Abstract: Objective Comparison of the bacterial quantification of culture-positive and culture-negative lower respiratory tract specimens by the multiplex quantitative polymerase chain reaction (MQ-PCR) molecular diagnostic method. Methods Sputum or bronchoalveolar lavage fluid (BALF) was collected from patients with lower respiratory tract infection (LRTI) in the Clinical Microbiology and Infection Laboratory of China-Japan Friendship Hospital from November 2019 to March 2021, and 12 target pathogens were detected by traditional culture and the MQ-PCR methods. The differences in bacterial quantification between culture-positive and culture-negative groups were compared. Results A total of 125 respiratory samples were collected that met the inclusion criteria, of which 101 were positive for the MQ-PCR. Among the 101 positive samples detected by the MQ-PCR, 31 samples were bacterial culture-positive, and 70 samples were negative for culture. In addition, the target pathogens were detected by the MQ-PCR 33 times in the culture-positive group and 144 times in the culture-negative group. The median of the bacterial load in Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae, which were the most detected, were 1.44×10⁸ copies/mL vs 1.22×10⁷ copies/mL,7.10×10⁸ copies/mL vs 4.83×10⁶ copies/mL,and 4.28×10⁷ copies/mL vs 5.77×10⁴ copies/mL in the culture-positive group and culture-negative group, and the P values were all less than 0.05. Conclusion The MQ-PCR can accurately quantify the target pathogens, and the median of the bacterial load in the culture-positive group is higher than that in the culture-negative group.

Key words: multiplex quantitative PCR, bacteria, lower respiratory tract infection