Background/Objectives: Conjugation of polyethylene glycol (PEG) to L-asparaginase (ASP) reduces immunogenicity and prolongs the half-life of this biological drug used to treat acute lymphoblastic leukemia (ALL). However, some patients develop anti-drug antibodies (anti-ASP and anti-PEG), which may impair efficacy through neutralization or faster elimination and may cause hypersensitivity reactions. Their clinical impact remains debated, partly due to variability in analytical methods and cut-points often defined by statistical rather than clinical outcomes. Methods: To address this issue, we used two assays to measure anti-PEG antibodies in 27 sera from 25 children with ALL who showed reactions to PEG-ASP—either clinical allergy with reduced ASP activity (n = 18) or allergic-like reactions without major activity loss (n = 9)—and in 25 sera from 19 children with ALL but without clinically relevant reactions. Sera from 16 healthy children were used as controls. This design enabled assessment of sensitivity and specificity using clinically relevant cut-points. Results: Anti-PEG IgG (but not IgM), measured by ELISA, correlated with reduced drug activity, showing 83% sensitivity and 91% specificity. We also developed a surface plasmon resonance (SPR) assay, capable of detecting simultaneously both anti-PEG and anti-ASP antibodies and analyzing binding kinetics. ROC analysis of SPR data showed 78% sensitivity and 94% specificity. Conclusions: Overall, these findings highlight the importance of clinically defined cut-points, support a key role of anti-PEG antibodies in hypersensitivity with reduced drug activity, and suggest that measuring anti-PEG antibodies may help guide therapeutic decisions.

This report presents the case of a 16-year-old girl admitted with fever and right upper quadrant abdominal pain simulating acute appendicitis. Diagnostic laparoscopy revealed perihepatic violin-string adhesions and pelvic inflammation and led to diagnose Fitz-Hugh-Curtis syndrome (FHCS) secondary to pelvic inflammatory disease (PID). Vaginal swabs confirmed Chlamydia trachomatis and Ureaplasma urealyticum infection. Targeted postoperative antibiotic therapy led to complete clinical resolution. The case highlights the diagnostic challenges of FHCS in adolescents and underscores the importance of including it in the differential diagnosis of acute right upper quadrant pain.