A 40-year-old patient suffers from influenza. On the 5th day of illness there are pain behind sternum, cough with sputum, inertness. Temperature is 39,5°C. Face is pale. Mucosa of conjunctivas and pharynx is hyperemic. Heart rate is 120/min, breathing rate is 38/min. In the lower lung segments shortening of percussion sound and moist rales (crackles) can be detected. What additional investigation should be performed first of all to specify the diagnosis?