This study first showed the efficacy of surveillance culture in detecting the causative bacteria of VAP in neonates. In adult patients, surveillance culture is considered important . In several studies on adult VAP cases in the intensive care unit (ICU), surveillance culture of TA samples was found to be effective in identifying causative bacteria, with a concordance rate of up to 35–85% , and the efficacy of this method is still investigated.
Michel et al. reported that the sensitivity of identifying causative bacteria using TA samples every 72–96 h was 83% . According to a systematic review by Brusselaer et al., the sensitivity and specificity of surveillance culture using lower respiratory tract specimens were 75% and 92%, respectively. Hence, it has a high accuracy in assessing the causative bacteria of VAP . Conversely, Carlos et al. compared two groups receiving VAP treatment: one treated according to the results of surveillance culture using TA samples and the other treated according to treatment guidelines . The treatment efficacy rates were 77.4% in the surveillance culture group and 97.9% in the guideline treatment group. These results showed that whether surveillance culture is effective in evaluating the causative bacteria of adult VAP is controversial. Ichikawa et al. have reported the efficacy of surveillance culture for late-onset bacterial infection among neonates . However, of 23 episodes, only three involved pneumonia, and the efficiency of surveillance culture in VAP was unclear.
Based on the concordance rate of surveillance culture and causative bacteria in our study, when the collection time of specimens was closer to the onset of VAP, the concordance rate was higher. Furthermore, tracheal secretion was more likely to match with the causative bacteria than the NC culture. However, neither of the two had significant differences. Regarding the appropriate sample collection time, Delclaux et al. have reported that in 16 of 24 VAP adult cases, the same causative bacteria produced colony in tracheal tubes 2–6 days prior to the event . Based on meta-analysis results of adult VAP cases, surveillance culture performed close to the onset of VAP had higher sensitivity and specificity in identifying the causative bacteria of VAP than surveillance culture performed earlier. Hence, when the samples were collected close to the onset, the concordance rate might be higher . However, Boots et al. have shown that even surveillance culture using TA samples collected a day before the onset of VAP could yield negative results; therefore, samples should be collected at the time of onset .
In our hospital, bacterial culture results are available 4 days after sample submission. The concordance rate of clinically available TA from 4–10 days prior to the onset of VAP and the causative bacteria of VAP was 56%. This indicates a high rate of failure in the evaluation of causative bacteria based on surveillance culture alone. Katayama et al. have reported that Gram staining at the onset of VAP was effective in identifying the causative bacteria of VAP in the NICU . In our study, the concordance rate of Gram staining and the causative bacteria was as high as 91%, thereby confirming the efficacy of the additional use of Gram staining. Notably, Gram staining does not reveal the bacterial species, even if it is positive or negative or a bacillus or coccus. Moreover, it cannot identify whether it is multidrug resistant. Thus, concurrently performing surveillance culture and Gram staining can lead to a more precise identification of the causative bacteria. Therefore, we assessed the causative bacteria using combined Gram staining and surveillance culture of TA collected on the day of VAP onset. We considered surveillance culture as an indicator of causative bacteria identification only if the color of the stained bacteria (positive or negative) and the shape (bacillus or coccus) were similar those in the surveillance culture collected 4–10 days prior to the onset of VAP. This shows the advantage of using an early approach against drug-resistant bacteria.
In our hospital, MRSA and Pseudomonas aeruginosa were the drug-resistant causative bacteria of VAP. Similarly, the sensitivity and specificity of surveillance culture in VAP caused by MRSA in the adult ICU were 70% and 92%, respectively. This result indicated that surveillance culture is effective . If these resistant bacteria are detected on surveillance culture, broad-spectrum antibiotics should be considered.
The current study had some limitations. First, the small sample size was small, and it had a retrospective design. Second, the conditions of resistant bacteria could differ between facilities; therefore, data on MRSA and Pseudomonas aeruginosa could be influenced by different conditions.
The concurrent use of surveillance culture and Gram staining may be effective in identifying causative bacteria in VAP. However, prospective, randomized studies on the outcome of initial antibiotic therapy, rather than broad-spectrum antibiotics, based on the result of both surveillance culture and Gram staining should be conducted to provide more insight regarding the appropriate treatment for VAP in neonates.