Maternal Anti‐Ro Antibody Titers Obtained With Commercially Available Immunoassays Are Strongly Associated With Immune‐Mediated Fetal Heart Disease.

Objective: Anti‐Ro antibody–positive mothers are frequently referred for serial echocardiography due to the fetal risk of developing heart block and endocardial fibroelastosis. Little is known why only some and not all offspring develop these cardiac manifestations of neonatal lupus (CNL). This prospective study examined associations between anti‐Ro antibody titers and fetal CNL. Methods: Antibody‐positive mothers referred since 2018 for fetal echocardiography at risk of CNL (group 1; n = 240) or with CNL (group 2; n = 18) were included. Maternal antibody titers were measured with a chemiluminescent immunoassay (CIA). Additional testing on diluted serum samples was used to quantify anti–Ro 60 antibody titers above the analytical measuring range (AMR) of the standard CIA (≥1,375 chemiluminescent units [CU]). Results: Among 27 total mothers with a fetal diagnosis of CNL, all displayed anti–Ro 60 antibody titers that exceeded the AMR of the CIA at least 10‐fold. Of 122 mothers in group 1 who underwent additional anti–Ro 60 antibody testing, event rates of CNL (n = 9) were 0% (0 of 45) among mothers with anti–Ro 60 antibody titers from 1,375–10,000 CU, 5% (3 of 56) among mothers with titers from 10,000–50,000 CU, but 29% (6 of 21) among mothers with titers >50,000 CU (odds ratio 13.1, P = 0.0008). Of mothers in group 2 with a primary diagnosis of CNL, 0% (0 of 18 mothers) had anti–Ro 60 antibody titers <10,000 CU, 44% (8 of 18 mothers) had titers from 10,000–50,000 CU, and 56% (10 of 18 mothers) had titers >50,000 CU. Conclusion: CNL is associated with substantially higher anti‐Ro antibody titers than are obtained using a standard CIA. Enhancing the assay measuring range allows an improved specificity of identifying pregnancies at risk of CNL. [ABSTRACT FROM AUTHOR]