[The 459th case: arthralgia, fever, rash, and thrombocytopenia]

The differential diagnoses of reactive arthritis presenting as arthralgia should be considered as diverse disorders, especially when the symptoms cannot be fully explained by some definite diseases. Do not ignore the indication of bone marrow aspiration. We reported a 50-year-old woman who complained of arthralgia, recurrent fever and rash 9 months ago. Laboratory exams showed mild leukopenia, anemia, thrombocytopenia and increased lymphocyte proportion. She was treated with glucocorticoid after the diagnosis of connective tissue disease was suspected. Until platelet count abruptly decreased to very low level, the final diagnosis of acute lymphoblastic leukemia was made through bone marrow morphology, flow cytometry, and chromosome examination. Therefore, a small number of leukemia is not easily diagnosed by routine operations. Thus when diagnoses are not determined with recurrent symptoms, cautious observation and further examination are required to avoid misdiagnoses or missed diagnoses of acute leukemia.