Typical or atypical hemolytic uremic syndrome? That is the question

An 11-month-old boy with a clinical diagnosis of hemolytic uremic syndrome (HUS) associated with Shiga toxin-producing Escherichia coli infection (STEC-HUS) and a documented presence of both Shiga toxin (Stx) 1 and 2 in his stool showed an unexpected drop in platelet count during recovery (day 13 after diagnosis, day 10 after platelet nadir). Although clinically mild (from 305,000/mm³ to 216,000/mm³), this decline diverged from the expected platelet course observed in a cohort of 148 confirmed STEC-HUS patients treated at our center during the last decade and described in detail elsewhere, raising suspicion of an overlapping condition. Differential diagnoses were therefore investigated. ADAMTS13 activity and homocysteine levels were within normal range, and blood tests revealed a C3 level of 0.62 g/L. Given the unusual course of platelet count, which further dropped to 157,000/mm³, alongside the decreased C3 level, atypical HUS (aHUS) was suspected and treated accordingly with intravenous eculizumab. Platelet count peaked at 417,000/mm³ within 7 days, kidney function normalized, and the patient was discharged 23 days after admission. Genetic analysis revealed two heterozygous rare variants of uncertain significance: p.(Gly759Arg) and p.(Gly110Arg) in the complement factor H (CFH) and factor I (CFI) genes, respectively. Given the diagnostic uncertainty, C5 inhibition (C5i) was discontinued, but HUS relapsed 3.5 months later following a febrile upper respiratory tract infection. This case highlights the diagnostic challenge of discriminating aHUS when STEC infection coexists. As variants in complement regulatory genes (including pathogenic, likely pathogenic, or variants of unknown significance) are not uncommon in the general population, careful monitoring of platelet count using disease-specific reference trajectories may represent a clinically useful tool to detect early deviation from classical STEC-HUS evolution and prompt timely C5i, preventing severe consequences.