive IgM supports previous infection or vaccination. HAV RNA in serum or stool is used in selected early, reference, and public-health investigations.1
HEV. Anti-HEV IgM supports recent infection, and anti-HEV IgG supports prior exposure. The duration and performance of these markers vary among assays and populations. HEV RNA in blood or stool provides direct evidence of current infection. RNA testing is especially useful when an immunocompromised patient has a weak or absent antibody response or when chronic HEV is suspected. A late negative RNA result does not exclude earlier infection because viremia and stool shedding can decline before testing.2
Hepatitis B
CDC recommends HBsAg, anti-HBs, and total anti-HBc as the initial triple panel for adults who have never been screened. HBeAg, anti-HBe, IgM anti-HBc, and HBV DNA answer follow-up questions about recent infection and viral replication.3
| Marker | Laboratory meaning | Qualification |
|---|---|---|
| HBsAg | Current HBV infection | It may be transiently reactive within 30 days after hepatitis B vaccination; persistence for 6 months or longer supports chronic infection3 |
| Anti-HBs | Recovery with immunity or a vaccine response | CDC uses a concentration greater than 10 mIU/mL as evidence of immunity after a completed vaccine series; concentrations can wane in vaccine responders3 |
| Total anti-HBc | Current or past natural infection | It contains IgM and IgG anti-HBc, commonly persists for life, and is absent after vaccination alone3 |
| IgM anti-HBc | Recent infection, usually within 6 months | It can reappear during a severe flare or reactivation of chronic HBV3 |
| HBeAg | A pattern commonly associated with higher HBV replication | HBeAg-negative variants can still have substantial HBV DNA4 |
| Anti-HBe | Seroconversion after loss of HBeAg | This marker alone cannot establish low replication or low infectivity4 |
| HBV DNA | Circulating viral nucleic acid and replication level | Sensitive NAAT can become reactive before HBsAg; the assay, specimen, and reporting range govern interpretation4 |
The combined pattern of the initial triple panel determines the main screening interpretation.
| HBsAg | Total anti-HBc | Anti-HBs | Additional marker | Interpretation |
|---|---|---|---|---|
| Nonreactive | Nonreactive | Nonreactive | None | Susceptible when there is no documented completed vaccine series3 |
| Nonreactive | Nonreactive | Reactive | Vaccination history | Immune after vaccination3 |
| Nonreactive | Reactive | Reactive | None | Resolved natural infection; consider reactivation risk in an immunosuppressed setting3 |
| Reactive | Reactive | Nonreactive | IgM anti-HBc reactive | Acute infection3 |
| Reactive | Reactive | Nonreactive | IgM anti-HBc nonreactive | Chronic infection when persistence is established3 |
| Nonreactive | Reactive | Nonreactive | Assess history and HBV DNA as indicated | Isolated anti-HBc pattern: resolved infection with waned anti-HBs, occult infection, false reactivity, passive antibody in an infant, or an HBsAg mutant3 |
HBV DNA can become reactive before HBsAg. HBsAg appears several weeks after exposure, IgM anti-HBc usually appears near symptom onset, and anti-HBs appears after HBsAg clears during recovery. During the HBV window period, HBsAg has disappeared and anti-HBs has not appeared. IgM anti-HBc is the marker of recent infection. Total anti-HBc is also reactive because it includes IgM. HBV DNA testing can clarify current infection when the serologic pattern is incomplete or discordant.34
Blood-donor HBV testing uses product-specific federal requirements and is covered in Product Processing, Storage, Components, and Quality Control.
Hepatitis D and C
HDV requires HBV surface antigen to form infectious particles. Coinfection means acquisition of HBV and HDV together. Superinfection means acquisition of HDV by a person who already has HBV. Begin with total anti-HDV in an HBsAg-reactive patient according to the applicable testing policy. A reactive antibody result shows exposure, and HDV RNA establishes current viremia. WHO supports anti-HDV testing followed by RNA testing and describes reflex testing as a way to reduce missed follow-up.4 CDC surveillance criteria also separate total anti-HDV exposure evidence from HDV RNA evidence of current infection; surveillance criteria serve case classification and are not a clinical diagnostic algorithm.6
IgM anti-HDV can support recent infection, but it may be delayed, brief, or present during chronic infection and cannot establish current viremia by itself.6
HCV testing begins with an FDA-approved anti-HCV assay and reflexes a reactive result to HCV RNA NAAT. Anti-HCV usually becomes detectable 8 to 11 weeks after exposure. RNA can be present earlier, so recent exposure within 6 months, immunocompromise, or strong clinical suspicion can justify RNA testing after a nonreactive antibody result. Anti-HCV generally persists after spontaneous clearance or treatment and cannot distinguish current from resolved infection.5
| Anti-HCV | HCV RNA | Interpretation |
|---|---|---|
| Nonreactive | Usually not performed | No serologic evidence of exposure; test RNA when recent exposure or impaired antibody formation remains a concern |
| Reactive | Detected | Current HCV infection |
| Reactive | Not detected | No current HCV infection in most cases; repeat RNA when recent exposure, ongoing risk, or specimen concerns remain |
Quantitative HCV RNA reports viral load for clinical monitoring. A change is interpreted with the same assay’s measuring range and the clinical time point. Donor screening, perinatal testing, and treatment endpoints use their own current algorithms.
Epstein-Barr virus
Epstein-Barr virus (EBV) infects B lymphocytes through interactions that include the CD21 receptor. The resulting polyclonal B-cell response can produce heterophile antibodies. CDC advises against Monospot testing for general use because false-positive and false-negative results occur, especially in young children and early illness. EBV-specific antibody patterns provide a clearer answer when laboratory confirmation matters.7
| EBV marker | Time course | Interpretive use |
|---|---|---|
| VCA IgM | Appears early and usually disappears within 4 to 6 weeks | Supports primary infection when EBNA antibody is absent7 |
| VCA IgG | Appears during acute infection, peaks at about 2 to 4 weeks, and persists for life | Establishes exposure; timing requires the rest of the panel7 |
| Early antigen IgG | Often appears during acute illness and usually falls within 3 to 6 months | Supports recent activity, although about 20% of healthy people retain it for years7 |
| EBNA antibody | Standard immunofluorescence commonly becomes reactive 2 to 4 months after symptom onset and then persists | Its absence supports primary infection; VCA IgG plus EBNA supports past infection7 |
Paired acute and convalescent EBV sera add little because VCA antibodies often appear near the start of illness. A high VCA IgG result can occur in recent and remote infection, so a high concentration alone does not establish recent infection.7
The CBC and blood-film findings of infectious mononucleosis are covered in Benign Leukocyte Disorders.
Cytomegalovirus
Cytomegalovirus (CMV) persists after primary infection and may reactivate during immunosuppression. The laboratory question determines the test.8
| Laboratory question | Preferred evidence | Interpretation |
|---|---|---|
| Has a transplant donor or recipient encountered CMV? | CMV IgG | Donor and recipient IgG define the pretransplant risk group; D-positive/R-negative is the highest-risk solid-organ pairing8 |
| Is CMV DNA present or changing after transplant? | Quantitative CMV NAAT in plasma or whole blood | Use one specimen type and one assay for serial results; universal treatment thresholds are unavailable8 |
| Did an immunocompetent patient recently acquire CMV? | IgG seroconversion, supported by the complete antibody pattern | CMV IgM can persist, recur with reactivation, or react falsely; IgM alone leaves primary infection unresolved9 |
| Does a newborn have congenital CMV? | Saliva PCR within 3 weeks of birth, confirmed with urine PCR | Collect saliva more than 1 hour after breastfeeding; antibody testing cannot establish congenital CMV9 |
Quantitative CMV DNA is the main current blood method for transplant surveillance and monitoring. The pp65 antigenemia assay remains an alternative in selected settings. A positive blood NAAT shows DNAemia; tissue-invasive disease requires the appropriate clinical and tissue evidence. Culture and shell-vial methods have limited roles in current blood monitoring.8
Detailed transplant support and component selection are covered in Transplantation Immunology and Laboratory Support.
Varicella-zoster virus
PCR of material from a skin lesion is the preferred test for current varicella-zoster virus (VZV) infection, especially in an atypical, severe, or outbreak-associated presentation. Scabs, vesicular fluid, and cells from the lesion base provide useful material. A positive lesion PCR detects VZV DNA; the clinical pattern distinguishes primary varicella from herpes zoster.10
VZV IgM has poor specificity and cannot reliably separate primary infection from reinfection or reactivation. A fourfold IgG rise in paired sera can support recent infection, though it is less practical and may be insensitive in a vaccinated patient. A positive VZV IgG result documents antibody from past varicella or vaccination; it cannot distinguish those sources or confirm a current case. Commercial IgG assays can miss vaccine-induced antibody, so documented completion of the recommended vaccine series remains evidence of immunity after a later nonreactive commercial result.10
Rubella, measles, and mumps
These vaccine-preventable infections require clinical and public-health context because low disease prevalence increases the proportion of reactive IgM results caused by cross-reactivity or another illness. Coordinate a suspected case promptly with the state or local health department.
Rubella
Rubella testing separates immune-status assessment from evaluation of a suspected recent infection.
| Question | Test and interpretation |
|---|---|
| Is there serologic evidence of immunity? | Use rubella IgG. CDC uses greater than 10 IU/mL as the U.S. EIA cutoff; the laboratory applies its calibrated assay cutoff. IgM is unsuitable for immunity screening.11 |
| Is recent infection suspected? | Collect rubella IgM and IgG with clinical, exposure, and vaccination information. Seroconversion or a validated fourfold IgG rise in paired specimens can also support recent infection. Only about half of cases are IgM reactive on the day of rash; more than 90% are reactive by about day 5. Repeat an early negative sample when suspicion remains.11 |
| Is a reactive IgM credible during pregnancy? | Confirm through the public-health or reference-laboratory pathway. Repeat testing, a different assay format, RT-PCR in selected cases, and IgG avidity can resolve cross-reactivity, persistence, recent vaccination, and recent infection.11 |
| Does an infant have congenital rubella syndrome? | Infant rubella IgM, rubella RNA, or infant IgG that persists after maternal antibody should have declined can provide evidence. IgG at birth may be maternal.12 |
Low-avidity rubella IgG supports recent exposure when measured with an assay-specific cutoff; high avidity supports more distant infection or vaccination. Routine IgM screening of an asymptomatic, unexposed pregnant patient creates avoidable false-positive results.11
Measles
For suspected measles, collect serum and a throat or nasopharyngeal swab at first contact. Measles IgM provides presumptive evidence, while real-time reverse-transcription PCR (rRT-PCR) on the respiratory specimen can confirm viral RNA and is most sensitive near rash onset. A nonreactive IgM specimen collected within the first 3 days of rash can be repeated after day 3 when PCR is negative or unavailable and suspicion remains. Recent MMR vaccination can complicate serology; public-health genotyping can distinguish vaccine virus from wild-type virus in selected cases.13
Mumps
Mumps rRT-PCR on a buccal swab is the preferred confirmation method. Collect the buccal swab as soon as possible after parotitis begins. If collection occurs after day 3, add serum for IgM. IgM supports the diagnosis and may be weak or absent after vaccination. Collection timing and specimen quality affect the interpretation of a negative result.14
Human T-lymphotropic virus
Human T-lymphotropic virus type 1 (HTLV-1) is associated with adult T-cell leukemia/lymphoma and HTLV-1-associated myelopathy/tropical spastic paraparesis. HTLV-2 has no comparably established disease association. Both viruses integrate proviral DNA into host lymphocytes, and established infection produces persistent antibody. Seroconversion timing varies, especially with partial immune responses.15
| Step | Method | Interpretation |
|---|---|---|
| Screen | Combined anti-HTLV-1/2 enzyme or chemiluminescent immunoassay | A reactive result requires the current assay-specific supplemental algorithm15 |
| Supplemental serology | A validated line immunoassay or other method that measures antibodies to multiple viral proteins | Resolves many false-reactive screens and may differentiate HTLV-1 from HTLV-215 |
| Molecular resolution | Proviral-DNA PCR on peripheral blood mononuclear cells | Can resolve an indeterminate serologic pattern and differentiate viral type15 |
Western blot has produced many indeterminate patterns and is no longer a universal default for clinical confirmation. Laboratories follow the current validated screening and supplemental algorithm and report reactive, positive, negative, or indeterminate results according to that algorithm.15
U.S. blood-donor screening follows separate federal anti-HTLV-I/II screening and supplemental-testing rules covered in Product Processing, Storage, Components, and Quality Control.16
References
- Centers for Disease Control and Prevention. Clinical screening and diagnosis for hepatitis A. January 11, 2024. Accessed August 30, 2026.
- World Health Organization. Hepatitis E. Updated July 28, 2026. Accessed August 30, 2026.
- Centers for Disease Control and Prevention. Clinical testing and diagnosis for hepatitis B. Updated August 14, 2026. Accessed August 30, 2026.
- World Health Organization. Guidelines for the Prevention, Diagnosis, Care and Treatment for People With Chronic Hepatitis B Infection. World Health Organization; 2024. Accessed August 30, 2026.
- Centers for Disease Control and Prevention. Clinical screening and diagnosis for hepatitis C. January 31, 2025. Accessed August 30, 2026.
- Centers for Disease Control and Prevention. Hepatitis D virus 2025 case definition. Accessed August 30, 2026.
- Centers for Disease Control and Prevention. Laboratory testing for Epstein-Barr virus. April 10, 2024. Accessed August 30, 2026.
- Kotton CN, Kumar D, Manuel O, et al. The Fourth International Consensus Guidelines on the Management of Cytomegalovirus in Solid Organ Transplantation. Transplantation. 2025;109(7):1066-1110. doi:10.1097/TP.0000000000005374.
- Centers for Disease Control and Prevention. Laboratory testing for CMV and congenital CMV. April 15, 2024. Accessed August 30, 2026.
- Centers for Disease Control and Prevention. Laboratory testing for varicella-zoster virus. May 10, 2024. Accessed August 30, 2026.
- Centers for Disease Control and Prevention. Serology testing for rubella. June 10, 2024. Accessed August 30, 2026.
- Centers for Disease Control and Prevention. Chapter 15: congenital rubella syndrome. Manual for the Surveillance of Vaccine-Preventable Diseases. Updated February 10, 2026. Accessed August 30, 2026.
- Centers for Disease Control and Prevention. Laboratory testing for measles. June 12, 2024. Accessed August 30, 2026.
- Centers for Disease Control and Prevention. Laboratory testing for mumps. June 11, 2024. Accessed August 30, 2026.
- World Health Organization. Human T-Lymphotropic Virus Type 1: Technical Report. World Health Organization; 2021. Accessed August 30, 2026.
- US Food and Drug Administration. Use of Serological Tests to Reduce the Risk of Transfusion-Transmitted Human T-Lymphotropic Virus Types I and II (HTLV-I/II): Guidance for Industry. Published February 2020. Accessed August 30, 2026.