Introduction: Paroxysmal nocturnal haemoglobinuria is a rare disease, and predominantly affects young adults,
although pediatric cases have been reported. It is secondary to mutation in the X-linked gene of PIG-A, leading to
loss of GPI-anchored membrane proteins, making the cell vulnerable to hemolysis by complement system.
Case report: We report an 11-year-old female patient in follow-up since October 2017, with an initial diagnosis of
autoimmune hemolytic anemia despite DAT (direct antiglobulin test) persistently negative and maintaining haemolytic
activity despite corticosteroid therapy, with fatigue and mild to moderate thrombocytopenia. She was transferred from
hematologic service; a diagnostic review was performed and confirmed classic PNH by immunophenotyping. Due
to the underlying thrombocytopenia, an inventory was performed for bone marrow failures. The other differential
diagnoses of DAT negative hemolytic anemias (membrane diseases, enzymatic deficiencies and hemoglobinopathies)
were investigated and excluded, findings compatible with myelodysplasia were supported by medullary evaluation.
Discussion: PNH is a heterogeneous disease and should be considered in all cases of hemolytic anemia with negative
DAT, thrombosis in non-usual sites and in bone marrow failure syndromes. As it is a rare disease and still unknown
by most physicians, it is underdiagnosed and several patients are inadvertently given ineffective treatments due to
erroneous etiologic diagnoses. The present case is even more relevant given the rarity of the disease in a pediatric
context, with some clinical presentation peculiarities in this age group.