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 Table of Contents  
Year : 2017  |  Volume : 5  |  Issue : 2  |  Page : 57-61

Blood culture contamination rates in two district general hospitals in the Southeast of England

Department of Microbiology, East Sussex Healthcare Trust, Hastings, UK

Date of Web Publication19-Jan-2018

Correspondence Address:
Dr. Nadeem Sajjad Raja
Department of Microbiology, East Sussex Healthcare Trust, Hastings, East Sussex, TN37 7RD
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jpsic.jpsic_17_17

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Introduction: Blood culture is considered an important diagnostic tool to identify the causative agent of bloodstream infection (BSI) as well as to direct the definitive therapy. High contamination rate in any healthcare institution is directly linked with increased cost, use of unnecessary antibiotics and additional testing in the diagnostic laboratories. The audit team determined the blood culture contamination rate and the distribution of microorganisms causing BSIs in patients for the East Sussex Healthcare Trust, United Kingdom.
Materials and Methods: This is a retrospective study which analyses the blood culture results over 2 years period (January 2014 to December 2015). Positive blood cultures were grouped into significant, contaminant or unknown significance by reviewing patient clinical data collected at the time of positive blood culture and the microbiology records.
Results: A total of 11036 blood cultures were processed in the microbiology laboratory during the study (January 2014 to December 2015). A total of 1641 (14.9%) blood cultures were positive. Of 1641, 1298 (11.8%) and 286 (2.6%) were grouped as significant and contaminant, respectively. Fifty-six positive blood culture sets remained indeterminate due to the lack of the clinical information. The overall contamination rates in both hospitals in 2014 and 2015 were 0.9% and 1.7%, respectively. The average yearly contamination rate is 1.3%. Higher contamination rates were recorded from the paediatric, emergency, orthopaedics and surgical departments. A total of 1753 microorganisms were isolated. The most common isolated organisms include Escherichia coli 482, (27.5%), Coagulase-negative Staphylococcus, 274 (15.6%), Streptococcus species, 183 (10.4%), Staphylococcus aureus, 162 (9.2%), Klebsiella species, 135 (7.7%), Enterococcus species, 109 (6.2%) and Streptococcus pneumonia, 48 (2.7%). Thirty-three Candida species were isolated. The prevalence of methicillin-resistant S. aureus and the extended-spectrum b-lactamase producing Enterobacteriaceae were low.
Conclusion: The present data showed lower contamination rate in the Trust than the acceptable rates. It also supports the need for regular training and education of healthcare professional that collect blood culture where the contamination rates are high.

Keywords: Blood culture, bloodstream infection, contamination

How to cite this article:
Raja NS, O'Neill B. Blood culture contamination rates in two district general hospitals in the Southeast of England. J Patient Saf Infect Control 2017;5:57-61

How to cite this URL:
Raja NS, O'Neill B. Blood culture contamination rates in two district general hospitals in the Southeast of England. J Patient Saf Infect Control [serial online] 2017 [cited 2023 Mar 30];5:57-61. Available from: https://www.jpsiconline.com/text.asp?2017/5/2/57/223688

  Introduction Top

Blood culture remains an important diagnostic tool to identify the causative agent of bloodstream infection (BSI) as well as to direct the definitive therapy. It is estimated that approximately 20,000 deaths occur every day worldwide due to the sepsis. Blood culture contamination has been a common problem for decades which continues to be the cause of a diagnostic confusion and frustration for the clinicians and laboratory personnel alike.[1],[2] Blood cultures are important for determining the local prevalence of pathogens and resistance in severe infections. Prevalence of pathogens causing BSI, and their antibiotic susceptibility pattern do help to develop local antimicrobial guidelines. On receiving positive blood culture result, the clinicians should establish whether the isolated organism represents a clinically significant infection or a false positive result of no clinical significance. False positive blood cultures are associated with increased length of hospital stay, inappropriate initiation of antibiotics, additional laboratory testing and increased healthcare cost.[3] Research has found that it is virtually impossible to have a contamination free rate (0%) in the modern clinical settings.[4] However, the contamination rate between microbiology laboratories varies, from 0.96%–6%.[5] According to the Clinical and Laboratory Standards Institute guidelines, the maximum acceptable percentage of contaminated blood culture in any healthcare facility is 3%.[6]


We intend to determine the blood culture contamination rate and to determine the distribution of the organisms isolated from the blood cultures in East Sussex Healthcare Trust (ESHT), England, United Kingdom.

  Materials and Methods Top

Study sites and period

This is a retrospective study which analyses the blood culture results over 2 years period from 1 January, 2014 to 31 December, 2015 at ESHT which provides services at two district general hospitals to the half million population of the Southeast of England. All patients with positive blood culture were identified from the microbiology information system and included in this study. This study was approved by the Department of clinical effectiveness of ESHT.

Blood culture collection

The blood cultures were done for clinical purposes and were conducted according to the accepted clinical standards. The local clinical standard is the blood culture policy and procedure which was issued by the intravenous team in 2013 and reviewed in 2015. During the study, attending physicians ordered a pair of blood culture. A pair of blood culture consists of one aerobic and one anaerobic bottle. The blood cultures were collected by the medical staff, nursing staff, or dedicated phlebotomy teams. The skin was disinfected by rubbing single-use applicator or swab containing chlorhexidine 2% and alcohol 70% for 30 s, then allowed to air dry for at least 30 s before blood cultures were taken. Samples were added into blood culture bottles (BacT/ALERT, Biomerieux, Durham, NC, USA) and incubated at 37°C for 5 days. Microorganisms isolated from positive blood culture were identified by the conventional as well as automated methods in the clinical microbiology laboratory.


The consultants microbiologists in the department collect data on clinical parameters at the time of each positive blood culture, antibiotic therapy, identification and antibiotic sensitivity of isolated organisms, laboratory parameters such as C-reactive protein, white blood cell count, neutrophil count and clinical course. Based on these information and input from clinicians, blood cultures were grouped into significant, contaminant or unknown significance as described by Weinstein et al. and other studies.[7],[8]

  Results Top

During 2 years study, a total of 11,036 blood culture sets were processed in the clinical microbiology laboratory. Of 11,036 blood culture sets, 1641 (14.9%) were positive. Of 1641 positive blood cultures, 1298 (11.8%) and 286 (2.6%) were categorised as significant and contaminant, respectively. Fifty-six positive blood culture sets remained indeterminate due to the lack of the clinical information. The overall contamination rates in both hospitals in 2014 and 2015 were 0.9% and 1.7%, respectively. The average yearly contamination rate is 1.3%. The highest number of contaminant blood cultures were recorded from the paediatric department (31%) and critical care (31%); followed by the accident and emergency (20%), orthopaedics (17%) and surgical departments (16%). Low blood culture contamination rates were noted in haematology (5%) and medical (12%) units [Table 1].
Table 1: Blood culture contamination rate in different specialities in East Sussex Healthcare Trust (n=1641)

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A total of 1753 organisms were isolated during the study [Table 2]. Gram-negative organisms (52%) were predominant in this study. The most common microorganisms grew in the blood cultures were identified as in descending order,  Escherichia More Details coli 482 (27.5%), Coagulase-negative Staphylococcus (CoNS) 274 (15.6%), Streptococcus species 183 (10.4%), Staphylococcus aureus 162 (9.2%), Klebsiella species 135 (7.7%), Enterococcus species 109 (6.2%) and Streptococcus pneumonia 48 (2.7%). Thirty-three Candida species were also isolated. The most common Candida species was Candida albicans (16), followed by Candida glabrata (8), Candida parapsilosis (4) and Candida tropicalis (2). Three Candida species remained unidentified. A total of 44 (2.5%) Anaerobes were isolated from blood cultures, 24 and 13 of which were Bacteriodes and Clostridium species, respectively. In this audit, the prevalence of methicillin-resistant Staphylococcus auresus (MRSA) was low (11 cases) and only 56 extended spectrum β-lactamase (ESBL) producing Enterobacteriaceae were isolated [Table 3].
Table 2: Blood culture isolates during 2014 and 2015 (n=1753)

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Table 3: The incidence of methicillin-resistant Staphylococcus auresus and extended spectrum β-lactamase isolates from different specialities

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The most commonly isolated organisms in the contamination group [Table 4] were CoNS (274 isolates), Streptococcus species (183 isolates), Diphtheriods (33 isolates), Bacillus species (4 isolates) and Propionibacterium species (3 isolates). Of the total 274 CoNS, 93% were contaminant. Most frequently isolated organisms in the significant blood culture group [Table 5] were E. coli (482 isolates), S. aureus (158 isolates), Klebsiella species (135 isolates), Enterococcus species (105 isolates), S. pneumonia (48 isolates) and Pseudomonas aeruginosa (38 isolates). All isolated E. coli, Klebsiella species, S. pneumonia and Pseudomonas aeruginosa were pathogens for true bacteraemia. Four blood cultures with S. aureus remained undetermined while two isolated of Enterococcus species were contaminants.
Table 4: Prevalence of different organisms isolated as contaminant

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Table 5: Prevalence of different organisms isolated as pathogen

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  Discussion Top

Surveillance and monitoring of key indicators are important in improving the quality and safety of healthcare services. More important is identifying the area of the improvement and implementing the interventions that impact significantly on the key indicators. Quality improvement strategies in the clinical settings improve patient outcome and reduce healthcare cost.[9] A previous study reported that majority of blood culture contamination events occurred before processing of the sample at the laboratory and associated with specimen collection, specimen handling and other pre-analytical factors.[10]

The main finding of our audit related to good compliance with the Trust guidelines for blood culture sampling as it is clearly reflected in the low contamination rate (1.3%/year). This rate was comparatively low than international standards.[6] In contrast, the contamination rates of blood culture performed in paediatric, critical care unit and emergency department were high [Table 1]. It likely reflects the use of less scrupulous methods or focuses on aseptic technique or heavy workload or high turnover of patients when blood culture samples are collected in these settings. According to the literature, the contamination rates are high in intravenous lines blood cultures.[11] The teams might be drawing blood cultures from intravenous catheters because patients on critical care unit and paediatric department have intravenous lines. Hence, direct collection of blood culture from vein is recommended to reduce the contamination rate.[12]

Several factors play an important role to reduce the blood culture contamination rate such as dedicated phlebotomy service, teaching and training, use of tincture of iodine for skin preparation and an application of antiseptic to the top of the blood culture bottle before inoculation of the sample.[13],[14] Lower blood culture contamination rates have been reported in blood culture taken by the dedicated phlebotomy or medical technologist teams as compared to the other ward-based staff.[15]

In this study, Gram-negative organisms (52%) were predominant. On the contrary, two other studies showed the prevalence of Gram-positive organism causing BSI.[2],[16] The most prevalent organisms are E coli, CoNS, Streptococcus species and S. aureus [Table 1]. In this study, the leading cause of blood culture contamination was CoNS followed by Diphtheroides, Bacillus, Propionibacterium as similar in previous studies.[12],[17] Small number of CoNS (13) found to be pathogen in this study. Diphtheroides species and Prionibacterium species are normally considered commensal bacteria of the skin, conjunctiva, mouth and upper respiratory tract and are rarely considered pathogens.[18],[19] One study from the United States reported Propionibacterium acnes causing central nervous system (CNS) and prosthetic joint infections (PJI). Of 140 P. acnes, 16 CNS infections were caused by this organism, and in PJI, 19 out of 31 were confirmed as the causative agent.[18]

The growing concern about MRSA bacteraemia is that it shows higher resistance to several antibiotics. Hence, there are limited options to treat this infection. S. aureus bacteraemia reporting to the Department of Health of the UK has become compulsory since April 2001. This surveillance system was enhanced to web-based reporting by all acute Trusts in the country in October 2005. The main purpose for this surveillance was to reduce the MRSA bacteraemia rates to 50% by 2008. This campaign has definitely been successful in reducing the MRSA bacteraemia cases.[20] This case series reports 11 cases (0.63% of total isolated organisms) of MRSA bacteraemia, which accounted 6.2% of S. aureus and 0.6% of total isolates. The higher isolation rates are from emergency department followed by medical, surgical and critical. Perovic et al.[21] from South Africa reported that of 2709 S. aureus bacteraemia from 13 academic centres from June 2010 to July 2012, 46% (1231) were found to be MRSA. MRSA isolation rate in our Trust is extremely low. The low incidence of MRSA bacteraemia may be due to good infection prevention and control practice, quick MRSA screening of the risk patients, timely delivery of MRSA results and without any delay initiation of MRSA decolonisation therapy.

ESBLs organisms are multidrug-resistant organism, and they cause nosocomial infections. They also pose a real challenge to treat these infections.[22] In this study, 66 (4% of total positive blood cultures) cases of extended-spectrum of β-lactamase (ESBL) producing Enterobacteracae were recorded. García-Gómez et al.[23] reported a total of 3.4% (124 of 3409) prevalence of ESBL and other beta-lactamase producing bacteria causing bacteraemia. The prevalence of ESBL producing organisms in our study is consistent with this study.

  Conclusion Top

The present data showed lower contamination rate in the ESHT than the acceptable rates. However, high contamination rates were recorded in paediatrics, critical care and emergency departments. It likely reflects the use of less scrupulous methods or focus to aseptic technique or heavy workload or high turnover of patients when blood culture samples are collected in these settings. It remains important to keep the contamination rate low by following the local guidelines. On the hand, regular training of the healthcare professionals in high contamination rate areas involved in blood culture sampling on aspects such as the maintenance of strict aseptic precautions while collecting and transferring the sample, number of sets of the sample, volume in each bottle and taking the samples before starting an antimicrobial therapy is important.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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Weinstein MP, Towns ML, Quartey SM, Mirrett S, Reimer LG, Parmigiani G, et al. The clinical significance of positive blood cultures in the 1990s: A prospective comprehensive evaluation of the microbiology, epidemiology, and outcome of bacteremia and fungemia in adults. Clin Infect Dis 1997;24:584-602.  Back to cited text no. 7
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Mamishi S, Pourakbari B, Ashtiani MH, Hashemi FB. Frequency of isolation and antimicrobial susceptibility of bacteria isolated from bloodstream infections at children's medical center, Tehran, Iran, 1996-2000. Int J Antimicrob Agents 2005;26:373-9.  Back to cited text no. 16
Altindis M, Koroglu M, Demiray T, Dal T, Ozdemir M, Sengil AZ, et al. Amulticenter evaluation of blood culture practices, contamination rates, and the distribution of causative bacteria. Jundishapur J Microbiol 2016;9:e29766.  Back to cited text no. 17
Burnham JP, Shupe A, Burnham CD, Warren DK. Utility of strain typing of Propionibacterium acnes in central nervous system and prosthetic joint infections to differentiate contamination from infection: A retrospective cohort. Eur J Clin Microbiol Infect Dis 2017;36:2483-9.  Back to cited text no. 18
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Perovic O, Iyaloo S, Kularatne R, Lowman W, Bosman N, Wadula J, et al. Prevalence and trends of Staphylococcus aureus bacteraemia in hospitalized patients in South Africa, 2010 to 2012: Laboratory-based surveillance mapping of antimicrobial resistance and molecular epidemiology. PLoS One 2015;10:e0145429.  Back to cited text no. 21
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  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]


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