ABSTRACT
I. Introduction The clinical diagnosis of many respiratory infections can be made relatively easily using clinical and radiographic tools, but determining the etiological diagnosis can be much more challenging. This difficulty is largely due to the limitations of conventional diagnostic tests that, despite many advances, are still suboptimal. For example, even in the most rigorous prospective studies an etiological diagnosis is not made in up to half of the patients. In the real world the diagnostic rate is much less. For some respiratory infections, such as ventilator-associated pneumonia, making a syndromic diagnosis can also be difficult. In these situations, clinicians often look for other laboratory markers to supplement clinical decision-making.
