Usefulness of Lymphocyte ratio and CRP Level for Early Detection of Surgical Site Infection Following spinal surgery

Background: To investigate the significant laboratory markers for early diagnosis of surgical site infection after spinal surgery. And determine the diagnostic cut-off values of these markers Methods: A total of 67 patients participated in the study: 11 patients who developed surgical site infection after spinal surgery (SSI Group) and 56 patients were compared with the infected group in terms of agegender, operating time and intraoperative blood loss (Non-SSI Group). The white blood cell (WBC) count , WBC differential , C-reactive protein (CRP) and erythrocyte sedimentation rate(ESR) were determined before and 1, 3 and 7 days postoperatively . Then, we determine the diagnostic cutoff for these markers by using the receiver operating characteristic curve. Results: The CRP, ESR and WBC were significantly higher in the SSI group at 3 and 7 days postoperatively. The lymphocyte ratio at 3 days postoperatively was significantly lower in the SSI Group. Using the receiver operating characteristic curvelymphocyte ratio <11.5% at 3 days postoperatively (sensitivity 90.9%, specificity 75.4%, area under the curve [AUC] 0.919), and C-reactive protein level >26 mg/dL at 7 days postoperatively (sensitivity 90.9%, specificity 87.7%, area under the curve [AUC] 0.954) were the significant laboratory marker for early detection of SSI Conclusion: Lymphocyte ratio<11.5% at 3 days and C-reactive protein levels>26.5mg/dl at 7 days after spinal surgery are reliable markers of SSI.


Background
With the increase of spinal surgery Surgical site infection (SSI), which was first described as a clinical entity by Turnbull in 1953 1 , remains a common and costly complication of spinal surgery. SSI infection rates of 0.7 to 16% after instrumented spinal fusion have been reported 2,3 . Complications associated with SSI after spinal surgery include pseudoarthrosis, deterioration of neurological function, sepsis, and even death.
When Surgical site infection does occur, early diagnosis and treatment will greatly improve outcomes and shorten post-operative recovery. It should be based on indicators of systemic infection, laboratory data, imaging techniques [4][5][6][7] , and local findings such as tenderness, swelling, redness, and purulent discharge 8 to diagnose SSI. Of the clinical findings associated with SSI, there are several laboratory markers available for early detection because of their objectivity and convenience, For instance, C-reactive protein (CRP), erythrocyte sedimentation rate(ESR) and white blood cell (WBC) count and differential count [8][9][10] .
Hence, we consider that reliable laboratory markers are needed for the early detection of SSI after spine decompression surgery. The present study aimed to identify the most signicant laboratory marker and to determine an appropriate cut-off values for these diagnostic markers using receiver operating characteristic (ROC) curve for early detection of SSI.

Methods
The study protocol was approved by the institutional review board of Shaanxi Provincial surgery of the lumbar spine, or were under 20 years of age. We also excluded patients who did not undergo laboratory tests on 1, 3 and 7 days after surgery.
We collected data include CRP, ESR, WBC count, and neutrophil and lymphocyte ratio before surgery and 1, 3, and 7 days postoperatively. CRP and ESR were measured by latex agglutination method, and WBC was measured by automatic cell counter. The ratio of neutrophils and lymphocytes was calculated from WBC percentage difference.
Statistical analyses were conducted using IBM SPSS 12.0 for Windows (Microsoft Corporation, Redmond WA). Data are expressed as mean ± standard deviations (SDs).
Continuous variables are presented as means and standard deviations. Categorical variables are summarized as the number and percentage of the total study population.
Normally distributed continuous variables were compared with a two-sided independent ttest. Categorical variables were combined with Chi-square. We determined appropriate diagnostic cutoffs of these selected markers using the ROC curve. Statistical significance was defined as P < 0.05.

Demographics and Operative Circumstances
The final sample consisted of 67 patients and was divided into 11 patients who developed deep SSI (SSI group) and 56 who did not (non-SSI group). 7 male and 4 female were included in the SSI group, while 23 male and 33 female were in the non-SSI group. Mean age of pa-tients in the SSI Group was 55.82±20.89 years and in the non-SSI group was 55.55±14.43 years. Operating time in the SSI group was 223.64±90.36 minutes (range 120~420),and in the non-SSI group was 189.74±80.70 minutes (range 60~450); intraoperative blood loss in the SSI group was 600.00±303.32ml(range 200~1000),and in the non SSI group was 442.11±251.21 ml (range 100~1000). There were no significant differences in the age, sex, operating time, and intraoperative blood loss between the two groups ( Table 1).

Outcomes in the SSI Group
With the 11 SSI patients, 10 patients conducted debridement, 8 of them removed their instrumentation. The other 1 patient had antibiotics treated without surgury. In 10 patients who underwent reoperation, the responsible bacteria (staphylococcus aureus in 4 cases, klebsiella pneumonia in 3 cases, pseudomonas aeruginosain 2 cases and 1 unknown) were detected at the surgical site.). The blood culture result of drug-treated patients is Staphylococcus aureus.
After surgery or medication treated all patients recovered ( Table 2).

Biochemical Markers
There were no significant differences between the SSI and no-SSI groups for all chemical markers before surgery. Post-operative 1 day, the WBC count in the SSI group (15.65 ± 6.05) was significantly higher compared to the no-SSI group (12.07 ± 3.95) (t = -2.51, p = 0.01), but there was no significant differences in CRP, ESR, neutrophil ratio and lymphocyte ratio. (Fig.1) Post-operative 3 day (Fig.2), the WBC count in the SSI group (12.55 ± 3.78) was no significant differences compared to the no-SSI group (10.29± 3.65) (t = -1.69, p = 0.096).
The neutrophil ratio in the SSI group (0.84 ± 0.07) was significantly higher compared to the no-SSI group (0.75 ± 0.10) (t = -2.77, p = 0.007). The lymphocyte ratio in the SSI group (0.06± 0.03) was significantly lower compared to the no-SSI group (0. 16  Post-operative 7 day (Fig.3), there was no significant differences in the WBC count. The neutrophil ratio in the SSI group (0.79 ± 0.10) was significantly higher compared to the no-SSI group (0.72 ± 0.10) (t = --2.391, p = 0.020). The lymphocyte ratio in the SSI group (0.12 ± 0.07) was significantly lower compared to the no-SSI group (0. 18   However, not all patients can afford additional assessments (e.g. CT, MRI, PET) because these methods are very expensive and requires a lot of time [13][14][15] . Therefore, postoperative laboratory markers are often used to screen SSI due to their objectivity, low cost and convenience 12,16 .
The most widely used biochemical markers for early diagnosing SSI are CRP, ESR, and the WBC count and differential. The difference between ESR and CRP increases after surgery can distinguish between infected and non-infected patients. Larsson et al 17 . were the first to report changes in the CRP level in lumbar posterior lumbar surgery. they found that the level reached to peak on post-operative day 3, and dropped to normal (less than 10 mg/l) within 21 days after surgery. However, Takahashi et al 18 . reported that the postoperative CRP level peaked on day 2, and Aono et al 19 . reported that it peaked on day 4. The peak postoperative CRP levels should be between the 2nd and 4th day after surgery. It is believed that CRP levels have risen again or are still at peak levels 5 days after surgery, suggesting that SSI is more likely to occur.In our result, we found that the level of CRP in the SSI group increased significantly 3 days after operation and remained high 7 days after operation. CRP in non-infectious group increased significantly 3 days after operation and decreased gradually 7 days after operation (Fig.1 D). ESR in infection group and noninfection group increased significantly 3 days after operation, but decreased slightly 7 days after operation (Fig.1 E). Erythrocyte sedimentation rate increased to peak levels about five days after surgery, followed by a slow and irregular decrease 17 . Therefore, CRP is more suitable than ESR for evaluating infection. In our research, one day after operation, CRP increased in all patients. In non-SSI patients, CRP began to decline on days 3 and 7 after surgery. However, in infected patients, the third day after surgery increased further and decreased on the 7th day after surgery. We determine the diagnostic cutoff for CRP by using the receiver operating characteristic curve. If CRP is still > 26 mg/dl in 7 days after operation, infection should be highly suspected and antibiotics should be changed or increased.
Several factors have been reported in the literature that affect postoperative CRP levels, including blood loss, preoperative CRP levels, and spinal surgery segments. For example, surgery in the lumbar region is associated with higher postoperative CRP levels compared to surgery in other areas. 20 . Given the uncertainty of the use of inflammatory markers as a diagnostic tool for SSI, additional tools are needed to distinguish between infected and non-infected patients at the postoperative stage. ROC results showed that NLR> 5.19 at 4 days the sensitivity and specicity of the NLR were 61.5% and 77.6%, the AUC was 0.708 and NLR> 3.85 at 7 days the sensitivity and specicity of the NLR were 69.2% and 62.7%; the AUC was 0.663.
In our result one day after surgical invasion, neutrophils increase and lymphocytes decrease in all patients. In non-SSI patients, neutrophils begin to decrease and lymphocytes increase on day 3 after surgery. However, in infected patients, further increase in neutrophils and decrease in lymphocytes were detected on day 3 (Fig.1 B.C).
Therefore, we determine the diagnostic cutoff for lymphocytes ratio by using the receiver operating characteristic curve. If the lymphatic ratio was less than 11.5% at 3 days after operation, and infection was highly suspected, which was similar to the previous literature. This may be an important indicator of SSI after posterior lumbar surgery.
Based on the results of the study, we analyzed the sensitivity and specificity of five laboratory indicators for early detection of SSI. There are two laboratory markers to be founded for its sensitivity and specificity. The first indicator was CRP level >26 mg/dl on day 7 after surgery, the sensitivity and specificity were 90.9% and 87.7%, the AUC was 0.954. the other indicator was lymphocyte ratio <11.50% on the third day after surgery. the sensitivity and specificity were 90.9% and 75.4%, the AUC was 0.919.
Our research has several limitations. First, this is a retrospective study. Therefore, there may be inherent bias associated with patient selection and missing patient information.
Second, chronic SSI patients may be included in the non-SSI group because they only focus on the results of the hospitalization exam, which may reflect a serious underestimation of the actual number of SSI cases. In future studies, we will expand into the population and follow-up time, and hope to reduce these problem.

Conclusion
In conclusion, the lymphocyte ratio at 3 days postoperatively with a cutoff of <11.50%, and CRP level at 7 days postoperatively with a cutoff of >26 mg/dL,are the most reliable laboratory markers for early SSI following lumbar surgery, because they have a high sensitivity and can be measured early. Availability of data and materials All data generated or analyzed during this study are included in this article. We confirm that the Availability of data and materials refers to the raw data generated and used for this study. Meanwhile, the datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Authors' contributions WBW was in charge and contributed to all stages of the present study.; DPD and LQG were responsible for participated in the design of the study, made revisions of the manuscript and approved the final version. WBW and SJD contributed to interpreting the data and writing the final manuscript; LW and JW were contributor in writing and editing the manuscript. All authors read and approved the final manuscript.

Funding
No funding was obtained for this study Ethics approval and consent to participate The study protocol was approved by the Institutional Review Board of Shaanxi provincial people Hospital (the ethics approval number provided by the board was (2014), No.027), and written informed consent was obtained from all study participants. Notes: Values are expressed as mean ± SD and the number of patients (%).