In this retrospective study we evaluated care for patients receiving OPAT, in a setting without a specialized OPAT-team in place, at a large teaching hospital in the Netherlands. Our main objective was to identify risk factors associated with readmission within 30 days after discharge. We found that patients discharged with vancomycin or aminoglycosides have a higher risk of readmission and only half of this group received TDM on a weekly base. Furthermore, infected prosthetic material was identified as a second risk factor for readmission. We found that a total 16% of the patients discharged with OPAT experienced complications.
Our first finding is in line with previous studies, which reported that patients treated with aminoglycosides  have a higher risk of readmission. Several studies have also found vancomycin to be a risk factor of readmission. [8, 16] One explanation for this finding is that aminoglycoside and vancomycin are nephrotoxic agents, which can cause kidney injury and therefore require close monitoring. Secondly, this study confirmed that infected prosthetic material to be second independent predictor for readmission. This finding has previously been described by Duggal, who observed that patients discharged with OPAT for prosthetic joint infections to have a high readmission rate within 12 weeks of discharge (73%). This is the first study to identify infected prosthetic material as an independent predictor of readmission in OPAT-patients. A different study found discharge to a skilled nursing facility to be associated with a higher risk of readmission. We were not able to confirm this finding in this study.
In this study we found that the majority of complications were line related (21/247). This is consistent with that of previous studies[11, 18]. Surprisingly, only 16 of the 247 (6%) patients were found to have antibiotic-related ADE documented. Previous studies evaluating the complication rate of antibiotic-related ADE in OPAT patients observed a higher complication rate of 15-20%, with one study even describing an ADE rate of 63%.
A possible explanation for this inconsistency may be the lack of adequate follow-up due to the retrospective nature of this study. We tried to correct for this factor by establishing the frequency of monitoring in this cohort. We observed that patients who were monitored on a weekly basis had a higher complication and readmission rate versus non-weekly monitored patients (23% versus 13% and 16% versus 5%). This could imply that complications go unnoticed in patients who did not receive weekly monitoring. This hypothesis is confirmed by Huck et al(2013), which found that less frequent monitoring, defined as non-availability of recommended test results, to be independently associated with readmission. Compared to the study of Huck et al (2013), in this study a broad definition of weekly monitoring was used, namely either telephone contact, laboratory testing or outpatient consultation). This discrepancy could lead to an overestimation of monitoring frequency.
In our study, therapeutic drug monitoring was performed poorly: only in 51% of cases, TDM was performed as indicated. Shah et al (2014) showed a similar therapeutic drug monitoring rate of 57% in the group of patients without ID supervision compared to a monitoring rate of 68.3% in patients with ID supervision. It is important that TDM is performed in these patients, due to the nephrotoxic effects of this group of antibiotics, when administrated in high doses. Intensive TDM offers the possibility to perform dosage adjustment and prevent adverse drug events. In our center, a specialized OPAT-team responsible for the follow-up after discharge is not present.
One of the OPAT quality indicators is frequent laboratory monitoring, as recommended by the IDSA update 2018. In a recent Delphi study, Berrevoets et al.(2019) tried to establish uniform advice for monitoring of laboratory results, which should be executed by the OPAT-team. Berrevoets et al. came to the conclusion that monitoring frequency of OPAT-patients should depend on the agent used, patients condition and comorbidities and duration of OPAT. This is in line with the recommendations of the IDSA guidelines. This study provides additional evidence in the risk factors for readmission in OPAT patients. Our findings emphasize the importance of appropriate monitoring in patients discharged with antimicrobials which require therapeutic drug monitoring. Additionally, introduction of an OPAT-team could lead to appropriate use of antibiotics, as previous studies have shown[11, 23], which could help increase antimicrobial stewardship.
This study contributes to understanding of the difficulties of OPAT. We identified independent risk factors in a large cohort of patients discharged with OPAT. These patients signify a respective proportion of the patients with OPAT. The results of our study highlight that OPAT teams should be aware of the higher risk of re-admission in this subgroup of patients. This study could be a great stepping stone for future research and improvement of OPAT-therapy.
Our study had several limitations. First, this study was a retrospective study. Due to the retrospective nature of this study, there remains a possibility that complications were underreported. Our study did give an overview of a large cohort of OPAT patients from 2016 until 2018. Subsequently, we also did not have access to any laboratory results outside of our academic hospital. This could underestimate the adherence to monitoring. An additional uncontrolled factor is the possibility of selection bias.
In conclusion, this study contributes to understanding of the difficulties of OPAT. We identified independent risk factors in patients discharged with OPAT. These patients signify a respective proportion of the patients with OPAT. The results of our study highlight that OPAT teams should be aware of the higher risk of re-admission in this subgroup of patients. The results of this study underscore the need to implement IDSA guidelines and provide OPAT care on the basis of quality indicators, by means of a specialized multidisciplinary team for OPAT.
Future research should focus on complications during OPAT and the frequency of monitoring especially TDM. Cost-effectiveness analysis should be integrated in future research in order to provide a solid business case for OPAT dedicated teams. Several questions, such as the appropriate quantity of laboratory monitoring, remain undetermined. More research using prospective trials is needed to answer these questions.