False interpretation of diagnostic serology tests for patients treated with pooled human immunoglobulin G infusions: a trap for the unwary

Discussion on hepatitis B core antibody testing

Positive HBV core antibody was found for all IgG products tested at neat concentration by all three different methods tested. At physiological concentrations, all IgG products tested were positive for HBV core antibody apart from Kiovig (where the two results were discrepant – see Table 2). HBV surface antibody was strongly positive for all IgG products tested at both neat and physiological concentrations, and HBV surface antigen was uniformly negative. There are published examples of similar cases to case 1, such as a patient on IgG with a positive HBV core antibody who was negative on repeat testing 4 weeks later once the IgG was stopped,¹ and a spuriously positive HBV core antibody and HBV surface antigen for a patient on IgG treatment.²

Occult HBV infection detected by HBV DNA testing has been shown in 0.1–2.4% (median 1%) of HBV surface antigen-negative, HBV core antibody-positive blood donors in Western countries such as the United States.³ HBV transmission by blood components negative for HBV surface antigen has occurred historically,^4–6 but is highly unlikely with current manufacturing methods. Risk of transmission of hepatitis B from plasma products appears to be negligible when concurrent HBV surface antibody is present in the blood above a certain level (100–200 mIU/ml), and it has been observed that blood containing detectable HBV surface antibody carries no increased risk of transmitting hepatitis B when compared with blood that lacks this antibody.⁷ Screening and prevention of blood donations from those donors positive for HBV core antibody is only feasible in areas of low endemic disease. Since commercial companies do not screen donors for HBV core antibody but do screen for HBV DNA and HBV surface antigen, it is possible that the positivity in commercial IgG reflects specific antibody from donors with cleared HBV infection.

A particular clinical scenario that could significantly impact patient care would be in a patient being prepared for immunosuppression, such as in the context of idiopathic thrombocytopaenic purpura (ITP). These patients are frequently treated with IgG and/or rituximab. Rituximab may cause reactivation of HBV even in patients with remote historical infection,^8–12 so testing for HBV serology prior to rituximab treatment is advised. In the context of IgG treatment, falsely positive HBV core antibody serology is likely and would complicate the decision to use rituximab therapy. Arnold et al¹³ published a case series on this clinical situation which indicated 11 ITP cases were seropositive for hepatitis B core antibody, of which 10 had received IgG. Seven of these 10 patients subsequently reverted to negative hepatitis B core antibody on repeat testing having discontinued IgG.

Discussion on syphilis serological testing

The syphilis serological results of commercial IgG products showed uniform negativity for syphilis enzyme immunoassay IgM, which is unsurprising given there is a negligible amount of IgM in IgG products (which are over 99% IgG). However, syphilis testing of products was uniformly positive at neat and physiological concentrations for the syphilis enzyme immunoassay for total (IgG and IgM) reactivity. The IgG TPPA results showed some disparity, with most neat solutions producing a positive result and most physiological products producing a negative result. It has previously been reported that there have been false positive serological results for syphilis in patients on IgG treatment. Constable et al¹⁴ reported a patient on IgG treatment for Guillain–Barre syndrome with positive syphilis serology (EIA and TPPA, but negative RPR) 24 hours after infusion, with subsequent testing at 8 and 13 weeks following infusion being negative. Similarly, Rossi et al¹⁵ described a pregnant patient who tested positive for syphilis from IgG therapy for neonatal alloimmune thrombocytopenia. This patient had negative serology testing prior to and 6 weeks following her last treatment, but she received antibiotic treatment during pregnancy for syphilis anyway.

In view of the screening questions, medical examination and serological screening for syphilis in donor populations, it is likely that the above positive syphilis serological results are the result of non-specific reactivity, as opposed to the possible true positivity for HBV core antibody. It is unlikely that there is a significant burden of syphilis in the donor populations as donors are usually regularly tested for syphilis. There remains a question as to whether treatment of syphilis in these patients during pregnancy is warranted in view of the likelihood of it relating to the infused IgG. The treatment for syphilis is safe and effective, and the outcomes of active syphilis for the fetus are potentially severe (stillbirth or severe disability). This decision should be made on an individual basis for each patient after taking a thorough sexual history.

Discussion on other microbiological serological tests

Serological testing of products for HIV and HTLV antibody were uniformly negative. False positive tests for toxoplasma serology have been found in patients on IgG treatment previously,¹⁶ and would be expected from a normal population of donors not screened for this infection.

Discussion on rheumatological serological testing

At physiological concentrations, most of the autoantibody tests were negative. Weak positive ANCA results were seen for two of the products. At neat (non-physiological) concentrations, ANCA, ANA, anti-cardiolipin IgG and anti-dsDNA variably tested positive. This is suggestive of non-specific binding in these assays, but could reflect low level positivity of these antibodies within the IgG products. Since there are only trace amounts of IgM in IgG products, the uniform negativity of testing products for IgM anti-cardiolipin antibody is expected.