False-positive Cultures Research Articles

Case of laboratory cross-contamination of Mycobacterium tuberculosis in the broth-based culture system

We experienced a case of laboratory cross-contamination of Mycobacterium tuberculosis on the broth based culture system. These false-positive cultures were confirmed by analysis of DNA fingerprinting, RFLP method, which showed the same pattern in three specimens with that of the first manipulated specimen in our laboratory on that day, out of 7 specimens examed. We found possible several process causing cross-contamination where mixture of the foreign body could occur in buffer or NALC-NaOH. False-positive cultures for Mycobacterium tuberculosis may lead to unnecessary, potentially toxic, costly treatment, and changing the treatment strategy. So we must critically interpret a single positive culture, especially by liquid media.

The Sum of the Parts Is Greater Than the Whole: Reducing Blood Culture Contamination

Editorials1 February 2011The Sum of the Parts Is Greater Than the Whole: Reducing Blood Culture ContaminationJi Yeon Kim, MD, MPH and Eric S. Rosenberg, MDJi Yeon Kim, MD, MPHFrom Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114.Search for more papers by this author and Eric S. Rosenberg, MDFrom Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114.Search for more papers by this authorAuthor, Article, and Disclosure Informationhttps://doi.org/10.7326/0003-4819-154-3-201102010-00010 SectionsAboutFull TextPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinkedInRedditEmail Determining whether a positive blood culture represents contamination at the time of collection or a true bloodstream infection is a common, time-consuming problem for physicians and clinical microbiologists. Contamination rates for routine blood cultures are estimated to range from 0.6% to more than 6% of all cultures, potentially resulting in unnecessary hospital admissions, prolonged length of stay, increased laboratory costs, and inappropriate use of antimicrobial therapy (1). False-positive blood cultures may increase laboratory costs by about 20% and are associated with a 39% increase in intravenous antibiotic charges (2); 1 university-affiliated hospital estimated the annualized cost of 6% contaminated blood ...

Clinical and economic impact of contaminated blood cultures within the hospital setting

Blood cultures have an important role in the diagnosis of serious infections, although contamination of blood cultures (i.e. false-positive blood cultures) is a common problem within the hospital setting. The objective of the present investigation was to determine the impact of the false-positive blood culture results on the following outcomes: length of stay, hotel costs, antimicrobial costs, and costs of laboratory and radiological investigation. A retrospective case-control study design was used in which 142 false-positive blood culture cases were matched with suitable controls (patients for whom cultures were reported as true negatives). The matching criteria included age, comorbidity score and month of admission to the hospital. The research covered a 13-month period (July 2007 to July 2008). The findings indicated that differences in means, between cases and controls, for the length of hospital stay and the total costs were 5.4 days [95% CI (confidence interval): 2.8-8.1 days; P<0.001] and £5,001.5 [$7,502.2; 95% CI: £3,283.9 ($4,925.8) to £6,719.1 ($10,078.6); P<0.001], respectively. Consequently, and considering that 254 false-positive blood cultures had occurred in the study site hospital over a one-year period, patients with false-positive blood cultures added 1372 extra hospital days and incurred detrimental additional hospital costs of £1,270,381 ($1,905,572) per year. The findings therefore demonstrate that false-positive blood cultures have a significant impact on increasing hospital length of stay, laboratory and pharmacy costs. These findings highlight the need to intervene to raise the standard of blood-culture-taking technique, thus improving both the quality of patient care and resource use.

Reducing Blood Culture Contamination by a Simple Informational Intervention

Compared to truly negative cultures, false-positive blood cultures not only increase laboratory work but also prolong lengths of patient stay and use of broad-spectrum antibiotics, both of which are likely to increase antibiotic resistance and patient morbidity. The increased patient suffering and surplus costs caused by blood culture contamination motivate substantial measures to decrease the rate of contamination, including the use of dedicated phlebotomy teams. The present study evaluated the effect of a simple informational intervention aimed at reducing blood culture contamination at Skåne University Hospital (SUS), Malmö, Sweden, during 3.5 months, focusing on departments collecting many blood cultures. The main examined outcomes of the study were pre- and postintervention contamination rates, analyzed with a multivariate logistic regression model adjusting for relevant determinants of contamination. A total of 51,264 blood culture sets were drawn from 14,826 patients during the study period (January 2006 to December 2009). The blood culture contamination rate preintervention was 2.59% and decreased to 2.23% postintervention (odds ratio, 0.86; 95% confidence interval, 0.76 to 0.98). A similar decrease in relevant bacterial isolates was not found postintervention. Contamination rates at three auxiliary hospitals did not decrease during the same period. The effect of the intervention on phlebotomists' knowledge of blood culture routines was also evaluated, with a clear increase in level of knowledge among interviewed phlebotomists postintervention. The present study shows that a relatively simple informational intervention can have significant effects on the level of contaminated blood cultures, even in a setting with low rates of contamination where nurses and auxiliary nurses conduct phlebotomies.

Skin disinfection with a single‐step 2% chlorhexidine swab is more effective than a two‐step povidone‐iodine method in preventing bacterial contamination of apheresis platelets

Skin commensal bacteria account for most septic reactions after apheresis platelet (PLT) transfusion. Consequently, we evaluated the effectiveness of two skin disinfection methods in preventing bacterial contamination of PLT collections. Three regional blood centers evaluated a one-step 2% chlorhexidine/70% isopropyl alcohol (2% CHX/IPA) skin disinfection method (trial group), while 32 blood centers (control group) continued to use a two-step povidone-iodine (P-I) method. Aerobic quality control bacterial culture results and adverse donor events were compared between April 1, 2009, and December 31, 2009. The rate of initial positive bacterial cultures was significantly lower in the trial regions compared to control regions in 2009 (143 vs. 321 per million: odds ratio [OR] 0.44, 95% confidence interval [CI] 0.23-0.86]). Trial regions also tended to have lower rates of both true-positive cultures (100 vs. 214 per million: OR 0.47, 95% CI 0.21-1.03) and false-positive (contamination) cultures (43 vs. 96 per million: OR 0.45, 95% CI 0.14-1.49). No differences were seen in the corresponding periods in 2007 and 2008 when all centers used P-I skin preparation. Allergic reactions were reported after 89 of 73,247 apheresis procedures (0.12%) in the trial regions representing a 16-fold increase (OR 15.9, 95% CI 9.9-25.6) compared to control regions. The 2% CHX/IPA single-step skin swabs are more effective in preventing bacterial contamination of apheresis PLT components than a two-step P-I skin disinfection method and may reduce the risk of septic transfusion reactions. Skin irritation and allergic reactions were more likely among donors in trial regions, but reactions were generally mild and self-limiting.

The Effect of Inoculum Volume on the Microbiologic Detection of Naturally Occurring Staphylococcus Aureus Intramammary Infections

Currently no standard definitions for the diagnosis of Staphylococcus aureus intramammary infection (IMI) exist. As a result, criteria applied in research to diagnose S. aureus IMIs have varied making comparisons between published works difficult. The goal of the current study was to define the optimal inoculum volume used in the diagnosis of naturally occurring S. aureus IMIs. Microbiologic results from 2 field studies examining S. aureus IMIs were used to examine the effects of inoculum volume on the microbiologic detection of S. aureus. A total of 1,583 milk samples were included in the analysis, and the results of using a 0.01-ml and a 0.1-ml inoculum are presented. Using a 0.01-ml inoculum resulted in a sensitivity of 91% (95% confidence interval [CI]: 88.6-93%) and a specificity of 99.4% (95% CI: 98.6-99.8%). Using the larger 0.1-ml inoculum resulted in a sensitivity of 96.8% (95% CI: 95.2-97.9%) and a specificity of 99.3% (95% CI: 98.4-99.7%). All false-positive samples were from S. aureus-negative quarters in S. aureus-positive cows. There were no false-positive cultures from S. aureus-negative cows. Of the false-negative samples, the majority (77%) were from 6 of the 34 S. aureus-positive quarters. Results from the current study of naturally occurring S. aureus IMIs support the hypothesis that, when using quarter level milk samples, a S. aureus IMI is most accurately diagnosed using a 0.1-ml inoculum. Regardless of inoculum volume, a single quarter sample culture that is positive with S. aureus (>or=1 colony-forming unit) is sufficient to diagnose a S. aureus IMI.

Guidelines on Blood Cultures

Just over one-third of sepsis patients have positive blood cultures, mainly due to inadequate sampling volumes (50% of adults have < 1.0 CFU/mL blood) and the prior use of antibiotics. However, 20-30% of sepsis patients are given inappropriate empirical antibiotics. The Clinical and Laboratory Standards Institute guidelines recommend paired culture sets to help discriminate between contaminant organisms and true pathogens; four 10-mL bottles (2 sets) should be used for the initial evaluation to detect about 90-95% of bacteremias and six 10-mL bottles (3 sets) should be used to detect about 95-99% of bacteremias. It has also been shown that the positivity rate increased by 15-35% with resin-based media in patients on antibiotics. For diagnosing catheter-related bloodstream infections, differential time-to-positivity is one method recommended to help determine whether the catheter is truly the source of infection. The proper training of personnel with regard to drawing an appropriate blood volume and the importance of clear labeling of culture bottles is also of critical importance. Furthermore, if the contamination rate is relatively high, hiring dedicated staff who are well-trained in order to get a lower blood culture contamination rate may be cost-effective. It is because high false-positive blood culture rates due to contamination are associated with significantly increased hospital and laboratory charges.

Molecular Evidence of False-Positive Cultures for Mycobacterium tuberculosis in a Taiwanese Hospital With a High Incidence of TB

Isolation of Mycobacterium tuberculosis (MTB) from the clinical specimens of patients with suspected TB remains the gold standard for diagnosis of TB. However, false-positive MTB cultures can occur as a result of laboratory contamination. After reviewing the medical records of 400 TB cases identified during January 2008 to January 2009 by the infection control unit of a university-affiliated hospital in Taipei, Taiwan, five patients were considered as clinically suspected false-positive cases and were referred to a mycobacteriology laboratory for confirmation. Spoligotyping and mycobacterial interspersed repetitive unit-variable number tandem repeat analyses were performed for all the suspected isolates and all other isolates cultured on the same day as the five suspected isolates. Three cases were confirmed as false-positive culture cases based on the laboratory investigation. The culture from one of these cases (index case 1) grew multidrug-resistant TB. Another patient (index case 2) received an extended course of anti-TB treatment after he was considered to have failed treatment because of the false-positive MTB culture result. No anti-TB medication was given for index case 3. All three cases with false-positive cultures had only one positive culture specimen among multiple consecutive specimens submitted for cultures. In addition, specimens of the false-positive cultures were all negative for acid-fast smears. False-positive MTB cultures should be suspected in the following situations: when growth is observed on only one specimen among multiple specimens submitted; when it is positive in only one culture medium, especially in broth; or when there is only one specimen submitted. False-positive MTB cultures can be further confirmed with modern molecular typing techniques.

Serious Bacterial Infections in Febrile Outpatient Pediatric Heart Transplant Recipients

The purpose of this study was to describe the incidence of serious bacterial infections (SBIs) in febrile outpatient pediatric heart transplant recipients and to assess the utility of using white blood cell (WBC) indices to identify patients at low risk for bacteremia. A retrospective study was conducted on all heart transplant recipients followed at a single children's hospital. All outpatient visits from January 1, 1995, to June 1, 2007, in which fever was evaluated were reviewed. Patients with history of a primary immunodeficiency, receiving concurrent chemotherapy, or having had a stem cell or small bowel transplant were excluded. Demographic, historical, physical examination, laboratory, and radiographic data were then recorded. Sixty-nine patients had 238 individual episodes of fever evaluation; of these, 217 (91.2%) had blood cultures drawn with results available in their initial evaluation. There were six (2.8%) true-positive blood cultures and eight (3.7%) false-positive cultures. Chest radiography was done in 185 evaluations (77.8%), and 44 episodes of pneumonia (23.8%) were diagnosed. Of 112 urine cultures done, one (0.9%) was positive. Neither of two lumbar punctures performed were positive. In non-ill-appearing children without indwelling central lines or focal bacterial infections (pneumonia, cellulitis), the incidence of bacteremia was 1.2%. In children with a focal bacterial infection, the rate of bacteremia was 6.3%. WBC indices were not significantly different between bacteremic and nonbacteremic patients. A band-to-neutrophil ratio (BNR) of > or =0.25 and a published guideline for identifying low-risk infants using WBC indices identified all bacteremic patients, each with a sensitivity of 100% (95% confidence interval [CI] = 48% to 100% and 54% to 100%, respectively). The incidence of bacteremia was low in febrile, outpatient pediatric heart transplant patients, especially in those who were not ill-appearing and did not have a focus of serious infection. Two different low-risk criteria performed well in identifying the bacteremic patients, although given the low number of true-positive cultures, the CIs for the sensitivities of these tests were extremely wide, and neither test could be reliably used at present. A prospective multicenter study is required to confirm the low incidence of bacteremia and low-risk criteria in this population.

Blood culture contamination in a district general hospital in the UK: a 1-year study

Blood culture remains a gold standard to identify patients with bacteraemia. High contamination rates in blood culture are associated with prolonged hospital stay, inappropriate antimicrobial therapy and extra work in the microbiology laboratory. The blood culture contamination rate and the distribution of organisms from patients tested for bloodstream infections were determined for Southend University Hospital, UK. The study was conducted retrospectively over a 1-year period from August 2006 to July 2007 by reviewing patient clinical data collected at the time of positive blood culture and the microbiology records from Southend University Hospital. During the study period, 7203 blood cultures were processed in the microbiology laboratory. Of the total, 1254 (17.4%) were positive. A total of 535 were shown to be contaminated, giving an overall contamination rate of 7.4%. A total of 703 (9.8%) blood cultures were determined as genuinely positive. The highest numbers of false-positive blood cultures were recorded from the Accident and Emergency, Medical and Paediatric units. A total of 1380 organisms including bacteria and fungi were isolated. Coagulase-negative Staphylococci (547 isolates, 39.3%) were the most commonly isolated organisms, followed by Escherichia coli (132 isolates, 9.6%), Staphylococcus aureus (130 isolates, 9.4%), and Streptococcus species (85 isolates, 6.2%). The prevalence of methicillin-resistant Staphylococcus aureus was low, whereas the extended-spectrum β-lactamase producing E. coli and Enterobacter species was high and Klebsiella species was low. The study indicates a high contamination rate of blood cultures and supports the need for regular training and education of healthcare professionals who collect blood samples.

Blood cultures: towards zero false positives

The culture of micro-organisms from blood is an essential laboratory test for the diagnosis of bacteraemia. Early positive results provide valuable diagnostic information on which appropriate antimicrobial therapy can be initiated. Like any tests, however, false-positive blood culture results can limit the utility of this important tool. These false-positive results arise due to contamination. Reports from NHS trusts and equipment suppliers suggest these contamination rates could be as high as 10% (Department of Health (DH), 2007). A variety of strategies have been investigated and employed to decrease contamination rates. This article describes the utility of a variety of approaches to reduce the number of false-positive percutaneous blood cultures specifically in the adult population.

Evaluation of Low-Colony-Number Counts of Mycobacterium tuberculosis on Solid Media as a Microbiological Marker of Cross-Contamination

Low-colony-number counts on solid media are considered characteristic of cross-contamination, although they are normally observed in true-positive cultures from some groups of patients. The aim of this study was to evaluate low-yield growth cultures as a microbiological marker for cross-contamination. We evaluated 106 cultures with <15 colonies from 94 patients, and the proportions of false-positive cultures were 0.9% per sample and 1.1% per patient, which indicates that low-yield growth is not a reliable marker of cross-contamination.

Impact of Blood Cultures Drawn by Phlebotomy on Contamination Rates and Health Care Costs in a Hospital Emergency Department

We conducted a prospective comparison of blood culture contamination rates associated with dedicated phlebotomists and nonphlebotomy staff in the emergency department (ED) at Parkland Memorial Hospital in Dallas, TX. In addition, hospital charges and lengths of stay were determined for patients with negative, false-positive, and true-positive blood culture results. A total of 5,432 blood culture collections from two ED areas, the western wing of the ED (ED west) and the nonwestern wing of the ED (ED nonwest), were evaluated over a 13-month period. Phlebotomists drew 2,012 (55%) of the blood cultures in ED west while nonphlebotomy staff drew 1,650 (45%) in ED west and 1,770 (100%) in ED nonwest. The contamination rates of blood cultures collected by phlebotomists were significantly lower than those collected by nonphlebotomists in ED west (62/2,012 [3.1%] versus 122/1,650 [7.4%]; P < 0.001). Similar results were observed when rates between phlebotomists in ED west and nonphlebotomy staff in ED nonwest were compared (62/2,012 [3.1%] versus 100/1,770 [5.6%]; P < 0.001). Comparison of median patient charges between negative and false-positive episodes ($18,752 versus $27,472) showed $8,720 in additional charges per contamination event while the median length of stay increased marginally from 4 to 5 days. By utilizing phlebotomists to collect blood cultures in the ED, contamination rates were lowered to recommended levels, with projected reductions in patient charges of approximately $4.1 million per year.

The University of Michigan Specialist–Hospitalist Allied Research Program: Jumpstarting hospital medicine research

Clinical research has developed slowly in most academic hospitalist programs, possibly because of a failure to recognize the important role of specialists in the diagnosis and management of complex medical patients as well as their expertise in clinical research. Ideally, a successful hospital-based clinical research program will need to partner hospitalists with specialists. The University of Michigan's Specialist-Hospitalist Allied Research Program (SHARP) was designed to jumpstart hospital-based clinical and translational research at a major academic medical center by pairing specialists and hospitalists to ask and answer novel research questions. SHARP is codirected by a hospitalist and a subspecialist and includes key personnel such as a hospitalist investigator, a clinical research nurse, a research associate, and a clinical epidemiologist. The program is guided by an oversight committee that includes institutional research leadership. Two initial projects have already been supported. The first, a collaboration between infectious disease specialists and hospitalists, is a prospective trial of antiseptic agents and techniques to reduce false-positive blood cultures. The second pairs geriatricians and clinical pharmacists with hospitalists to prospectively study techniques to reduce medication errors around the time of hospital discharge. Although initial pilot projects are single-institution studies, SHARP's goal is to expand its clinical research to include multicenter investigation. Metrics to evaluate SHARP include the number of successfully completed projects, extramural grants submitted and funded, and peer-reviewed publications. A successful hospital-based clinical research program combines hospitalists and specialists in a collaborative environment to identify optimal strategies for delivering inpatient care.

Evaluation of Conventional Microbiological Procedures and Molecular Genetic Techniques for Diagnosis of Infections in Patients with Implanted Orthopedic Devices

Evaluation of Conventional Microbiological Procedures and Molecular Genetic Techniques for Diagnosis of Infections in Patients with Implanted Orthopedic Devices

A reliable transformation method and heterologous expression of β-glucuronidase in Lentinula edodes

A simple and reliable mushroom transformation procedure based on electroporation of basidiospores or mycelial fragments was developed. This method eliminated the problem of protoplast preparation, the transformation efficiency were 30–150 transformants per μg DNA and the hygromycin resistant marker gene and gus were expressed in Lentinula edodes successfully. No false positive antibiotic-resistant cultures were detected by PCR amplification and the β-glucuronidase (GUS) expression was maintained stable during mitotic cell division without selection pressure for more than 6 months. Southern analysis of transformants indicated the integration of gene might occur by non-homologous recombination. Using the glyceraldehyde-3-phosphate dehydrogenase ( gpd) promoter with the first intron of gpd gene, the average GUS activity in L. edodes reached 144.6 ± 3.9 U mg − 1 soluble protein, while only 30.1 ± 0.7 U mg − 1 soluble protein was detected for those without the intron. The percentage of GUS in total soluble protein was 5.67 × 10 − 4 (0.06%) for the transformant with the highest GUS activity. This rapid and convenient electroporation procedure offers a new approach for the genetic manipulation and tool to tag genes of important edible mushroom species.

Review of Clinical Trials of Skin Antiseptic Agents Used to Reduce Blood Culture Contamination

False-positive blood culture results may lead to prolonged hospitalization, inappropriate antibiotic administration, and increased healthcare costs. We conducted a review of the literature to assess the effect of skin antiseptic agents on the rate of false-positive blood culture results. We found no clear evidence to suggest which antiseptic should be used to prevent false-positive results. Studies suggest, however, a possible benefit from the use of prepackaged skin antiseptic kits and alcohol-containing antiseptics.

The residual risk of sepsis: modeling the effect of concentration on bacterial detection in two‐bottle culture systems and an estimation of false‐negative culture rates

Septic reactions continue to be reported with culture-tested platelet (PLT) products, probably due to false-negative results associated with inadequate sampling of low-concentration bacteria. The mechanism of test failure was modeled and false-negative rates were estimated utilizing published data. The effect of concentration on the probability of sampling one or more viable bacteria in an 8-mL sample of a 300-mL product was determined. The ratio of single- versus dual-bottle-positive tests in repeated cultures utilizing two-bottle systems was used to assess mean bacterial concentrations and to predict false-negative tests. Published reports reveal a mean residual risk of sepsis of 2.3 per 10(5) products tested. Modeling the effect of concentration predicts that 50 and 95 percent of samples are detected at 0.09 and 0.36 CFU per mL for organisms that grow under both aerobic and anaerobic conditions, and suggest a 50 and 5 percent false-negative culture rate at these concentrations. Reanalysis of published studies documenting single-bottle growth of nonfastidious organisms indicate that low bacterial concentrations are frequently encountered and predict false-negative cultures in products contaminated with common bacteria at rates that varied from 4 to more than 75 percent. The model highlights the weakness of the AABB definition of false-positive culture results at low bacterial concentrations, especially for organisms that grow poorly in aerobic storage conditions. Continuing reports of septic reactions after transfusion of culture-tested apheresis PLTs, and frequent single-bottle-positive results when testing with two-bottle systems, suggest appreciable levels of false-negative results with a commercially available bacterial detection system (BacT/ALERT, bioMéreiux) as implemented.

Effect of Topical Emollient Treatment of Preterm Neonates in Bangladesh on Invasion of Pathogens Into the Bloodstream

Topical emollient therapy may reduce the incidence of serious infections and mortality of preterm infants in developing countries. We tested whether emollient therapy reduced the burden of pathogens on skin and/or prevented bacterial translocation. Neonates <33 wk gestational age were randomized to treatment with sunflower seed oil (SSO) or Aquaphor or the untreated control group. Skin condition score and skin cultures were obtained at enrollment and on d 3, 7, and weekly thereafter, and blood cultures were obtained for episodes of suspected nosocomial sepsis. For analysis, blood cultures were paired with skin cultures obtained 0-3 d before the blood culture. Skin condition scores at 3 d were better in patients treated with either emollient compared with untreated controls; however, skin flora was similar across the groups. The SSO group showed a 72% elevated odds of having a false-positive (FP) skin culture associated with a negative blood culture (i.e. skin flora blocked from entry into blood) compared with the control group. Topical therapy with SSO reduced the passage of pathogens from the skin surface into the bloodstream of preterm infants.

Molecular epidemiology of tuberculosis: toy or tool? A review of the literature and examples from Central Europe

Genotyping has become an indispensable tool in medical microbiology and epidemiology. One of the first targets has been Mycobacterium tuberculosis. Over the past 15 years approximately 900 pertinent publications have substantiated the value of the genotyping approach for tuberculosis control. New insights into the understanding of the natural history of tuberculosis, especially regarding the frequencies of reactivation, reinfection or multiple infection entailed adaptations of pathophysiological concepts. However, assessment of recent transmission, outbreak analysis, and detection of laboratory contamination still form the genuine scope of genotyping. Detection of unsuspected clusters of cases can provide clues to search for further, undetected cases. Uncovering false positive cultures spares the risks and costs of unnecessary treatment and may reveal systematic laboratory weaknesses. Several European countries already profit from nationwide prospective fingerprinting. After providing genotyping results to public health officials, these were able to document epidemiological links for substantially more tuberculosis patients. On a global scale, strain families and particular strains have been identified, characterised and traced in their spread. The importation of Beijing-genotype multidrug-resistant M. tuberculosis into Central European countries will be described here as an example. The goal for further developments is the ability to compare isolates for epidemiological purposes in a single step that also comprises species determination, drug resistance testing and detection of pathogenicity factors.