Techniques for noninvasive diagnosis of lower respiratory tract infections. Which tests to order, when to consider invasive procedures.

Although sputum culture and Gram's staining have been the traditional methods for determining the cause of lower respiratory tract infections, oropharyngeal contamination and improper sputum collection can limit their usefulness. Nevertheless, these noninvasive techniques remain a rapid means of gathering diagnostic clues. Alternative approaches include acid-fast sputum stains, direct immunofluorescence examination, enzyme immunoassays, DNA probes, and serologic testing. However, for critically ill patients, invasive procedures (such as bronchoscopy and thoracentesis) can provide more definitive diagnoses to guide selection of antimicrobial therapy.