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To assess the accuracy of direct examination and quantitative cultures of BAL to diagnose pneumonia with or without antibiotic treatment.Experimental rat models.Pneumonia was induced by intratracheal inoculation of S. pneumoniae (10(9) cfu/ml) or P. aeruginosa (10(8)cfu/ml). Controls (n = 10) received sterile inoculum. Study animals received penicillin (n = 19) or saline (n = 18) (pneumococcal model); amikacin (n = 13), ceftazidime (n = 11), or saline (n = 13) (Pseudomonas model). BAL was assessed 48 h after infection. The animals were killed for histopathological analysis.All study animals developed pneumonia, which was more extensive in the pneumococcal than in the Pseudomonas model. In pneumococcal pneumonia the sensitivity of BAL cultures (10(3) cfu/ml or higher) was 77.8% with saline and 21.0% with penicillin. In the Pseudomonas ceftazidime group all specimens were negative, precluding diagnosis. The sensitivity of cultures with amikacin was 23.1% vs. 30.8% with saline. In the pneumococcal model intracellular organism (ICO) count of 2% or higher had a sensitivity of 100% for detecting pneumonia with saline and 57.9% with penicillin. In the Pseudomonas model the sensitivity of ICO was 69.2% with both amikacin and saline and 36.3% with ceftazidime. The sensitivity of neutrophil count above 50% in pneumococcal pneumonia was 77.8% and 64.7% with saline and penicillin, respectively, and 69.2%, 61.5%, and 81.8% with saline, amikacin, and ceftazidime, respectively, in Pseudomonas pneumonia.BAL-positive intracellular organisms were more accurate than cultures for the diagnosis of recent pneumonia, and were less affected by antibiotic treatment.