A 32-year-old man has been hospitalized with fever episodes that are accompanied by excessive sweating and occur every 48 hours. Twelve years ago he was in military service in Tajikistan, where he had a case of malaria. Objectively, he sclerae are subicteric, his skin is pale, th liver is dense and enlarged by 2 cm, the spleen is enlarged by 5 cm. What test is necessary to clarify and confirm the diagnosis in this case?