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Int Orthop. 2009 June; 33(3): 847–850.
Published online 2008 June 3. doi:  10.1007/s00264-008-0573-4
PMCID: PMC2903079

Language: English | French

Asymptomatic urinary tract colonisation predisposes to superficial wound infection in elective orthopaedic surgery

Abstract

There is no evidence surrounding the benefits, effects or clinical outcomes treating asymptomatic urinary tract colonisation. A series of 558 patients undergoing elective admission for orthopaedic surgery were recruited prior to surgery and were screened for urinary tract infection (UTI). Patients had their urine dipstick tested and positive samples were sent for culture and microscopy. Patients with a positive urine culture were treated with antibiotics prior to surgery; 85% of dipsticks tested were positive, while only 7% of the urine samples were culture positive. Over 36% of patients with a pre-operative UTI show some form of post-operative delayed wound healing or confirmed infection versus 16% in the other subgroup giving a relative risk of wound complications of 2:1 (p < 0.02). We have established that patients who present to pre-admission with urinary tract colonisation are a high risk subgroup for wound infection post-operatively.

Résumé

L’analyse d’urine est une pratique pré-opératoire habituelle en chirurgie orthopédique. Il n’y a pas d’évidence à traiter, a priori, les infections du tractus urinaires. Méthode: une série de 558 patients admis en chirurgie orthopédique et hospitalisés avant une intervention chirurgicale ont bénéficié d’une analyse urinaire systématique. Les patients présentant des tests positifs ont bénéficié par ailleurs d’une culture et d’un examen au microscope. Les patients présentant des cultures urinaires positives (plus de 105 colonies par mml) ont été traités par antibiotiques avant l’intervention chirurgicale. Résultats: 85% des analyses étaient positives, 7% seulement présentaient des cultures positives. 36% des patients qui avaient une infection urinaire pré-opératoire ont montré, en post-opératoire des problèmes de cicatrisation ou d’infection, versus 16% dans l’autre sous-groupe ceci donnant, pour cet autre sous-groupe un risque relatif de complications profondes de 2/1 (p < 0,02). En conclusion, les cultures d’urine en pré-opératoire doivent être poursuivies chez tous les patients devant bénéficier d’une chirurgie orthopédique d’autant que les patients qui présentaient à l’admission une infection du tractus urinaire étaient à haut risque de complications profondes en post-opératoire.

Introduction

Pre-operative urine screening is commonly viewed as an essential investigation included in the pre-operative assessment protocols of many hospitals throughout the UK [1]. It has been well established that urinary sepsis may cause septic arthritis both in the presence [1, 4, 7] and absence [1, 810] of surgical intervention and the benefits surrounding treatment of symptomatic urinary tract infections (UTIs) is established. There is however little data surrounding the consequences of asymptomatic urinary tract colonisation (UTC). UTIs have been established as a cause for prolonged post-surgical hospitalisation and as a cause for post-operative wound infections [6]. There is no data surrounding the relative risk of complication to a patient undergoing elective orthopaedic surgery with asymptomatic UTC.

We studied a cohort of 600 patients undergoing elective orthopaedic surgery, managed throughout their admission according to current best practice guidelines [1] in order to establish the rate of pre-operative urinary colonisation and the effect on post-operative outcomes.

Methods and materials

We evaluated 600 consecutive patients who were pre-assessed for elective orthopaedic arthroplasty surgery according to established trust guidelines. In line with current national guidelines [1] all patients underwent a pre-operative multi-disciplinary assessment. A standardised pro forma was used for history taking and clinical evaluation was carried out by a junior doctor. Screening for UTIs and methicillin-resistant Staphylococcus aureus (MRSA) screening were also performed. Details of concurrent disease, age and previous admissions were recorded on a standard pro forma. All patients had their urine screened with dipstick testing. This has previously been shown to be a reliable screen [5], and positive samples were sent for culture and microscopy. In line with local policy 105 organisms/ml was defined as a positive culture. Patients who were symptomatic for a UTI were excluded from the study.

In patients in whom a positive urine culture was obtained the patient was contacted directly and the culture results forwarded to their general practitioner (GP), who was asked to treat the patient prior to surgery. No further urine sample was sent for culture following pre-assessment, although all patients had completed an appropriate course of antibiotic therapy. All patients were admitted to the elective centre (barring those positive for MRSA) and strict infection control methods were implemented [3]. The data for the study were collected as part of an ongoing prospective audit process.

All surgery was undertaken in laminar air flow theatres and local policy includes three doses of prophylactic cefuroxime perioperatively. All patients were managed according to standardised arthroplasty agreed care pathways. Patients were followed up after their surgery and the need for any additional intervention such as wound washout or wound swabs was recorded.

All patients underwent daily dressing inspection after their arthroplasty surgery and wound inspection as necessary. The Centres for Disease Control [2] criteria for clinical diagnosis of wound infection were used for diagnosis of superficial infection. Wound swabs were sent for cultures in all cases of suspected infection. The data were recorded on a standardised pro forma by the investigators following the discharge of each patient. Patients’ notes were reviewed after discharge and divided into three clinical groups:

  1. Uneventful surgery
  2. Superficial wound infection, cultures negative
  3. Superficial wound infection, cultures positive

Statistical analysis was undertaken using contingency tables and chi-squared analysis. Statistical significance was defined as a p value of 0.05 or less.

Results

Six hundred patients undergoing elective orthopaedic surgery were initially recruited into the study: 42 patients were disregarded either because their operation was eventually cancelled, or because their notes were not available for review; 558 patients were successfully followed up. The mean age was 62, and the M:F ratio was 1:1.36.

The age distribution was calculated for each cohort and there were no significant differences in either the distribution or the skew between the three subgroups (Fig. 1). Of the urine dipstick screens, 85% were positive, while only 7% of the subsequent mid-stream urine samples were culture positive. Urine cultures were sent positive in 7.0% (n = 39/560) of cases, and one sample was lost in the laboratory.

Fig. 1
Age distribution of sample and subgroups. a Whole sample: number: 558; mean: 61.9; standard deviation: 17.4; skew: -0.73; excess kurtosis: -0.26. b Uncomplicated sample: number: 449; mean: 60.1; standard deviation: 17.0; skew: -0.66; excess kurtosis: ...

At final follow-up the patients were divided into cohorts based on their wound status post-operatively. Of the patients, 81% (n = 452) had uneventful surgery, 13% (n = 61) had a superficial infection, but did not culture a microbe, and 6.7% (n = 38) had a positive wound swab. In the patients with an uneventful post-operative course, 5.3% had asymptomatic UTC. In the groups with superficial infection, 13.3% of culture negative patients and 18.4% of culture positive patients were colonised with urinary tract pathogens at pre-assessment screening.

The cohorts were then further subdivided by pre-operative urine culture results (Fig. 2). The relative risk factors for wound complications in the urine culture positive cohort versus the normal population was 2.4:1. There was a statistically significant difference between the rates of post-operative infection in the colonised group and the control (chi-squared analysis p < 0.02). The relative risk for wound culture positive surgical site infection was 3:1 in the UTC group as compared to the control group.

Fig. 2
Outcome by pre-operative screening status

Interestingly four of five post-operative infections in the urine culture positive cohort were anaerobic infections, where only 25% (n = 10/40) were anaerobic in the urine culture negative group. Although the cohort is too small for formalised statistical analysis this implies a likely difference. In four cases (80%) of urine culture positive post-operative wound infection the same bacterium was cultured from the wound swabs as the pre-operative urine.

Discussion

Over 40% of patients with a pre-operative UTI show some form of superficial wound infection compared to 16% in the other subgroup. This gives a relative risk of wound complications of 2:1. Further analysis shows there was also a significant (p < 0.05) increase in positive wound cultures in the UTC group; 53% of wound cultures were positive versus 37% in those without a cultured UTI. These results support the view that asymptomatic UTC is a risk factor for wound infection post-arthroplasty surgery.

Superficial surgical site infection is difficult to define, and although the rates of superficial infection appear high in this series they are in line with other published reports using similar criteria [11]. There were only two early deep infections in the cohort, both of which required open washout and débridement of the wound.

It is possible that the presence of pre-operative UTC is a confounding factor and indicative of poorer physiological status, rather than a discrete risk factor for post-operative infection. However, all the patients were of similar age distribution, and analysis yielded no statistically significant link between other co-morbidities and risk of UTC. In view of the potentially significant increase in anaerobic infections, and previously documented cases of septic arthritis secondary to UTI [9], it seems likely that these bacteria are not secondarily colonising the patient.

The protocol used throughout this study, and for several years prior to the study, can be seen to have several flaws. Dipstick testing in our series as previously reported [5] was only 5% specific for UTC, making it a poor screening test. In common with widely accepted practice patients were referred back to their GP for treatment of their UTI, and the GP left to decide on appropriate antibiotic therapy. The patients operations went ahead without any further screening. In view of a greater than twofold risk of surgical complications in those with a UTI, it would be advisable for all patients to have a negative urine culture prior to surgery. In the light of these data practice has been changed in our unit. Whilst further work is required to determine the most appropriate treatment regimen for this problem, we have established that UTC is a significant factor contributing to post-operative superficial wound infection.

Previous studies have demonstrated that orthopaedic infection rates may be reduced by cutting pre-operative stay and implementation of multi-modal infection control policies [6]. This is the first study to quantify the effect of UTC on post-operative wound complications, and we have demonstrated that simple outpatient management of UTIs does not completely control the risks in these patients.

Our results demonstrate that urine culture is a vital part of a pre-operative assessment, and that patients who are asymptomatic, but urine culture positive have a higher risk of post-operative wound complications. Pre-operative UTC has a significant link with post-operative wound complications.

In the light of this study pre-operative urine culture should be undertaken in all orthopaedic arthroplasty patients. It should be recognised that patients who present with UTC, even if treated, are a high-risk subgroup for wound infection post-operatively.

References

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Articles from International Orthopaedics are provided here courtesy of Springer-Verlag