There has been much recent debate on the utility of drain tip cultures for predicting and diagnosing postoperative SSIs. Studies published prior to 2004 concluded that drain tip culture results were positively correlated with SSI diagnoses in cases of total hip arthroplasty. However, these studies all had small sample sizes (approximately 100 patients), making the conclusions of these retrospective analyses controversial, as small sample sizes can sometimes lead to fallacious results. In 2009, Petsatodis et al. [5] noted that a statistically significant evaluation would require a cohort of no fewer than 1,000 patients. Toward this end, in 2015, Takada et al. [6] evaluated the relationship between drain tip cultures and SSIs in 1,380 patients who underwent hip arthroplasty. Interestingly, they found no significant correlation between the results of drain tip culture and the incidence of SSI using diagnostic criteria similar to the criteria we used here.
Our study, which included 1,112 patients who underwent primary hip arthroplasty, agrees with the results of previous studies that do not support using routine drain tip culture after hip replacement to diagnose postoperative infection [5, 6, 9]. The low positive predictive value suggests that the prognostic value of drain tip cultures is minimal, the likelihood of false-negative results is high, and, overall, that this test is not effective for predicting or diagnosing an SSI in postoperative patients.
The false-positive results in the data could be the result of drain tip contamination during the process of drain tip removal, transport, or culturing. Drain tip contamination has not been emphasized in other studies; however, based on the type of bacteria we found and the lack of clinical findings indicative of an infection, we suggest that drain tip contamination is a possible cause of false-positive culture results. Relying on the results of drain tip cultures could therefore lead to the unnecessary use of antibiotics, thus increasing the risk of side effects and the costs of healthcare.
Based on the low prognostic and diagnostic value of drain tip cultures, we recommend that alternate techniques be evaluated for the prediction and diagnosis of SSIs. It is critical to recognize that the total cost of performing the 1,204 drain tip cultures in our study was at a minimum US$3,688, with positive results incurring additional costs. The costs associated with drain tip cultures will increase as the number of people undergoing hip arthroplasty increases, raising additional questions about the feasibility of drain tip culture.
We also evaluated the incidence of SSI with patient age, gender, BMI, and history of hypertension, diabetes, and steroid use. Age, gender, and the tested comorbidities were not statistically significantly correlated with the development of SSI. However, preexisting conditions that we did not assess here may still impact postoperative outcomes. For example, previous geriatric or bariatric surgery may lead to higher incidences of postoperative complications due to the potential degradation of the patients’ immune system. Understanding the relationships between patient comorbidities and the incidence of SSI may allow for patient-specific SSI screening procedures to be introduced in the future.
There are limitations associated with all the studies considered here, including the present study. First, all studies were retrospective in nature, although in our case the use of a standard and uniform clinical protocol throughout the study period mitigated potential confounds associated with variation among surgeons or procedures. Second, all SSIs diagnosed in patients undergoing primary hip arthroplasty were superficial in nature, meaning that the infection only involved the skin or subcutaneous tissue and did not extend down to the muscle, fascia, or the organ space. We identified only one case of deep tissue infection in a patient who underwent revision hip arthroplasty. It is therefore unclear from our results whether drain tip cultures were effective at predicting deep tissue or organ/space SSIs. Third, we recognize that it is difficult to compare across studies because of variation in postoperative antibiotic protocols, asepsis techniques, implant types, and surgical approaches.
A fourth limitation is that the timing of drain removal may have an impact on the risk of contamination. For example, we cannot exclude the possibility that drain tip cultures were contaminated by skin flora, which may not result in infection; skin flora contamination is likely to have contributed to findings reported in all similar studies that have been published to date [10, 11]. The length of time that the drain remains in the operative site could also affect the likelihood of developing an SSI and the risk of skin flora contamination of the drain tip culture [12]. It is therefore important to standardize the time at which the drain is removed.
Other problems include the lack of universal guidelines for the diagnosis and prevention of SSIs and the additional expense of drain tip cultures, which may act as a barrier to their clinical use [13]. In addition, given the low incidence of SSI associated with hip arthroplasty procedures, an ideal study would include a substantially larger patient cohort in which a significant number of patients tested positive for SSI. A larger study would provide a more definitive conclusion about the prognostic and diagnostic value of drain tip cultures. Because drain tip cultures are also routinely used in spinal procedures and knee arthroplasty, continuing to evaluate the effectiveness of drain tip cultures is important for the broader field of orthopedic surgery.
Given the present studies and the general lack of statistical support for the routine use of drain tip cultures, other diagnostic tools should be explored as potential alternatives. Such tools may include screening for hematoma, as previous studies have observed a correlation between the incidence of hematoma and SSIs, or diagnostic tools such as physical examination [14].