Between Neglect and Technology: The Importance of the Rk39 Test in the Immediate Diagnosis of Visceral Leishmaniasis

Visceral leishmaniasis (VL), also known as kala-azar, is a severe and neglected infectious disease caused by protozoa of the genus Leishmania and transmitted by sandflies. In Brazil, it represents an important public health problem, accounting for more than 94% of cases in Latin America, especially in the North and Northeast regions, with progressive expansion into urban and periurban areas. Despite its lethal potential, diagnosis is still frequently delayed due to the diversity of clinical manifestations and low clinical suspicion in non-endemic areas. Early recognition is crucial, particularly in the presence of the classic clinical picture of prolonged fever, hepatosplenomegaly, weight loss, and pancytopenia. The advent of rapid immunochromatographic tests, such as those detecting the rK39 antigen, revolutionized VL management by enabling bedside diagnosis with sensitivity and specificity greater than 90%. We report the case of a 38-year-old man, born in Bonfim, Bahia, admitted to a tertiary hospital in São Paulo, with complaints of fever, abdominal pain associated with increased abdominal volume, hyporexia, asthenia, dry cough, and weight loss of 20 kg over four months. Physical examination revealed hepatosplenomegaly and moderate malnutrition. Laboratory tests showed pancytopenia (Hb: 7.5 g/dL; leukocytes: 2,830/mm³; platelets: 29,000/mm³) and hypoalbuminemia (1.2 g/dL). Given the compatible clinical and epidemiological context, a rapid immunochromatographic test for VL was performed, yielding a positive result. Treatment with liposomal amphotericin B was initiated (cumulative dose of 40 mg/kg), with favorable evolution and significant clinical and laboratory improvement. This report highlights the importance of clinical vigilance for neglected diseases, even outside endemic areas, and reinforces the role of rapid tests as essential tools for early diagnosis, contributing to reduced time to treatment initiation and lower morbidity and mortality rates. Additionally, it underscores the need for public policies that expand access to timely diagnosis and effective therapies for vulnerable populations.