Fever of unknown origin in pediatrics: causes and clinical characteristics in a single centre experience

Fever of unknown origin (FUO) is a common condition worldwide in children that remains a diagnostic challenge. The causes of FUO vary depending on the patient's age, residency, and the time of study. Therefore, this study aimed to identify the common aetiologies of FUO at Alexandria University Children’s Hospital under the current diagnostic abilities and newly emerging diseases. The current prospective observational study included all children fulfilling the definition of FUO admitted at Alexandria University Children's Hospital from January 2020 to December 2021 using a steps approach for investigations. A total of 110 children with fever of unknown origin (FUO) were included in this study; the average duration of fever was 36.76 ± 31.73 days. In most of the enrolled cases 105/110 (95.4%) the definite etiology of FUO was identified. The common causes were collagen vascular diseases (30.9%), infectious diseases (28.2%), miscellaneous conditions (19.1%), and malignancy (17.3%). Among collagen vascular diseases, systemic lupus erythematosus (SLE) (47.1%) and systemic onset juvenile idiopathic arthritis (sJIA) (38.2%) were the most common. In the infectious category, Katayama fever (16.1%), brucellosis (12.9%), and urinary tract infection (UTI) (12.9%) were the most frequently observed. Post-Covid MIS-C (52.4%) was the most common in the miscellaneous category. Children in the infectious category had significantly higher neutrophil [5.76 (2.28–7.92) × 10^3/µl] and lymphocytic counts [ 4.2 (2.04–5.91) × 10^3/µl]; (P < 0.001 and < 0.010 respectively). Moreover, in the collagen category the median lymphocytic count was lower [1.95 (1.47–2.73) × 10^3/µl] with a significantly (P < 0.010) higher neutrophil/ lymphocyte ratio [2.30 (1.53–3.91)]. Collagen vascular diseases, infectious diseases, miscellaneous, and malignancy were the most common causes of FUO. Katayama fever, urinary tract infections (UTI), and brucellosis were the most common causes in the infectious category. Post-Covid MIS-C and hemophagocytic lymphohistiocytosis were the most common diagnoses in the miscellaneous category.