Hepatitis A Serology

Hepatitis A virus (HAV) is a non-enveloped RNA virus of the picornavirus family, spread by the faecal oral route through contaminated food and water, and person-to-person contact. Serology detects IgM anti-HAV (acute infection) and total anti-HAV (IgG and IgM, indicating past infection or vaccination) by chemiluminescent or enzyme immunoassay.

HAV RNA by PCR is used in selected cases, such as outbreak investigation and immunocompromised patients. The incubation period is 2 to 6 weeks (average 4 weeks), and IgM anti-HAV is positive at symptom onset, peaking in the first month and persisting for 3 to 6 months, sometimes up to a year.

Hepatitis A causes an estimated 1.4 million cases each year worldwide. Infection in young children is usually asymptomatic or mild (70% under 6 years), while 70% of adults have jaundice. Mortality is under 0.5% overall but rises to 1 to 2% in older adults and patients with chronic liver disease, and fulminant liver failure occurs in about 0.5%.

Seroprevalence in adults is over 90% in low income countries with poor sanitation, but under 30% in high income countries, where outbreaks occur in men who have sex with men, people who inject drugs, people experiencing homelessness and after contaminated food.

Indications. Acute hepatitis with jaundice, dark urine, nausea, fever and raised transaminases (ALT often over 1000 U/L). Outbreak investigation and contact tracing. Travellers or food handlers with hepatitis. Pre-vaccination screening in people from high prevalence countries or those at risk of earlier infection, and post-exposure management.

Chronic liver disease (immunity check before vaccination). Immunity assessment in healthcare workers when required. Acute liver failure of unknown cause.

Specimen. Serum 1 mL, no fasting. Stable for a week at 2 to 8 °C and frozen for longer storage. Stool samples are used for HAV RNA PCR in outbreak settings.

Reference values (assay dependent).

• IgM anti-HAV: negative under 0.8 index (signal to cut off ratio), positive over 1.0, equivocal in between.
• Total anti-HAV: negative under 20 mIU/mL (immunity at 10 to 20 mIU/mL by assay), positive over 20 mIU/mL.
• HAV RNA: not detected.

Interpretation.

• A positive IgM anti-HAV in a patient with compatible hepatitis confirms acute hepatitis A. Sensitivity is over 95% and specificity over 98% when used for symptomatic patients.
• Isolated positive IgM without symptoms or raised transaminases is often a false positive (cross-reacting antibodies in other viral infections or autoimmune disease), and confirmation uses HAV RNA or repeat serology.
• Total anti-HAV after vaccination reflects protective antibody, and a level over 10 to 20 mIU/mL is considered protective. Routine post-vaccination testing is not needed in healthy people.
• Persistent IgM beyond 6 months raises the question of relapsing hepatitis A (up to 20%) or false positivity.
• Relapsing hepatitis A occurs in 3 to 20% within weeks to months and resolves without chronic disease. Prolonged cholestatic hepatitis (jaundice for over 12 weeks) also occurs.
• Hepatitis A does not become chronic.

Critical values and findings needing immediate action.

• Acute hepatitis A with INR over 1.5, hepatic encephalopathy, bleeding or ALT falling with rising bilirubin: acute liver failure, urgent transfer to a liver unit and transplant assessment.
• Acute hepatitis A in pregnancy, older adults, patients with chronic liver disease (particularly hepatitis B or C) or immunosuppression: admission and close monitoring for liver failure.
• Contacts of a confirmed case: post-exposure prophylaxis with hepatitis A vaccine (or immunoglobulin for infants, older people and those with chronic liver disease or immunodeficiency) within 14 days of exposure.
• Food handler or healthcare worker with acute hepatitis A: exclude from work for 7 days after jaundice onset, and notify public health.
• Outbreak setting: notify the public health authority and trace contacts.

Interferences and pitfalls.

• IgM anti-HAV can be falsely positive in other acute viral infections (Epstein-Barr virus, cytomegalovirus), autoimmune hepatitis, rheumatoid factor and heterophile antibody, particularly where hepatitis A prevalence is low. Testing is therefore restricted to patients with clinical or biochemical hepatitis.
• Passive antibody from immunoglobulin or blood products can cause a positive total anti-HAV for several months.
• Vaccinated people may have undetectable antibody with protective immunity (immune memory), and revaccination is not usually needed.
• Total anti-HAV in infants under 12 months may reflect maternal antibody.
• A negative IgM very early in illness (first days) can occur, and repeat testing is needed if suspicion persists.
• Acute hepatitis needs a full viral panel: hepatitis B surface antigen and IgM core antibody, hepatitis C antibody and RNA, hepatitis E IgM, and exclusion of drugs, alcohol and autoimmune causes.
• Blood borne transmission is rare, and serology cannot identify the source, so food and travel histories are taken.
• Faecal shedding peaks before symptoms and continues for 1 to 2 weeks after jaundice, longer in children.

Special groups.

• Children: usually asymptomatic or mild, and a source of household spread. Vaccination is given from 12 months in many countries.
• Pregnancy: acute hepatitis A can be associated with preterm labour, and vaccine is considered safe in pregnancy when indicated. Vertical transmission is rare.
• Older adults and chronic liver disease: increased risk of fulminant hepatitis, so vaccination is given in chronic liver disease, including hepatitis B and C.
• Immunocompromised patients and people with HIV: reduced vaccine response, so antibody levels are checked after vaccination.
• Regions: high endemicity in sub-Saharan Africa, South Asia and parts of Latin America, with early childhood infection and lifelong immunity. Improved sanitation raises the susceptible adult population and outbreak risk. Travellers to endemic areas are vaccinated.

Follow up tests. Liver function tests with ALT, AST, bilirubin, alkaline phosphatase and albumin, INR, full blood count, hepatitis B (HBsAg, anti-HBc IgM), hepatitis C antibody, hepatitis E IgM and RNA, HIV, Epstein-Barr virus and cytomegalovirus serology if IgM is unexpected, HAV RNA PCR with genotyping in outbreaks, liver ultrasound, repeat transaminases and bilirubin weekly until improvement, and anti-HAV after recovery to document immunity.