Culture-negative versus culture-positive in pyogenic spondylitis and analysis of risk factors for relapse

Abstract Objectives This study aims to compare and analyze the clinical features, diagnosis, treatment and prognosis of culture-negative and culture-positive primary pyogenic spondylitis. Methods In a retrospective analysis, 202 cases of adult primary pyogenic spondylitis with complete clinical data in our hospital from January 2013 to January 2020 were divided into two groups according to bacterial culture results: culture negative (n = 126) and culture positive (n = 76). We compare the clinical characteristics, diagnosis, treatment and prognosis of patients with different culture results. Results The culture positive rate was 37.62% (76/202). There were no significant differences in age, gender, affected segment, spinal abscess, diabetes mellitus, course of disease, surgery, recurrence, and follow-up time between the two groups (p>.05). There were statistically significant differences in hospital admission erythrocyte sedimentation rate (ESR), admission C-reactive protein (CRP), admission white blood cell (WBC) count, discharge ESR, discharge CRP, ESR decline rate, CRP (p<.05). There were statistically significant differences in the rate of decline, hospitalization days, and body temperature ≥38 °C (p<.05). Higher CRP levels on admission, antibiotic treatment time <6 weeks, and body temperature ≥ 38 °C are independent risk factors for infection recurrence. Conclusions The culture-negative group's admission WBC, admission ESR, admission CRP, discharge ESR, discharge CRP, ESR decline rate, CRP decline rate, and hospital stay were lower than the culture positive group, the difference was statistically significant (p<.05). The independent risk factors for infection recurrence are higher CRP levels in hospital admission, antibiotic treatment time <6 weeks, and body temperature ≥ 38 °C.


Introduction
Pyogenic spondylitis is a type of non-specific spondylitis caused by bacterial infection, mainly involving the spine and intervertebral discs.Clinically, it can be divided into a group of diseases such as discitis, vertebral osteomyelitis, and epidural abscess. 1 At present, the number of patients with purulent spondylitis exceeds that of tuberculous spondylitis. 2Some recent studies have found that the number of patients with purulent spondylitis with negative bacterial culture is increasing year by year. 3,4Current literature on culture negative infection is mostly focused on infected prosthetic joints such as the knee, 5 and there are few studies on spondylitis with negative bacterial culture.The identification of microorganisms is the key to guiding the standardized treatment of pyogenic spondylitis.Culture-positive cases have been treated with relatively standardized treatments, among which staphylococcal inflammation is the most common. 6Studies have reported the clinical features and results of culture-negative purulent spondylitis. 7However, the treatment of patients with culture-negative purulent spondylitis mainly relies on empirical medication. 8This study focuses on the analysis of the clinical features, diagnosis, treatment and prognosis of culture-negative purulent spondylitis.

Patient selection
It is a prospective study done between 2013 and 2020.All patients' data were anonymized.This study was approved by the Research Review Committee and the Ethical Review Committee of the Affiliated Hospital of Qingdao University.Informed consents were obtained from the patients and prior to inclusion in the study.
The study included a culture-negative group (n ¼ 126) and a culture positive-group (n ¼ 76) with confirmed diagnosis of pyogenic spondylitis.There was no standardized antibacterial therapy.Those with positive cultures were treated with sensitive antibiotics by drug sensitivity test, and those with negative cultures were treated with antibiotics with broad antimicrobial spectrums.The outcome is divided into treatment success and relapse.Criteria for successful treatment: (1) inflammation indicators (ESR, CRP) are controlled within the normal range and body temperature is normal; (2) tissue bacterial culture is negative; (3) there is no evidence of spinal infection.The criteria for relapse are: (1) after the initial improvement, the clinical symptoms and inflammatory indicators (ESR, CRP) re-appear; (2) after the treatment is completed, the clinical symptoms, imaging and bacteriological examinations consistent with pyogenic spondylitis appear again.

Statistical analysis
Data were analyzed with SPSS 20.0 statistical software (SPSS Inc., Chicago, IL).The measurement data were expressed by mean and standard deviation.When the two groups of data met the normal distribution and homogeneity of variance, the Student t test was used, and the Mann-Whitney U rank sum test was used when the conditions were not met.Categorical variables used chi-square test, multi-categorical variables used row-multiplied list chi-square test, Fisher's exact test was used when the conditions of chi-square test were not met, and Mann-Whitney U rank sum test was used for ordinal categorical variables.For the risk factors with significant differences in univariate analysis, the binary logistic regression model was used to analyze.p<.05 was considered statistically significant (two-sided test).

Demographics and clinical characteristics
A total of 453 patients with spinal infections were screened during the study.Among them, post-spine infection (n ¼ 46), spinal tuberculosis (n ¼ 114) and brucellosis spondylitis (n ¼ 75), spinal fungal infection (n ¼ 11), mixed spinal infection (n ¼ 5), total 251 cases.A total of 202 patients with purulent spondylitis were included in this study, of which 76 (37.62%) were culture positive (see Table 1).The culture-positive group involved five cases of cervical spine, 18 cases of thoracic spine, and 53 cases of lumbosacral spine.The culture-negative group involved 13 cases of cervical spine, 25 cases of thoracic spine, and 84 cases of lumbosacral spine (see Table 2).

Treatment process
Among the 202 patients, 23 with epidural abscess were treated with emergency surgery.Tissue cultures were taken during the operation, and 16 cases were culture positive.The remaining 179 patients had complete blood cultures, and 20 were positive.The remaining 159 patients underwent CT-guided puncture of infected lesions under strict aseptic conditions, and 40 cases were positive.All patients were routinely given vancomycin þ imipenem or vancomycin þ fluoroquinolones or fluoroquinolones þ third-generation cephalosporin antibiotics intravenously, strict bed rest, nutritional support and other conservative treatments.Those who are culture-positive should be treated with sensitive antibiotics based on the results of drug susceptibility, and those who are culture-negative will continue to be treated with broad-spectrum antibiotics, and antibiotics will be adjusted according to changes in the disease.The patient's body temperature, ESR, CRP, and WBC changes were monitored regularly.Indications for surgical treatment: conservative treatment is ineffective, combined with spinal instability, impaired spinal cord nerve compression, and progressive symptoms.Indications for conservative treatment: conservative treatment is effective, no spinal instability, no spinal cord nerve compression damage, asymptomatic progressive aggravation, etc.

Pathogenic microorganisms
There were 76 cases with positive bacterial culture, of which 43 cases were Gram-positive bacteria, and Staphylococcus aureus (n ¼ 23) accounted for the largest proportion.There were 33  cases of gram-negative bacteria, and Escherichia coli (n ¼ 16) accounted for the largest proportion (see Table 3).

Treatment outcome and relapse
The length of stay in the culture-negative group was lower than that of the culture-positive group, and the difference was statistically significant (p<.05).There was no significant difference in the duration of antibiotic treatment and follow-up time between the two groups (p>.05).In the culture-negative group, 107 cases underwent surgery combined with antibacterial treatment, 19 cases underwent antibacterial treatment alone, and eight cases relapsed.In the culture-positive group, 59 cases underwent surgery combined with antibacterial treatment, 17 cases underwent antibacterial treatment alone, and 10 cases relapsed.Since our hospital is a large tertiary medical center, it receives more patients with difficult and critical illnesses, patients referred by lower-level hospitals, and patients who have failed conservative treatment, so there are more surgical patients.The course of intravenous antibiotics is 2-6 weeks, and then it is changed to oral antibiotics for 4-8 weeks (see Table 4).

Risk factors associated with recurrence of infection
The variables related to recurrence of purulent spondylitis are shown in Tables 5 and 6.In univariate analysis, patients with body temperature !38 C, antibiotic treatment time <6 weeks, and elevated CRP showed recurrence of infection.In the multivariate logistic analysis, body temperature !38 C, antibiotic treatment time <6 weeks, and higher admission CRP levels were independent risk factors for infection recurrence (see Tables 5  and 6).

Discussions
Pyogenic spondylitis is mainly caused by the spread of bacteria through the blood.The arterial route is more common than the venous route.The spinal blood supply is abundant, and it is more susceptible to bacterial spread and infection. 9In this study, we compared the clinical characteristics, diagnosis and treatment, and prognosis of the culture-positive group and the culture-negative group.We found that the inflammatory markers (WBC, ESR, CRP) of the culture-negative group were lower, and the diagnosis was often delayed.Previous studies 10 speculated that the culture-negative group had a lower pathogenic microorganism inoculation, so the signs of infection and inflammation markers is lower than that in the culture-positive group, which is similar to the conclusion drawn by Lee et al., 7 which is also similar to the conclusion of Kim et al. 11 Studies have shown 11 that body temperature of 37.8 C is highly correlated with infection recurrence.Our research shows that body temperature !38 C is an independent risk factor for recurrence.
There was no statistically significant difference in diabetes between the two groups (p>.05), but Park et al. studied diabetes as a risk factor for purulent spondylitis. 12The 79 patients (79/ 126) in the culture-negative group clearly indicated that they had used antibiotics early and were infected with a lower dose of pathogenic bacteria.Therefore, the body can effectively deal with a small amount of pathogenic bacteria, the increase of ESR and CRP was small.Research by Jean et al. 13 showed that taking non-steroidal anti-inflammatory drugs can also cover up clinical In this study, after treatment, ESR and CRP showed a downward trend compared with those in hospitalization.After treatment, the rate of decline of ESR, CRP, ESR, and CRP of the culturepositive group was higher than that of the culture-negative group, and the difference was statistically significant (p<.05).However, the decline rate of ESR is lower than the decline rate of CRP.CRP has the characteristics of fast rise and fast decline. 14RP levels change significantly before and after treatment, and ESR levels fluctuate slowly and smoothly after treatment.Studies have shown 11 that higher CRP levels are associated with higher recurrence rates.Our study has similar findings that higher CRP levels are independent risk factors for recurrence.Several studies have described the problem of the diagnosis rate of pyogenic spondylitis.In the study of Hopkinson and Patel, 10 the positive rate of blood culture was 52% and the positive rate of biopsy was 67%.In this study, the positive rate of blood culture (20/179) and the positive rate of puncture (40/159) were lower than those of open.The positive rate of biopsy (16/ 23) may be related to the patient's use of antibiotics before bacterial culture and the small amount of puncture specimens.According to reports, 15,16 17 the author believes that there are several possible explanations for the negative culture of pyogenic spondylitis: (1)  antibiotics before taking the specimen; (2) low-dose or low-grade infection; (3) false-negative biopsy of the infection site.In addition, we should also pay attention to the identification of spinal tuberculosis, brucellosis spondylitis, etc. 18 The optimal treatment time of antibiotics has always been a controversial topic.Only one published randomized controlled trial 19 showed that 6 weeks of antibiotic treatment for pyogenic spondylitis is no less effective than 12 weeks, but this study excludes microbial culture.In negative patients, the results may be affected.The American Academy of Infectious Diseases believes that antibiotic treatment should last at least 6 weeks. 20n this study, most of the total antibiotic treatment time (185/ 202) were 6 weeks or more.In most of relapsed patients in this group, total antibiotic treatment time was less than 6 weeks.Studies have shown that treatment of less than 6 weeks is associated with an increase in the recurrence rate. 11,12Similarly, our research shows that the total duration of antibiotic treatment <6 weeks is an independent risk factor for recurrence.
There is still no strong evidence to guide the use of antibiotics for culture-negative pyogenic spondylitis. 19As Staphylococcus aureus accounts for the highest proportion in pyogenic spondylitis, Kim et al. 21suggested that when choosing empirical antibiotics, regardless of demographic or clinical characteristics, they included an effective anti-Staphylococcus aureus drug.In order to obtain a good treatment effect, it is necessary to carry out antibacterial treatment of pathogenic bacteria.Studies have shown that PCR technology can be used to detect bone and joint infections in culture-negative patients, 22 thereby increasing the detection rate.
So far, there are no guidelines for conservative treatment and surgical treatment of purulent spondylitis. 23For those patients whose conservative treatment is ineffective or whose symptoms are progressively worsening, Pola et al. 24 and Kamal et al. 25 recommended to use antibacterial therapy combined with surgery to treat pyogenic spondylitis.Pola et al. 26 tried minimally invasive surgery to treat pyogenic spondylitis, and obtained certain clinical effects.

Conclusions
This study compared the two groups of patients before and after treatment.Before treatment, there were no significant differences in age, gender, affected segment, spinal abscess (paravertebral abscess, psoas major abscess, epidural abscess), diabetes, and course of disease between the two groups before treatment (p>.05).Before treatment, the WBC, CRP and ESR of the culture-negative group were lower than that of the culture-positive group, and the difference was statistically significant (p<.05).After treatment, there were no statistically significant differences in surgery, follow-up and infection recurrence rates between the two groups (p>.05).The culture-negative group ESR, CRP, ESR decline rate, CRP decline rate, and hospital stay were lower than the culture-positive group, the difference was statistically significant (p>.05).The decline rate of ESR in both groups was slower than that of CRP.The independent risk factors for infection recurrence are higher CRP levels in hospital admission, antibiotic treatment time <6 weeks, and body temperature !38 C.

Limitations
The research method is retrospective; the sample size is small, and the conclusion has certain limitations.A more credible conclusion needs to be reached in a multicenter, large sample randomized controlled trial.

Table 1 .
Clinical features of 202 cases of purulent spondylitis.

Table 2 .
Spinal segment distribution of culture-negative group and culture-positive group.

Table 3 .
Classification of bacteria in the culture-positive group (n ¼ 76).

Table 4 .
Comparative analysis of treatment and prognosis of the two groups.

Table 5 .
Analysis of risk factors for relapse.
CT-guided needle biopsy or open biopsy can improve the diagnosis rate.Based on the research of Kim et al.,