Diagnostic Odyssey of Patients with the Rare Immunodeficiency Activated PI3 Kinase Delta Syndrome (APDS): Case study from expert and patient surveys

Hermann Wolf^1,2Fabio Candotti³Roman Spelsberg⁴Georgios Sogkas⁵Hans-Holger Bleß⁴Kirsten H Herrmann⁶Fabian Hauck^7*

• ¹Sigmund Freud Private University Vienna (SFU), Faculty of Medicine, Vienna, Austria
• ²Clinical Immunology Office, Vienna, Austria
• ³Division of Immunology and Allergy, Lausanne University Hospital and University of Lausanne, Lausanne, Switzerland
• ⁴fbeta GmbH, Berlin, Germany
• ⁵Hannover Medical School (MHH), Department of Rheumatology and Immunology, Hannover, Germany
• ⁶Pharming Group NV, Leiden, Netherlands
• ⁷Division of Pediatric Immunology and Rheumatology, Department of Pediatrics, Dr. von Hauner Children’s University Hospital, Ludwig-Maximilians-Universität München, Munich, Germany

Introduction: Activated phosphoinositide 3-kinase delta syndrome (APDS) is an inborn error of immunity first described in 2013. With an estimated prevalence of 1–2 per 1,000,000 individuals, it is considered an ultra-rare disease. The aim of this survey was to explore the diagnostic and therapeutic challenges of patients with APDS from the patients` and physicians` perspective in Austria, Germany, and Switzerland. Methods: A qualitative case study approach was applied. Semi-structured interviews were conducted with six patients or legal guardians of children with APDS, and four clinical immunologists with direct experience in APDS care. Transcripts were analyzed using inductive content analysis. Results/discussion: The interviews revealed a median diagnostic delay of several years, mainly due to the rarity and phenotypic heterogeneity of APDS and the involvement of multiple specialties prior to referral to an immunologist. Many patients initially received symptomatic treatment before an underlying immune disorder was suspected. Physicians emphasized the decisive role of genetic testing for confirmation, while patients frequently described the diagnosis as a "lucky coincidence". Both groups highlighted structural barriers including limited awareness, fragmented care, and delayed access to targeted therapy. Early recognition of APDS requires specific education across specialties, wider access to genetic testing, and the development of standardized diagnostic and disease activity tools. Strengthening interdisciplinary care pathways and timely initiation of APDS-specific therapy may substantially improve outcomes in this ultra-rare immunodeficiency.