Table. 10—

Diagnostic approach for the most common specific agents in lower respiratory tract infections

Pathogens	Specimen	Rapid tests	Conventional tests	Comments
*Streptococcus pneumoniae*	Blood		Blood culture	Positive in 4–18% of cases when collected within 4 days
	Sputum	Gram stain	Culture	Only purulent samples acceptable. Can be obtained in 35–40% of patients; informative if >90% Gram +ve. Diplococci most relevant if Gram stain is informative
	BAL, PSB	Gram stain	Culture	Quantitative cultures
	Pleural exudates	Gram stain	Culture	Very specific, only considered if less invasive methods nondiagnostic
	TNA
	Urine	Antigen test		Sensitivity 50–80% of bactaeremic cases, lacks specificity in children, more evaluation necessary
*Haemophilus influenzae*	Blood		Blood culture	Less frequently positive than for Streptococcus pneumoniae
	Resp. specimens	Gram stain	Culture
Other G+ve or G-ve bacteria	Resp. specimens	Gram stain	Culture	Culture if predominant morphology in Gram stain
Legionella spp.	Urine	Antigen test		Sensitivity 66–95%
	Resp. specimens	NAAT	Culture	On appropriate media, late results
	Serum		Serology	Acute and convalescent specimens. Retrospective diagnosis
*Chlamydia pneumoniae, Mycoplasma pneumoniae*	Resp. specimens	NAAT	Culture	On appropriate medium, late results; low sensitivity
	Serum		IgG, IgM	Acute and convalescent specimens. Lack sensitivity, specificity not appropriate for individual patient management, retrospective results
Viruses, influenza, RSV, adeno, parainfluenza, rhino	Resp. specimens	Direct antigen test	Virus isolation	Requirement for appropriate infrastructure. Virus isolation results less sensitive than NAAT
Viruses, influenza, RSV, adeno, parainfluenza, rhino		NAAT

RSV: respiratory syncytial virus; BAL: bronchoalveolar lavage; PSB: protected specimen brush; TNA: total nutrient admixture; Resp.: respiratory; NAAT: nucleic acid amplification test, not generally available yet and not Food and Drug Association cleared; Ig: immunoglobulin.