A 54-year-old patient complains of weakness, weight loss despite the unchanged appetite, frequent urination, skin itch for six months. Some time ago the patient underwent treatment for furunculosis. She hasn't been examined recently. Objectively: malnutrition, dry skin with signs of scratching. Small lymph nodes can be palpated in the axillary regions. Changes in the internal organs are absent. What testing must be administered in the first place?