Parvovirus B19 Serology

Parvovirus B19 is a small single stranded DNA virus that infects erythroid progenitor cells through the P antigen (globoside) receptor. It spreads by respiratory droplets, blood products and from mother to foetus. Serology measures B19 IgM (recent infection) and B19 IgG (past infection and immunity) by enzyme immunoassay or chemiluminescence.

B19 DNA by quantitative PCR in serum, plasma, bone marrow, amniotic fluid or foetal blood detects viraemia and is the test of choice in immunocompromised patients, in aplastic crisis before antibody appears, and in foetal infection.

Infection causes erythema infectiosum (fifth disease, slapped cheek rash) in children, an arthropathy in adults (especially women), transient aplastic crisis in people with chronic haemolysis, chronic red cell aplasia in immunodeficiency, and hydrops fetalis. About 50 to 60% of adults are seropositive, rising to over 80% in older age. Susceptible pregnant women have a 1 to 2% chance of infection in an epidemic, with foetal loss in 5 to 10% of infections before 20 weeks, and hydrops in about 3%.

Indications. Symmetrical polyarthropathy of hands, wrists and knees with or without rash. Sudden anaemia, reticulocytopenia and pallor in sickle cell disease, thalassaemia, hereditary spherocytosis or other haemolytic states (aplastic crisis). Persistent anaemia in an immunocompromised patient (HIV, transplant, chemotherapy, congenital immunodeficiency).

Pregnant woman exposed to a rash illness or with a rash. Foetal hydrops, unexplained foetal anaemia or intrauterine death. Rash illness with negative rubella and measles tests. Unexplained hepatitis, myocarditis or pancytopenia.

Specimen. Serum 1 mL, no fasting. Stable for days at 2 to 8 °C. EDTA blood or plasma 2 to 3 mL for DNA PCR. Amniotic fluid 5 to 10 mL or foetal blood in specialist units.

Reference values (assay dependent). B19 IgM and IgG: negative (index under 0.8 or under 9 U/mL), equivocal, positive (index over 1.1). B19 DNA: not detected (quantitative results in IU/mL).

Interpretation.

• IgM appears 10 to 12 days after infection (at the rash or arthropathy stage), peaks at 30 to 60 days and persists for 2 to 6 months. IgG follows within days and persists for life.
• In erythema infectiosum and arthropathy, the rash and joint symptoms are immune mediated and viraemia has already cleared, so DNA can be negative while IgM is positive.
• In aplastic crisis, viraemia is high (over 10^10 IU/mL) and antibody is absent at presentation, so PCR is diagnostic and IgM follows a few days later.
• In immunocompromised patients, persistent anaemia with a high DNA load and absent or weak antibody indicates chronic infection, treated with intravenous immunoglobulin.
• In pregnancy, maternal IgM positivity with low IgG avidity confirms recent infection. IgG positive and IgM negative before or at exposure indicates immunity (over 99% protection), and no further action is needed. Susceptible women with exposure are retested at 4 weeks.
• Foetal infection is monitored by serial ultrasound with middle cerebral artery peak systolic velocity (to detect anaemia) every 1 to 2 weeks for 8 to 12 weeks after maternal infection, and by foetal blood sampling with intrauterine transfusion when anaemia or hydrops occurs.
• Low level B19 DNA can persist for months to years in tissue and blood of healthy people (up to 1000 IU/mL), and does not indicate acute infection.

Critical values and findings needing immediate action.

• Sudden pallor, tiredness and haemoglobin fall with reticulocytes under 1% in haemolytic anaemia: aplastic crisis, admission, transfusion support and isolation of the patient (highly infectious in the days before the crisis).
• Foetal hydrops, ascites, pericardial effusion or raised middle cerebral artery velocity after maternal infection: urgent foetal medicine referral for intrauterine transfusion.
• Persistent severe anaemia in an immunodeficient patient with high B19 DNA: intravenous immunoglobulin.
• Pregnant healthcare worker or teacher exposed to B19 with a negative IgG: urgent IgM and IgG, and repeat in 4 weeks.
• Non-immune pregnant woman or immunodeficient patient in contact with a patient in aplastic crisis: isolation precautions.

Interferences and pitfalls.

• B19 IgM may be false positive from cross-reactivity (rubella, measles, Epstein-Barr virus, cytomegalovirus, autoimmune disease, rheumatoid factor), and can persist for months.
• Antibody is absent at the viraemic stage of aplastic crisis, so IgM and IgG alone will miss early disease. PCR is required.
• DNA assays differ in sensitivity (genotype 2 and 3 can be missed by some), and results are not interchangeable.
• Very low IgG in immunosuppressed patients can be negative despite infection.
• Intravenous immunoglobulin gives passive antibody for months.
• Rash illnesses with similar presentation include rubella, measles, enteroviruses, roseola, scarlet fever and drug rash.
• Arthropathy from B19 resembles early rheumatoid arthritis, and the rheumatoid factor and anti-CCP may be transiently positive.
• A positive result in a healthy pregnant woman without exposure or symptoms needs confirmation (avidity and PCR) before intervention.
• Isolation of children with erythema infectiosum is not required once the rash is present, because they are no longer infectious.

Special groups.

• Children: usually mild illness. Haemoglobinopathies call for blood count monitoring in outbreaks.
• Pregnancy: the highest risk of foetal loss and hydrops is before 20 weeks. Most pregnancies are normal after maternal infection.
• Older adults: arthropathy and chronic anaemia are rare presentations.
• Immunocompromised patients: persistent anaemia, pancytopenia or hepatitis can be due to B19, and antibody tests are unreliable.
• Regions: sickle cell disease is common in sub-Saharan Africa, and B19 aplastic crisis is a recognised cause of severe anaemia in sickle cell children, with limited access to PCR.

Follow up tests. Full blood count with reticulocytes, blood film, B19 DNA PCR, bone marrow examination (giant pronormoblasts), haemolysis screen, rubella, measles and enterovirus tests for differential diagnosis, rheumatoid factor and ANA if arthropathy persists, foetal ultrasound with middle cerebral artery Doppler, foetal blood sampling, immunoglobulin levels, and HIV test in chronic infection.