Biological markers to determine eligibility in trials for community-acquired pneumonia: a focus on procalcitonin.

Clinical features such as cough, sputum production, fever, and the presence of a new lung infiltrate seen on radiograph are not specific to respiratory tract infection, nor do they define the need for antibiotic therapy. Therefore, investigators have looked for biological markers that can supplement clinical information to determine whether the etiology of the infection is more likely bacterial, needing antibiotic therapy, or viral. There are studies of a number of biological markers in serum and bronchoalveolar lavage fluid, including cytokines, acute-phase reactants, and immunoglobulins. The 2 most promising markers in serum are C-reactive protein and procalcitonin (PCT). PCT is a hormokine, produced primarily by parenchymal cells in response to microbial toxins and in response to certain host inflammatory mediators (interleukin-1beta, tumor necrosis factor-alpha, and interleukin-6). Because PCT is down-regulated in the presence of viral infection, PCT seems most promising for defining the need for antibiotic therapy among patients with radiographic evidence of pneumonia. Studies using the highly sensitive Kryptor assay have shown that PCT guidance can lead to the safe withholding of antibiotics among patients with low PCT levels (<0.25 microg/L) and no clinical signs of severe illness. In addition, serial measurements of PCT have been reported to correlate with clinical response to therapy and may be able to guide short durations of therapy. In the future design of trials for community-acquired pneumonia, we may want to exclude patients with low PCT levels, because they are unlikely to benefit from antibiotic therapy. On the other hand, inclusion of patients with low PCT values creates heterogeneity in the study population and confounds the interpretation of clinical trial end points.