Laboratory diagnosis of pneumonia due to Streptococcus pneumoniae

The diagnostic reliability of the Quellung reaction of sputum and the gram stain-directed sputum culture (SC) for bacteriologic diagnosis of pneumonia due to Streptococcus pneumoniae was determined. Detection of pneumococcal polysaccharide in sputum by counterimmunoelectrophoresis (CIE) or isolation of pneumococci from blood or pleural fluid (BPF-positive) was considered indicative of definite pneumococcal infection. Specimens were obtained from 211 patients with community-acquired bacterial pneumonia. Of 119 CIE-positive specimens, the Quellung reaction was positive for 117 and the SC was positive for 96. However, 35 CIE-negative specimens were positive by QR, SC, or both, a result suggesting that CIE is a less sensitive indicator of pneumococcal infection than originally assumed. CIE, QR, and SC were positive in similar numbers of BPF-positive cases. All BPF-positive and CIE-positive cases were detected by both QR and SC. Use of both the QR and directed SC offers a rapid, accurate bacteriologic diagnosis of pneumococcal pneumonia.