A 27-year-old patient has a severe headache, nausea and vomiting. Objectively: body temperature is 38,9°C, there is a haemorrhagic stellate rash on the legs. The patient takes meningeal pose in bed. Meningeal symptoms are strongly positive. Deep reflexes are brisk, uniform. Pathological reflexes are absent. It has been suspected that the patient has epidemic cerebrospinal meningitis. Which of additional tests should be performed in the first place to verify the diagnosis?