Heterophilic antibodies leading to falsely positive D-dimer concentration in an adolescent

BACKGROUND: We present the case of a 15-year-old adolescent with suspected pulmonary embolism and repeatedly elevated D-dimer levels. KEY CLINICAL QUESTION: We aim to determine the cause for elevated D-dimer levels in a patient without venous thromboembolism. CLINICAL APPROACH: When the D-dimer measurement was repeated with different assays, D-dimer levels were within the normal reference interval. Dilution series with assay diluent or low-affinity antibody blocking reagents either did not or only partially decreased the D-dimer value using the original reagent kit. CONCLUSION: Analyses suggested the presence of interfering heterophilic antibodies in patient plasma, a known phenomenon with immunoturbidimetric D-dimer assays, which is rarely described. Prior to drawing this conclusion, the patient underwent extensive diagnostic testing, which led to uncertainty and discomfort for the health care providers, the patient, and their family.