Sputum cultures are the most appropriate additional study in this patient. He presents with severe community-acquired pneumonia (CAP) and meets criteria to warrant hospital admission with an age great than 65 years and respiration rate greater than 30/min, which give him a CRB-65 score of 2. Because of his hemodynamic parameters, including hypotension and tachycardia, ICU management is appropriate. In addition to blood cultures, patients with severe CAP should have sputum cultures ordered. The diagnostic yield of sputum cultures varies widely, and culture data for ambulatory patients with uncomplicated CAP do not positively influence outcomes relative to empiric antibiotic therapy. Therefore, sputum cultures are not recommended for these patients. However, in patients being hospitalized and particularly for those with severe disease requiring ICU admission, possible complications of CAP, or multiple underlying comorbidities, sputum cultures may yield diagnostic information useful in guiding treatment. This patient has severe CAP requiring ICU admission; therefore, blood and sputum cultures are indicated.

Although chest CT has higher sensitivity than chest radiography for diagnosing CAP, no evidence shows that CT for initial diagnosis improves clinical outcomes. However, CT is particularly effective for detecting pleural effusion and empyema, cavitary lung lesions, and hilar lymphadenopathy and might be an appropriate diagnostic study if clinical suspicion for one of these complications develops.

C-reactive protein (CRP) is an inflammatory marker that tends to be increased to greater levels in bacterial pneumonia than viral pneumonia and may be helpful in differentiating the cause of pneumonia in patients in whom the diagnosis is unclear. Additional studies regarding use of CRP for this purpose are needed, and it would not be helpful in this patient with a clinical picture consistent with bacterial pneumonia.

Procalcitonin is a circulating molecule produced by cells as a response to bacterial toxins. Therefore, serum procalcitonin levels tend to be elevated in bacterial infections, and this test can be used as an adjunctive test along with other factors to help differentiate viral from bacterial infection. However, this patient has clear evidence of a bacterial cause of his pneumonia, so this study would not yield significant clinical information.