On the 4-th day of injections a 60 y.o. patient felt pain and tissue induration in the left buttock. Objectively: the skin in the superexternal quadrant of the left buttock is red and hot, palpation reveals a painful infiltrate 6x6 cm large with softening in the centre. Body temperature is 37,9°C. What action is necessary to di- agnose an abscess?