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Accuracy of Gram's stain in identifying pneumococci in sputum
Abstract:
We prospectively examined the accuracy of Gram-stained sputum for identifying pneumococci in 42 patients with community-acquired pneumonia. We considered the Gram's stain positive if a preponderant flora or more than ten Gram-positive lancet-shaped diplococci were seen per oil immersion (x1,000) field. These criteria were met in 18 (62%) of 29 specimens that actually contained pneumococci as determined by a positive quellung reaction, or culture on plates or in mice. The Gram's stain was negative in 11 of 13 specimens that did not contain pneumococci. Of the 20 specimens yielding positive Gram's stains, 18 contained pneumococci. A positive Gram-stained smear of sputum strongly suggests the presence of pneumococci, but will miss 38% of specimens containing pneumococci. Deviation from these criteria substantially reduces the overall accuracy of the test.
Insights
Gram-stained sputum accurately suggests pneumococci in pneumonia but misses 38% of cases. Strict criteria are vital for reliable Gram stain results in diagnosing bacterial pneumonia.
Area of Science:
- Microbiology
- Infectious Diseases
- Pulmonary Medicine
Background:
- Community-acquired pneumonia (CAP) is a significant global health concern.
- Early and accurate identification of causative pathogens is crucial for effective treatment.
- Streptococcus pneumoniae remains a leading cause of CAP.
Purpose of the Study:
- To prospectively evaluate the diagnostic accuracy of Gram-stained sputum smears for identifying pneumococci in CAP patients.
- To determine the sensitivity and specificity of Gram stain criteria for pneumococcal detection.
- To assess the clinical utility of sputum Gram stain in the etiological diagnosis of CAP.
Main Methods:
- Prospective study involving 42 patients diagnosed with community-acquired pneumonia.
- Sputum samples were analyzed using Gram staining, quellung reaction, and culture (plates and mice).
- Specific criteria were defined for a positive Gram stain: preponderant flora or >10 Gram-positive lancet-shaped diplococci per field.
Main Results:
- Gram stain correctly identified pneumococci in 18 out of 29 (62%) culture-confirmed specimens.
- The test was negative in 11 out of 13 specimens without pneumococci.
- A positive Gram stain yielded 18 true positives out of 20 positive results, indicating high specificity but missing 38% of pneumococcal cases.
Conclusions:
- A positive Gram-stained sputum smear is a strong indicator of pneumococcal presence in CAP.
- Sputum Gram stain, when adhering to strict criteria, offers valuable diagnostic information but has limitations in sensitivity.
- Deviation from established Gram staining criteria significantly compromises its accuracy for pneumococcal identification.