The demonstration of Leishmania amastigotes in splenic or bone marrow aspirates provides a definitive diagnosis of visceral leishmaniasis by directly visualizing the parasite within host cells. This method has the highest sensitivity and specificity compared to serological tests, which may yield false positives due to past exposure or cross-reactivity. Splenic aspiration, while more invasive, offers greater diagnostic yield than bone marrow. Cultures and imaging studies like chest X-ray do not detect the parasite and are not useful for diagnosis. Thus, microscopic identification of amastigotes remains the gold standard for confirming visceral leishmaniasis.

Reference: Harrison's Principles of Internal Medicine, Jameson et al., 20th Edition.