Controlled evaluation of the volume of blood cultured in detection of bacteremia and fungemia

RETROSPECTIVE ANALYSIS OF BLOOD CULTURES AND THEIR ASSOCIATION WITH CLINICAL FINDINGS AND OUTCOME IN GREEN SEA TURTLES (CHELONIA MYDAS) AT A FLORIDA SEA TURTLE REHABILITATION FACILITY, 2017-2020

Septicemia is commonly suspected of sea turtles entering rehabilitation. However, blood culture results of green sea turtles (Chelonia mydas) are infrequently reported in the literature. Aerobic blood cultures were performed for intake examinations of 167 green sea turtles undergoing rehabilitation at Brevard Zoo's Sea Turtle Healing Center, Melbourne, Florida, USA from 2017 to 2020. The incidence of positive cultures during intake examinations was 24% (40/167). The most common bacterial isolates identified were Vibrio alginolyticus, Shewanella algae, Achromobacter xylosoxidans, Photobacterium damselae, Sphingomonas paucimobilis, and Vibrio parahaemolyticus. There was a statistically significant association (P < 0.05) between culture status and evidence of external injury. There was no significant association between culture status and Caryospora-like coccidia infection, or fibropapillomatosis. Culture-positive turtles had significantly lower (P < 0.05) total white blood cell, lymphocyte, monocyte, total protein, albumin, and calculated globulin values compared to turtles with negative blood cultures. Significantly more culture-positive turtles died in rehabilitation compared to culture-negative (P = 0.042). Positive blood cultures suggestive of septicemia are commonly found during intake examinations at a Florida sea turtle rehabilitation facility.

Pathophysiology of sepsis

PURPOSE OF REVIEW

To provide a current overview of sepsis pathophysiology.

RECENT FINDINGS

The emphasis on sepsis pathophysiology has moved away from the pathogen - the initiating factor - and instead is focussed upon the abnormal and exaggerated host response. Instead of targeted eradication of the infection, the host response activates or suppresses multiple downstream pathways, leading to multiple organ dysfunction.

SUMMARY

Sepsis represents a dysregulated host response to infection leading to organ dysfunction. Here, the pathogen triggers an initial exaggerated inflammatory-immune response that leads to activation or suppression of multiple endothelial, hormonal, bioenergetic, metabolic, immune, and other pathways. These, in turn, produce the circulatory and metabolic perturbations resulting in organ dysfunction. This review will provide an overview of underlying mechanisms and propose that these processes, whereas superficially viewed as dysfunctional, may actually be adaptive/protective in the first instance, though spilling over into maladaptation/harm depending on the magnitude of the host response.

Critical factors in the recovery of pathogenic microorganisms in blood

BACKGROUND

The critical factors for optimal recovery of microbial pathogens from blood are not only the basis for how we perform blood cultures on a daily basis, but are also important for development of all current blood-culture systems. Because much of this research was conducted and published between 1975 and 2010 there is a general sense that many physicians and scientists may not be familiar with it, but it is critical for performing and interpreting blood cultures.

OBJECTIVES

To review the critical factors in the laboratory recovery and isolation of pathogenic microorganisms in blood.

SOURCES

Literature review of published papers, select reviews and updated guidelines.

CONTENT

This review focuses on the critical factors that affect the recovery isolation of pathogenic microorganisms from individuals with bloodstream infections. Contemporary blood-culture systems, and current methods for blood-culture collection, have been designed to incorporate these critical factors so as to optimize recovery and isolation of pathogens while minimizing contamination.

IMPLICATIONS

It is important for microbiologists and practitioners to understand how and why these critical factors affect current blood-culture practices to improve patient management. Future research should emphasize which of these critical factors will still play a role in the era of molecular diagnostic tests, which will no longer be relevant, and which new critical factors have yet to be defined.

Best Practices of Blood Cultures in Low- and Middle-Income Countries

Bloodstream infections (BSI) have a substantial impact on morbidity and mortality worldwide. Despite scarcity of data from many low- and middle-income countries (LMICs), there is increasing awareness of the importance of BSI in these countries. For example, it is estimated that the global mortality of non-typhoidal Salmonella bloodstream infection in children under 5 already exceeds that of malaria. Reliable and accurate diagnosis of these infections is therefore of utmost importance. Blood cultures are the reference method for diagnosis of BSI. LMICs face many challenges when implementing blood cultures, due to financial, logistical, and infrastructure-related constraints. This review aims to provide an overview of the state-of-the-art of sampling and processing of blood cultures, with emphasis on its use in LMICs. Laboratory processing of blood cultures is relatively straightforward and can be done without the need for expensive and complicated equipment. Automates for incubation and growth monitoring have become the standard in high-income countries (HICs), but they are still too expensive and not sufficiently robust for imminent implementation in most LMICs. Therefore, this review focuses on "manual" methods of blood culture, not involving automated equipment. In manual blood cultures, a bottle consisting of a broth medium supporting bacterial growth is incubated in a normal incubator and inspected daily for signs of growth. The collection of blood for blood culture is a crucial step in the process, as the sensitivity of blood cultures depends on the volume sampled; furthermore, contamination of the blood culture (accidental inoculation of environmental and skin bacteria) can be avoided by appropriate antisepsis. In this review, we give recommendations regarding appropriate blood culture sampling and processing in LMICs. We present feasible methods to detect and speed up growth and discuss some challenges in implementing blood cultures in LMICs, such as the biosafety aspects, supply chain and waste management.

Improvement of blood culture contamination rate, blood volume, and true positive rate after introducing a dedicated phlebotomy team

The introduction of dedicated phlebotomy teams certified for blood collection has been reported to be highly cost-effective by reducing contamination rates. However, data on their effects on blood volume and true positive rate are limited. Therefore, we investigated the effect of replacing interns with a phlebotomy team on blood culture results. We performed a 24-month retrospective, quasi-experimental study before and after the introduction of a phlebotomy team dedicated to collecting blood cultures in a 2700-bed tertiary-care hospital. The microbiology laboratory database was used to identify adult patients with positive blood culture results. During the study period, there were no changes in blood collection method, blood culture tubes, and the application of antiseptic measures. Blood volume was measured by the BACTEC™ FX system based on red blood cell metabolism. A total of 162,207 blood cultures from 23,563 patients were analyzed, comprising 78,673 blood cultures during the intern period and 83,534 during the phlebotomy team period. Blood volume increased from a mean of 2.1 ml in the intern period to a mean of 5.6 ml in the phlebotomy team period (p < 0.001). Introduction of the phlebotomy team also reduced contamination rate (0.27% vs. 0.45%, p < 0.001) and led to a higher true positive rate (5.87% vs. 5.01%, p < 0.05). The increased true positive rate associated with the phlebotomy team involved both gram-positive and gram-negative bacteria. The introduction of a dedicated phlebotomy team can increase blood volumes, reduce blood culture contamination rate, and increase true positive rate.

Quantitative N-glycoproteomics reveals altered glycosylation levels of various plasma proteins in bloodstream infected patients

Bloodstream infections are associated with high morbidity and mortality with rates varying from 10-25% and higher. Appropriate and timely onset of antibiotic therapy influences the prognosis of these patients. It requires the diagnostic accuracy which is not afforded by current gold standards such as blood culture. Moreover, the time from blood sampling to blood culture results is a key determinant of reducing mortality. No established biomarkers exist which can differentiate bloodstream infections from other systemic inflammatory conditions. This calls for studies on biomarkers potential of molecular profiling of plasma as it is affected most by the molecular changes accompanying bloodstream infections. N-glycosylation is a post-translational modification which is very sensitive to changes in physiology. Here we have performed targeted quantitative N-glycoproteomics from plasma samples of patients with confirmed positive blood culture together with age and sex matched febrile controls with negative blood culture reports. Three hundred and sixty eight potential N-glycopeptides were quantified by mass spectrometry and 149 were further selected for identification. Twenty four N-glycopeptides were identified with high confidence together with elucidation of the peptide sequence, N-glycosylation site, glycan composition and proposed glycan structures. Principal component analysis, orthogonal projections to latent structures-discriminant analysis (S-Plot) and self-organizing maps clustering among other statistical methods were employed to analyze the data. These methods gave us clear separation of the two patient classes. We propose high-confidence N-glycopeptides which have the power to separate the bloodstream infections from blood culture negative febrile patients and shed light on host response during bacteremia. Data are available via ProteomeXchange with identifier PXD009048.

Multicenter Clinical Evaluation of BacT/Alert Virtuo Blood Culture System

BacT/Alert Virtuo is an advanced, automated blood culture system incorporating improved automation and an enhanced detection algorithm to shorten time to detection. A multicenter study of the investigational Virtuo system (bioMérieux, Inc., Durham, NC) compared to BacT/Alert 3D (BTA3D) for detection of bacteremia/fungemia in four bottle types, SA and FA Plus (aerobic) and SN and FN Plus (anaerobic), was performed in a clinical setting with patient samples in a matched system design clinical trial. Blood was added to paired aerobic or anaerobic bottles, with the volume in each bottle in each pair required to be ≤10 ml and with the volumes required to be within 30% of each other. Of 5,709 bottle sets (52.5% aerobic pairs and 47.5% anaerobic pairs), 430 (7.5%) were positive for bacterial or fungal growth, with 342 (6.0%) clinically significant and 83 (1.5%) contaminated. A total of 3,539 sets (62.0%) were volume compliant, with 203 sets (5.7%) clinically significant. The positivity rates for volume-compliant bottle pairs determined by the two systems were comparable, with 68.7% of clinically significant isolates detected by both instruments, 15.7% by Virtuo only, and 15.7% by BTA3D only. Virtuo detected microbial growth nearly 2 h sooner overall than BTA3D (mean, 15.9 h versus 17.7 h). Shorter time to detection by Virtuo was related to organism group, with the time to detection being significantly shorter for enteric Gram-negative bacilli and enterococci (means, 3.6 h and 2.3 h shorter, respectively). This large clinical study demonstrated that the Virtuo blood culture system produced results comparable to those seen with the long-established BTA3D system, with significantly shorter time to detection.

Changes in plasma protein levels as an early indication of a bloodstream infection

Blood culture is the primary diagnostic test performed in a suspicion of bloodstream infection to detect the presence of microorganisms and direct the treatment. However, blood culture is slow and time consuming method to detect blood stream infections or separate septic and/or bacteremic patients from others with less serious febrile disease. Plasma proteomics, despite its challenges, remains an important source for early biomarkers for systemic diseases and might show changes before direct evidence from bacteria can be obtained. We have performed a plasma proteomic analysis, simultaneously at the time of blood culture sampling from ten blood culture positive and ten blood culture negative patients, and quantified 172 proteins with two or more unique peptides. Principal components analysis, Orthogonal Projections to Latent Structures Discriminant Analysis (OPLS-DA) and ROC curve analysis were performed to select protein(s) features which can classify the two groups of samples. We propose a number of candidates which qualify as potential biomarkers to select the blood culture positive cases from negative ones. Pathway analysis by two methods revealed complement activation, phagocytosis pathway and alterations in lipid metabolism as enriched pathways which are relevant for the condition. Data are available via ProteomeXchange with identifier PXD005022.

Broad-Range PCR Coupled with Electrospray Ionization Time of Flight Mass Spectrometry for Detection of Bacteremia and Fungemia in Patients with Neutropenic Fever

Infection is an important complication in patients with hematologic malignancies or solid tumors undergoing intensive cytotoxic chemotherapy. In only 20 to 30% of the febrile neutropenic episodes, an infectious agent is detected by conventional cultures. In this prospective study, the performance of broad-range PCR coupled with electrospray ionization time of flight mass spectrometry (PCR/ESI-MS) technology was compared to conventional blood cultures (BC) in a consecutive series of samples from high-risk hematology patients. In 74 patients, BC and a whole-blood sample for PCR/ESI-MS (Iridica BAC BSI; Abbott, Carlsbad, CA, USA) were collected at the start of each febrile neutropenic episode and, in case of persistent fever, also at day 5. During 100 different febrile episodes, 105 blood samples were collected and analyzed by PCR/ESI-MS. There was evidence of a bloodstream infection (BSI) in 36/105 cases (34%), based on 14 cases with both PCR/ESI-MS and BC positivity, 17 cases with BC positivity only, and 5 cases with PCR/ESI-MS positivity only. The sensitivity of PCR/ESI-MS was 45%, specificity was 93%, and the negative predictive value was 80% compared to blood culture. PCR/ESI-MS detected definite pathogens (Fusobacterium nucleatum and Streptococcus pneumoniae) missed by BC, whereas it missed both Gram-negative and Gram-positive organisms detected by BC. PCR/ESI-MS testing detected additional microorganisms but showed a low sensitivity (45%) compared to BC in neutropenic patients. Our results indicate a lower concordance between BC and PCR/ESI-MS in the neutropenic population than what has been previously reported in other patient groups with normal white blood cell distribution, and a lower sensitivity than other PCR-based methods.

Rapid Diagnosis of Infection in the Critically Ill, a Multicenter Study of Molecular Detection in Bloodstream Infections, Pneumonia, and Sterile Site Infections

Supplemental Digital Content is available in the text.

Early identification of causative microorganism(s) in patients with severe infection is crucial to optimize antimicrobial use and patient survival. However, current culture-based pathogen identification is slow and unreliable such that broad-spectrum antibiotics are often used to insure coverage of all potential organisms, carrying risks of overtreatment, toxicity, and selection of multidrug-resistant bacteria. We compared the results obtained using a novel, culture-independent polymerase chain reaction/electrospray ionization-mass spectrometry technology with those obtained by standard microbiological testing and evaluated the potential clinical implications of this technique.

Cost-Effectiveness of 30- Compared to 20-Milliliter Blood Cultures: a Retrospective Study

The importance of blood culture (BC) volume for detection of bloodstream infections (BSIs) is documented. Recently, improved diagnostic sensitivity was demonstrated for 30- versus 20-ml BCs in adults (Cockerill FR, Wilson JW, Vetter EA, Goodman KM, Torgerson CA, Harmsen WS, Schleck CD, IIstrup DM, Washington JA, Wilson WR. Clin Infect Dis 38:1724-1730, 2004, http://dx.doi.org/10.1128/JCM.01314-11). Hospitals receive higher reimbursement for patients with documented septicemia. We determined the cost-effectiveness of 30-ml versus 20-ml BCs using results from our institution and previously published data. Positive BC results from 292 bacteremic episodes were reviewed. The costs of the reagents, equipment, phlebotomist, and technologist time were determined. The medical records department provided Medicare reimbursement (MR) data for patients with selected ICD-9 codes. These data provided an estimate of the annualized increase in MR versus costs associated with conversion to 30-ml BCs. MR for 464 annual primary BSIs was $24,808/episode. An expected 7.2% increase in BSIs detected using 30-ml BCs would add 34 additional cases annually and increase MR by $843,472. Comparative MR data for cases where septicemia complicated another diagnosis were available for 4 International Classification of Diseases, Ninth Revision (ICD-9) codes: laparoscopic cholecystectomy, biliary tract disorders, pneumonia, and cellulitis. The mean incremental MR was $9,667 per episode, which projected to a $483,350 revenue increase annually. The annual cost associated with conversion to 30-ml BCs was estimated to be $157,798. Thus, the potential net increase in hospital revenue would be $1,169,031 for 30-ml versus 20-ml BCs. Our results suggest that conversion to 30-ml BCs may not only improve patient care by detecting more BSIs but also increase hospital revenue substantially.

Correlation between mass and volume of collected blood with positivity of blood cultures

BACKGROUND

The collection of blood cultures is an extremely important method in the management of patients with suspected infection. Microbiology laboratories should monitor blood culture collection.

METHODS

Over an 8-month period we developed a prospective, observational study in an adult Intensive Care Unit (ICU). We correlated the mass contained in the blood vials with blood culture positivity and we also verified the relationship between the mass of blood and blood volume collected for the diagnosis of bloodstream infection (BSI), as well as we explored factors predicting positive blood cultures.

RESULTS

We evaluated 345 patients with sepsis, severe sepsis or septic shock for whom blood culture bottles were collected for the diagnosis of BSI. Of the 55 patients with BSI, 40.0% had peripheral blood culture collection only. BSIs were classified as nosocomial in 34.5%. In the multivariate model, the blood culture mass (in grams) remained a significant predictor of positivity, with an odds ratio 1.01 (i.e., for each additional 1 mL of blood collected there was a 1% increase in positivity; 95% CI 1.01-1.02, p = 0.001; Nagelkerke R Square [R(2)] = 0.192). For blood volume collected, the adjusted odds ratio was estimated at 1.02 (95% CI: 1.01-1.03, p < 0.001; R(2) = 0.199). For each set of collected blood cultures beyond one set, the adjusted odds ratio was estimated to be 1.27 (95% CI: 1.14-1.41, p < 0.001; R(2) = 0.221).

CONCLUSIONS

Our study was a quality improvement project that showed that microbiology laboratories can use the weight of blood culture bottles to determine if appropriate volume has been collected to improve the diagnosis of BSI.

Bacteremia in children: epidemiology, clinical diagnosis and antibiotic treatment

The diagnosis of bacteremia in children is important and it can be clinically challenging to recognize the signs and symptoms. The reported rates of bacteremia are higher in young children but with the increasing vaccine coverage, there has been a decrease in bacteremia due to the three vaccine preventable bacteria (Streptococcus pneumoniae, Haemophilus influenzae group b and Neisseria meningitidis). Notably, there have been increases in healthcare-associated bacteremias with a rise in Staphylococcus aureus and Gram negative bacteremias. This review provides a brief overview of the clinical diagnosis of bacteremia in children, focusing on the epidemiology, clinical characteristics, risk factors, antibiotic treatment, outcomes and preventative measures to reduce the incidence of bacteremia and improve morbidity and mortality.

Developments for improved diagnosis of bacterial bloodstream infections

Bloodstream infections (BSIs) are associated with high mortality and increased healthcare costs. Optimal management of BSI depends on several factors including recognition of the disease, laboratory tests and treatment. Rapid and accurate identification of the etiologic agent is crucial to be able to initiate pathogen specific antibiotic therapy and decrease mortality rates. Furthermore, appropriate treatment might slow down the emergence of antibiotic resistant strains. Culture-based methods are still considered to be the "gold standard" for the detection and identification of pathogens causing BSI. Positive blood cultures are used for Gram-staining. Subsequently, positive blood culture material is subcultured on solid media, and (semi-automated) biochemical testing is performed for species identification. Finally, a complete antibiotic susceptibility profile can be provided based on cultured colonies, which allows the start of pathogen-tailored antibiotic therapy. This conventional workflow is extremely time-consuming and can take up to several days. Furthermore, fastidious and slow-growing microorganisms, as well as antibiotic pre-treated samples can lead to false-negative results. The main aim of this review is to present different strategies to improve the conventional laboratory diagnostic steps for BSI. These approaches include protein-based (MALDI-TOF mass spectrometry) and nucleic acid-based (polymerase chain reaction [PCR]) identification from subculture, blood cultures, and whole blood to decrease time to results. Pathogen enrichment and DNA isolation methods, to enable optimal pathogen DNA recovery from whole blood, are described. In addition, the use of biomarkers as patient pre-selection tools for molecular assays are discussed.

Microbial monitoring and methods of sample collection: a GITMO survey (Gruppo Trapianto di Midollo Osseo)

The collection of microbiological samples represents an important aspect of care both for doctors as well as nurses. It is important to recognise and identify some key points, to avoid performing ‘unnecessary’ or ‘incorrect’ sampling, which may give useless or even misleading results, these are: the moment at which the sample is collected, the collection method and timing (if indicated). The comparison between the various nursing members of the Italian National bone marrow transplant group (GITMO), showed diversity of practice across all fields. A formal survey was therefore conducted within GITMO centres looking at the methods of microbiological sample collection. These results were compared with the literature, and in addition to the lack of homogeneity of practice within the centres, a lack of compliance with the recommendations was also observed. To evaluate the effectiveness of this survey in highlighting awareness of this issue and the presence of relevant guidelines, the questionnaire was repeated (with the same centres responding), which demonstrated no major changes in care practices.

Detection of Mycobacterium tuberculosis in blood by use of the Xpert MTB/RIF assay

We have developed a novel blood lysis-centrifugation approach for highly sensitive Mycobacterium tuberculosis detection in large volumes of blood with the Xpert MTB/RIF assay. One through 20 ml of blood was spiked with 0.25 to 10 CFU/ml of the M. tuberculosis surrogate M. bovis BCG. Multiple replicates of each sample were processed by a new lysis-centrifugation method and tested with the Xpert MTB/RIF assay. The assay was very sensitive with increased blood volumes. In the 20-ml samples, BCG was detected in blood spiked with 10, 5, 1, and 0.25 CFU/ml 100, 100, 83, and 57% of the time, respectively, compared to 100, 66, 18, and 18%, of the time, respectively, in 1-ml blood samples. Assay sensitivity was influenced by the type of anticoagulant used, with acid-citrate-dextrose solution B (ACD-B) providing the best results. A limit of detection of 10 CFU/ml was established with BCG spiked into ACD-B-treated blood, and 92, 36, and 33% of the samples with 5, 1, and 0.5 CFU/ml, respectively, were assay positive. The lysis buffer was stable both at room temperature and at 4°C for 2 months. The assay was tested with blood stored for 8 days without a change in sensitivity as measured by cycle threshold. This new assay format extends the capability of the Xpert MTB/RIF test, enabling up to 20 ml of blood to be tested rapidly for the presence of M. tuberculosis. This approach may be a useful method to detect extrapulmonary tuberculosis and the risk of death in immunocompromised patients.

Current approaches to the diagnosis of bacterial and fungal bloodstream infections in the intensive care unit

Healthcare systems spend considerable resources collecting and processing blood cultures for the detection of blood stream pathogens. The process is initiated with the collection of blood cultures that depend upon proper skin disinfection, collection of an adequate number of specimens and volume of blood, and prompt processing in a sensitive culture system. Complementing blood cultures and gaining in use are techniques such as nucleic acid amplification tests and mass spectroscopy that allow clinical laboratories to detect and identify organisms from blood cultures substantially faster than conventional systems. Furthermore, certain resistance mutations can be detected within hours of organism detection, thus providing valuable guidance to clinicians who strive to initiate the appropriate antimicrobial therapy as rapidly as possible, and who wish to discontinue unnecessary drugs expeditiously. Molecular and mass spectroscopy techniques are changing sepsis diagnosis rapidly and will provide far more specific information far more quickly, but the performance characteristics of these systems must be understood by intensivists who use such information to guide their patient management.

Current approaches to the diagnosis of bacterial and fungal bloodstream infections in the intensive care unit

Healthcare systems spend considerable resources collecting and processing blood cultures for the detection of blood stream pathogens. The process is initiated with the collection of blood cultures that depend upon proper skin disinfection, collection of an adequate number of specimens and volume of blood, and prompt processing in a sensitive culture system. Complementing blood cultures and gaining in use are techniques such as nucleic acid amplification tests and mass spectroscopy that allow clinical laboratories to detect and identify organisms from blood cultures substantially faster than conventional systems. Furthermore, certain resistance mutations can be detected within hours of organism detection, thus providing valuable guidance to clinicians who strive to initiate the appropriate antimicrobial therapy as rapidly as possible, and who wish to discontinue unnecessary drugs expeditiously. Molecular and mass spectroscopy techniques are changing sepsis diagnosis rapidly and will provide far more specific information far more quickly, but the performance characteristics of these systems must be understood by intensivists who use such information to guide their patient management.

Evaluation of the blood volume effect on the diagnosis of bacteremia in automated blood culture systems

BACKGROUND

Blood culture volume is the most important variable in detecting bacteremia and fungemia. However, the majority of hospitals in Taiwan do not meet the criteria for an ideal blood culture volume (8-10 mL per bottle, two bottles per set) during collection.

METHODS

The object of this study is to initiate an educational program for healthcare workers to increase blood volume collection and to evaluate the relationship between blood volumes and bacteremia recovery rate for detecting bacteremia and fungemia effectively by using the BD BACTEC 9240 blood culture system.

RESULTS

After education, the blood sample volume ≥5 mL group increased from 2.93% to 71.24%. For a total of 4,844 bottles, the relative improvement in recovery rate for detection has increased by 17.81% between the <5 mL group and the ≥5 mL group. The recovery rates for the low-volume (<3 mL), mid-volume (3-7 mL), high-volume (8-10 mL) and extreme high-volume (>10 mL) groups are 13.31%, 15.02%, 17.68%, and 14.96%, respectively.

CONCLUSION

With good blood collection practice, our study found that blood volume obtained was in direct proportion to recovery rate for the detection of bacteremia and fungemia.

Optimized pathogen detection with 30- compared to 20-milliliter blood culture draws

Using data from 23,313 patients, we assessed whether two blood culture sets of three bottles per set would detect more pathogens than two sets of two bottles per set and achieve similar sensitivity to collecting three sets of two bottles per set. We also compared the yield of aerobic and anaerobic bottles. Thirty milliliters of blood was distributed to one anaerobic and two aerobic bottles. Among 26,855 collections of ≥ 60 ml within 30 min, 1,379 (5.1%) were positive for a pathogen not requiring detection in more than one set to be considered a pathogen, with 72 additional distinct pathogens detected using two 30-ml compared to two 20-ml sets of one aerobic and one anaerobic bottle (increased yield, 7.9%; 95% confidence interval [CI], 6.2 to 9.8%). For conditional pathogens requiring detection in at least two positive blood cultures for classification as pathogens (i.e., otherwise classified as contaminants), there were 162 positive detections with two 30-ml sets, of which 16 would not have been detected by two 20-ml sets (increased yield, 11.0% [95% CI, 6.4 to 17.2%]). Among 134 subjects who had three sets of 30 ml each within a 30-min interval, there was complete concordance between 60 ml of blood drawn in the first two sets of 30 ml and three 20-ml sets (P = 1.0). One aerobic bottle plus one anaerobic bottle yielded more pathogens than two aerobic bottles for organisms requiring a single (P < 0.001) and two (P = 0.04) positive sets to be defined as pathogens. In conclusion, we showed that collection of two aerobic and one anaerobic blood culture bottles per set results in improved yield compared to two bottles per set. We also confirmed that an anaerobic bottle should be included in blood culture sets.

Conventional and molecular techniques for the early diagnosis of bacteraemia

Bloodstream infections account for 30-40% of all cases of severe sepsis and septic shock, and are major causes of morbidity and mortality. Diagnosis of bloodstream infections must be performed promptly so that adequate antimicrobial therapy can be started and patient outcome improved. An ideal diagnostic technology would identify the infecting organism(s) and their determinants of antibiotic resistance, in a timely manner, so that appropriate pathogen-driven therapy could begin promptly. Unfortunately, despite the essential information it provides, blood culture, the gold standard, largely fails in this purpose because time is lost waiting for bacterial or fungal growth. Several efforts have been made to optimise the performance of blood culture, such as the development of technologies to obtain rapid detection of microorganism(s) directly in blood samples or in a positive blood culture. The ideal molecular method would analyse a patient's blood sample and provide all the information needed to immediately direct optimal antimicrobial therapy for bacterial or fungal infections. Furthermore, it would provide data to assess the effectiveness of the therapy by measuring the clearance of microbial nucleic acids from the blood over time. None of the currently available molecular methods is sufficiently rapid, accurate or informative to achieve this. This review examines the principal advantages and limitations of some traditional and molecular methods commercially available to help the microbiologist and the clinician in the management of bloodstream infections.

Blood cultures: key elements for best practices and future directions

Bloodstream infections (BSI) cause significant morbidity and mortality among populations worldwide. Blood cultures (BCs) are regarded as the "gold standard" for diagnosis of bacteremia and are among the most important functions of the clinical microbiology laboratory. Significant changes in the methods and techniques of obtaining BCs have occurred since the first inception of BCs into clinical practice. Aside from significant improvements of established, conventional technology, new assays for diagnosis of bacteremia and fungemia, particularly those involving molecular techniques, are now available. BCs must be collected under sterile conditions and guidelines for appropriate collection, processing, and results reporting of BCs have been established. This review provides comprehensive information on optimal BC practices for laboratories, utilizing traditional approaches and emerging technology. As laboratories and clinicians must appreciate the key factors affecting the use of these techniques, improved communication between laboratory personnel and clinicians regarding such elements as duration of incubation, workup of contaminants and critical action value reporting will greatly improve the diagnostic approach to BSI.

The era of molecular and other non-culture-based methods in diagnosis of sepsis

Sepsis, a leading cause of morbidity and mortality throughout the world, is a clinical syndrome with signs and symptoms relating to an infectious event and the consequent important inflammatory response. From a clinical point of view, sepsis is a continuous process ranging from systemic inflammatory response syndrome (SIRS) to multiple-organ-dysfunction syndrome (MODS). Blood cultures are the current "gold standard" for diagnosis, and they are based on the detection of viable microorganisms present in blood. However, on some occasions, blood cultures have intrinsic limitations in terms of sensitivity and rapidity, and it is not expected that these drawbacks will be overcome by significant improvements in the near future. For these principal reasons, other approaches are therefore needed in association with blood culture to improve the overall diagnostic yield for septic patients. These considerations have represented the rationale for the development of highly sensitive and fast laboratory methods. This review addresses non-culture-based techniques for the diagnosis of sepsis, including molecular and other non-culture-based methods. In particular, the potential clinical role for the sensitive and rapid detection of bacterial and fungal DNA in the development of new diagnostic algorithms is discussed.

[Diagnostic reliability of anaerobic blood cultures in bacteremias from a critical care unit]

BACKGROUND AND OBJECTIVE

This is a descriptive study of bacteraemias diagnosed in ICU with an analysis of the diagnostic reliability of anaerobic blood cultures.

PATIENTS AND METHOD

Analysis of all positive blood cultures in an Intensive Care Unit from May 2005 to October 2007.

RESULTS

The overall incidence of true bacteraemia was 6,1% of admissions. Out of 100 patients, there were 73 bacteraemias and 52 contaminated cultures. Samples with contaminated cultures were drawn 6,27 days earlier (CI 95% 0,61-11,94 Sig:0,03) than true bacteraemia. Most frequent micro organisms were cocci gram positive: 43 cases (58,9%) (coagulase-negative staphylococci was the most frequent: 30 or 41% of all bacteraemia)) Sig=0,001 in relation with gram negatives. Anaerobic micro organisms were not detected. Candidemias were found in 10 cases (13,7%). The most frequent causes of bacteraemia in this study were catheter-related infections with 36 cases (49,3% Sig<0,005), followed by digestive origin infections in 14 (19,2%). Of all bacteremic episodes (73 bacteraemia), 66 (90,4%) were isolated in aerobic blood cultures, and 58 (79,5%) were in anaerobic ones, with a difference of 10,9% Sig:0,06. When only intrahospitalary bacteraemias were analyzed, there was a difference of 13,56% of more yields in aerobic blood cultures; Sig:0,04 (IC 95% 0,8%-26%)). Candidemias were isolated only in aerobic blood cultures; Sig:0,001. When analyzing coagulase negative staphylococci, there were 30 bacteraemias and 41 contaminated samples. 62 (87,3%) were isolated in aerobic blood cultures and 50 (70,4%) in anaerobic ones, with a difference of 16,9% Sig=0,01,CI 95%(3%-30%).

CONCLUSIONS

In the Critical Care Unit, it would be possible to change the anaerobic blood cultures by aerobic ones to diagnose bacteraemias of intrahospitalary acquisition. This fact should be analyzed with others studies.

Neonatal blood cultures: effect of delayed entry into the blood culture machine and bacterial concentration on the time to positive growth in a simulated model

AIMS

To determine if: time from blood culture inoculation to positive growth (total time to positive) and time from blood culture machine entry to positive growth (machine time to positive) is altered by delayed entry into the automated blood culture machine, and if the total time to positive differs by the concentration of organisms inoculated into blood culture bottles.

METHODS

Staphylococcus epidermidis, Escherichia coli and group B beta-haemolytic streptococci were chosen as clinically significant representative organisms. Two concentrations (> or =10 colony-forming units per millilitre and <1 colony-forming units per millilitre) were inoculated into PEDS BacT/Alert blood culture bottles and randomly allocated to one of three delayed automated blood culture machine entry times (30 min/8.5 h/15.5 h).

RESULTS

For all organisms at all concentrations, except the Staphylococcus epidermidis, the machine time to positive was significantly decreased by delayed entry. For all organisms at all concentrations, the mean total time to positive significantly increased with increasing delayed entry into the blood culture machine. Higher concentrations of group B beta-haemolytic streptococci and Escherichia coli grew significantly faster than lower concentrations.

CONCLUSION

Bacterial growth in inoculated bottles, stored at room temperature, continues although at a slower rate than in those blood culture bottles immediately entered into the machine. If a blood culture specimen has been stored at room temperature for greater than 15.5 h, the currently allowed safety margin of 36 h (before declaring a result negative) may be insufficient.

Guidelines for evaluation of new fever in critically ill adult patients: 2008 update from the American College of Critical Care Medicine and the Infectious Diseases Society of America

OBJECTIVE

To update the practice parameters for the evaluation of adult patients who develop a new fever in the intensive care unit, for the purpose of guiding clinical practice.

PARTICIPANTS

A task force of 11 experts in the disciplines related to critical care medicine and infectious diseases was convened from the membership of the Society of Critical Care Medicine and the Infectious Diseases Society of America. Specialties represented included critical care medicine, surgery, internal medicine, infectious diseases, neurology, and laboratory medicine/microbiology.

EVIDENCE

The task force members provided personal experience and determined the published literature (MEDLINE articles, textbooks, etc.) from which consensus was obtained. Published literature was reviewed and classified into one of four categories, according to study design and scientific value.

CONSENSUS PROCESS

The task force met twice in person, several times by teleconference, and held multiple e-mail discussions during a 2-yr period to identify the pertinent literature and arrive at consensus recommendations. Consideration was given to the relationship between the weight of scientific evidence and the strength of the recommendation. Draft documents were composed and debated by the task force until consensus was reached by nominal group process.

CONCLUSIONS

The panel concluded that, because fever can have many infectious and noninfectious etiologies, a new fever in a patient in the intensive care unit should trigger a careful clinical assessment rather than automatic orders for laboratory and radiologic tests. A cost-conscious approach to obtaining cultures and imaging studies should be undertaken if indicated after a clinical evaluation. The goal of such an approach is to determine, in a directed manner, whether infection is present so that additional testing can be avoided and therapeutic decisions can be made.

Timing of specimen collection for blood cultures from febrile patients with bacteremia

Bloodstream infections are an important cause of morbidity and mortality. Physician orders for blood cultures often specify that blood specimens be collected at or around the time of a temperature elevation, presumably as a means of enhancing the likelihood of detecting significant bacteremia. In a multicenter study, which utilized retrospective patient chart reviews as a means of collecting data, we evaluated the timing of blood culture collection in relation to temperature elevations in 1,436 patients with bacteremia and fungemia. The likelihood of documenting bloodstream infections was not significantly enhanced by collecting blood specimens for culture at the time that patients experienced temperature spikes. A subset analysis based on patient age, gender, white blood cell count and specific cause of bacteremia generally also failed to reveal any associations.

Is the volume of blood cultured still a significant factor in the diagnosis of bloodstream infections?

"The higher the volume of blood cultured the higher the yield of blood cultures" has been a well-accepted dictum since J. A. Washington II performed his classic work. This rule has not been questioned in the era of highly automated blood culture machines, nor has it been correlated with clinical variables. Our objective in this study was to complete a prospective analysis of the relationship between blood volume, the yield of blood cultures, and the severity of clinical conditions in adult patients with suspected bloodstream infections (BSI). During a 6-month period, random samples of blood cultures were weighed to determine the volume of injected blood (weight/density). Overall, 298 patients with significant BSI and 303 patients with sepsis and negative blood cultures were studied. The mean volume of blood cultured in patients with BSI (30.03 +/- 14.96 ml [mean +/- standard deviation]) was lower than in patients without BSI (32.98 +/- 15.22 ml [P = 0.017]), and more episodes of bacteremia were detected with <20 ml (58.9%) than with >40 ml (40.2%) of blood cultured (P = 0.022). When patients were stratified according to the severity of their underlying condition, patients with BSI had higher APACHE II scores, and higher APACHE II scores were related to lower sample volumes (P < 0.001). A multivariate analysis showed that in the group of patients with APACHE II scores of >/=18, higher volumes yielded higher rates of bacteremia (odds ratio, 1.04 per ml of blood; 95% confidence interval, 1.001 to 1.08). We conclude that the higher yield of blood cultures inoculated with lower volumes of blood reflects the conditions of the population cultured. Washington's dictum holds true today in the era of automated blood culture machines.

How reliable is a negative blood culture result? Volume of blood submitted for culture in routine practice in a children's hospital

OBJECTIVES

The primary aims of this study were to determine the volume of blood submitted for culture in routine clinical practice and to establish the proportion of blood cultures with a blood volume inadequate for reliable detection of bacteremia.

METHODS

The volumes of blood samples submitted for culture from infants and children up to 18 years of age were measured over a 6-month period. Blood cultures were deemed adequate submissions if they contained an appropriate (age-related) volume of blood and were submitted in the correct blood culture bottle type. During the study, an educational intervention designed to increase the proportion of adequate blood culture submissions was undertaken.

RESULTS

The volume of blood submitted in 1358 blood culture bottles from 783 patients was analyzed. Of the 1067 preintervention blood cultures, 491 (46.0%) contained an adequate blood volume and only 378 (35.4%) were adequate submissions on the basis of collection into the correct blood culture bottle type. After the intervention, there were significant increases in both the proportion of blood cultures containing an adequate blood volume (186 [63.9%] of 291 cultures) and the proportion of adequate submissions (149 [51.2%] of 291 cultures). Overall, blood cultures with an adequate blood volume were more likely than those with an inadequate blood volume to yield positive blood culture results (34 [5.2%] of 655 cultures vs 14 [2.1%] of 648 cultures). Similarly, adequate blood culture submissions were more likely than inadequate submissions to yield positive blood culture results (26 [5.1%] of 506 cultures vs 22 [2.8%] of 797 cultures).

CONCLUSIONS

In routine clinical practice, a negative blood culture result is almost inevitable for a large proportion of blood cultures because of the submission of an inadequate volume of blood. Even after an educational intervention, nearly one half of blood cultures were inadequate submissions.

Optimal testing parameters for blood cultures

The effects of volume of blood, number of consecutive cultures, and incubation time on pathogen recovery were evaluated for 37,568 blood cultures tested with the automated BACTEC 9240 instrument (Becton Dickinson Diagnostic Instrument Systems) at a tertiary care center over the period of 12 June 1996 through 12 October 1997. When the results for this study were compared with previous data published for manual broth-based blood culture systems and patient samples obtained in the 1970s and 1980s, the following were found: (1) the percentage increase in pathogen recovery per milliliter of blood is less, (2) more consecutive blood culture sets over a 24-h period are required to detect bloodstream pathogens, and (3) a shorter duration of incubation is required to diagnose bloodstream infections. Guidelines developed in the 1970s and 1980s for processing and culturing blood may require revision.

Diagnostic methods. Current best practices and guidelines for isolation of bacteria and fungi in infective endocarditis

As the etiological character of IE changes, the microbiological tools used to confirm the diagnosis have also evolved. Here the authors have reviewed the current methods for optimal laboratory diagnosis of bacterial and fungal endocarditis using traditional growth-based technologies and offered good practice guidelines and recommendations. Newer techniques will be required to improve sensitivity of detection for known organisms and to identify emerging or as-yet unknown pathogens (see article by Lepidi et al in this issue).

Clinical utility of blood cultures drawn from central vein catheters and peripheral venipuncture in critically ill medical patients

STUDY OBJECTIVE

To determine the sensitivity, specificity, and positive and negative predictive values of blood cultures obtained through a central vein catheter compared with peripheral venipuncture.

DESIGN

Prospective cohort study.

SETTING

A medical ICU (19 beds) from a university-affiliated urban teaching hospital.

PATIENTS

Between February 2001 and October 2001, 300 paired blood culture specimens were obtained from 119 patients (2.52 paired cultures per patient).

INTERVENTION

Prospective patient surveillance and data collection.

MEASUREMENTS AND MAIN RESULTS

Thirty-four paired culture results (11.3%; 95% confidence interval, 7.8 to 14.8%) were accepted as true-positives representing a true bacteremia. The sensitivity of catheter-drawn and peripheral venipuncture samples was 82.4% and 64.7%, respectively, and specificity was 92.5% and 95.9%. The positive predictive value was 58.3% for catheter-drawn samples and 66.7% for peripheral venipuncture samples, and the respective negative predictive values were 97.6% and 95.5%.

CONCLUSIONS

In critically ill medical patients, the negative predictive value of blood samples obtained by catheter draw or peripheral venipuncture for suspected bloodstream infection is good. However, the sensitivity of blood samples obtained by either catheter draw or peripheral venipuncture alone is not adequate to recommend the elimination of blood samples obtained from the other site. Clinicians should also be aware that additional blood samples may be necessary when interpreting positive blood culture results for common skin or central vein catheter contaminants.

What is the relevance of obtaining multiple blood samples for culture? A comprehensive model to optimize the strategy for diagnosing bacteremia

Through a heuristic and probabilistic approach, we evaluated blood culture operating characteristics (sensitivity, specificity, and predictive values) as a function of several pretest parameters, together with their variability. On the basis of a meta-analysis of quantitative data from the literature, a model was developed and an estimation of the operating characteristics through numerical simulations (Monte Carlo method) was performed. The model evaluates the influence of ordering and drawing parameters on the ability of blood culture to distinguish bacteremic from nonbacteremic patients, regardless of the causative species. By considering the total blood volume to be cultured (six 5-10-mL bottles), results were found to confirm the current guidelines. On the basis of this hypothesis, the results, together with an analysis of the literature, failed to show any benefit of a strategy that involves obtaining multiple samples. The best strategy when performing blood culture is to obtain blood for 6 bottles (for a total volume of 35-42 mL), preferably at the same time.

Blood cultures in newborns and children: optimising an everyday test

Effective use of blood cultures is a key component of the management of septic newborns and children. The technical and practical aspects of paediatric practice and the heightened susceptibility of children to infection because of immunological immaturity make automatic extrapolation of adult data difficult and potentially unfounded.

Diagnostic methods current best practices and guidelines for isolation of bacteria and fungi in infective endocarditis

This article addresses the clinical need for accurate detection of bacteria and fungi that may cause infective endocarditis (IE). The pathophysiology of bacteremia during endocarditis is reviewed to provide an understanding of how current diagnostic methods may be used to determine the etiologic diagnosis. The critical contribution of microbiologic findings to the current clinical diagnostic criteria is emphasized. Finally, "best practices" recommendations are presented to optimize the likelihood of reaching a correct etiologic diagnosis while minimizing the number of "culture-negative" cases.

Laboratory diagnosis of bacteremia and fungemia

Many of the variables that affect the laboratory diagnosis of bacteremia and fungemia have been addressed in this article. Whereas the scientific basis and principles for blood cultures are well-established, and the methodology has improved, the diagnosis of bacteremia and fungemia still depends greatly on the care that is taken in obtaining the specimens of blood and the skill of the clinician in interpreting positive results.

Bacteremia associated with naturally occuring acute coliform mastitis in dairy cows

OBJECTIVE

To determine the incidence of bacteremia in dairy cows with naturally occurring acute coliform mastitis (ACM) with a wide range of disease severity.

DESIGN

Cohort study.

ANIMALS

144 dairy cows with ACM from 6 herds.

PROCEDURE

Cows were examined at time of identification of ACM (time 0) and classified as having mild, moderate, or severe mastitis on the basis of rectal temperature, hydration status, rumen contraction rate, and attitude. Cows were reexamined at 24 or 48 hours. Bacteriologic culturing of milk and blood (30 ml), CBC, and serum biochemical analysis were performed at each time point. Appropriate samples were obtained at a single point from herdmates without mastitis (controls) that were closely matched for lactation number and days since parturition. Blood culture results were compared among severity groups and controls by use of chi2 tests, as was outcome of an ACM episode for cows grouped by blood bacterial isolates.

RESULTS

Bacteria were isolated from 52 blood samples from 46 of 144 (32%) cows with ACM, which was significantly more than control cows (11/156; 7.1%). Group-1 isolates (Escherichia coli, Pasteurella multocida, Mannheimia haemolytica, Klebsiella pneumoniae, Enterobacter agglomerans, and Salmonella enterica serotype Typhimurium) were identified in 20 of 144 (14%) cows with ACM and 0 of 156 control cows. Group-1 isolates were identified in 4.3, 9.1, and 42% of cows classified as having mild, moderate, and severe ACM, respectively. Escherichia coli and K pneumoniae milk and blood isolates obtained from the same cow were of the same genotype. Bacillus spp were identified in 21 of 144 (15%) cows with ACM, which was significantly more than control cows (3/156; 1.9%). Thirty-five percent of cows with a group-1 isolate died during the mastitis episode.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that bacteremia develops in a substantial proportion of cows with ACM. Classification of severity of disease is important for establishment of effective treatment protocols; parenteral antimicrobial treatment may be indicated in cows with ACM.

Utility of paired BACTEC MYCO/F LYTIC blood culture vials for detection of bacteremia, mycobacteremia, and fungemia

In previous bloodstream infection studies in Malawi, we inoculated blood from a single venesection into a single BACTEC MYCO/F LYTIC (MFL) vial. Inoculation of one vial, however, would be expected to reduce the sensitivity of bloodstream pathogen detection with MFL vials. To ascertain the degree of this loss of sensitivity, blood was drawn from each of 228 febrile, adult inpatients in Malawi and 5 ml of each blood sample was inoculated into each of two MFL vials. Of 228 paired vials, 51 (22%) were both positive, 172 (75%) were both negative, and 5 (3%) had discordant results. Bloodstream infection would have been detected in 11 (92%) of 12 patients with mycobacteremia and 38 (92%) of 41 patients with bacteremia had only one MFL vial been inoculated. Our study shows that a second MFL vial does not significantly increase diagnostic sensitivity.

Diagnostic investigations in 101 dogs with pyrexia of unknown origin

Records from 101 dogs presented for investigation of unexplained pyrexia were reviewed. The most common diagnosis was immune-mediated disease (22 per cent of cases), with immune-mediated polyarthritis accounting for 20 per cent of all diagnoses. The frequency of positive results obtained in investigative tests was also assessed. Cytological and radiological examinations provided a high diagnostic success rate, although routine haematology and plasma biochemistry were also useful screening tests. On the basis of these results it is suggested that, in the investigation of unexplained pyrexia, a diagnosis of immune-mediated polyarthritis should be excluded before less common diagnoses are considered.

Evaluation of the BacT/Alert and VITAL blood culture systems for the diagnosis of bacteremia

OBJECTIVE: To evaluate the detection of bacterial growth in the BacT/Alert (Organon Teknika) and VITAL (bioMérieux) automated blood culture systems. METHODS: In accordance with the protocol of study, 1021 blood sample pairs for culture were obtained from adult patients admitted to the Emergency Room and Intensive Care Unit. RESULTS: In total, 139 (13.6%) clinically significant blood cultures were detected, of which 79 (56.8%) were detected by both systems, 48 (34.5%) only by BacT/Alert and 12 (8.6%) only by VITAL (P cent0.0001). The BacT/Alert system detected positive blood cultures more rapidly for all groups of microorganisms. The VITAL system showed six false-negative blood cultures, while the BacT/Alert system showed none (P50.03). There was no significant difference between the number of false-positive blood cultures detected by the two systems. CONCLUSIONS: In our study, overall the BacT/Alert system achieved a better recovery of microorganisms than the VITAL system.

Update on detection of bacteremia and fungemia

The presence of microorganisms in a patient's blood is a critical determinant of the severity of the patient's illness. Equally important, the laboratory isolation and identification of a microorganism present in blood determine the etiologic agent of infection, especially when the site of infection is localized and difficult to access. This review addresses the pathophysiology and clinical characteristics of bacteremia, fungemia, and sepsis; diagnostic strategies and critical factors in the detection of positive blood cultures; characteristics of manual and instrument approaches to bacteremia detection; approaches for isolating specific microorganisms associated with positive blood cultures; and rapid methods for the identification of microorganisms in blood cultures.

Blood culture volume and detection of coagulase negative staphylococcal septicaemia in neonates

A prospective, blind study was carried out to determine: the amount of blood submitted for culture from neonates; whether small blood volumes resulted in false negative results; and whether there was a temporal relation between volume of blood cultured and time to positivity. Seventy three bottles were evaluated. They contained a median of 0.63 ml of blood. Twenty nine bottles (39.7%) contained less than 0.5 ml of blood; 21 bottles (28.8%) were positive. There were three false negative cultures, only one of which contained a blood volume below 0.5 ml. The median time to positivity was 22.4 hours. There was no correlation between blood volume cultured and time to positivity. Neonatal cultures frequently contain less than 0.5 ml of blood. False negative cultures are rare. Neonatal blood culture bottles need to be validated for blood volumes below 0.5 ml.

Volume of blood required to detect common neonatal pathogens

OBJECTIVE

To determine the minimum volume of blood and the absolute number of organisms required for detection of bacteremia and fungemia by an automated colorimetric blood culture system (BacT/Alert, Organon Teknika).

DESIGN

Common neonatal pathogens, Escherichia coli, Streptococcus agalactiae (group B streptococcus (GBS): one American Type Culture Collection (ATCC) strain and one clinical isolate), Staphylococcus epidermidis, and Candida albicans, were seeded into blood to produce bacteremia or fungemia with low colony counts (1 to 3 colony-forming units (CFU) per milliliter) and ultra-low colony counts (<1 CFU/ml). For each organism, 96 culture bottles were inoculated with either 0.25, 0.5, 1.0, or 4.0 ml of the two seeded blood concentrations. Blood culture bottles were incubated in the BacT/Alert device for 5 days, and time to positivity was noted when applicable. All bottles were subcultured on plated media.

DATA ANALYSIS

The Poisson statistic was used to calculate the probability of finding at least one viable CFU per inoculated culture bottle. The fraction of culture bottles with positive findings per group was divided by the probability of one or more organisms present to give the positivity index.

RESULTS

Plated subculture identified no growth of organisms not detected by the colorimetric detection system. The false-positive rate for the automated device was less than 1%. The positivity index for the GBS clinical isolate was 1.13, for the GBS ATCC isolate 0.96, for S. epidermidis 0.94, for C. albicans 0.97, and for E. coli 0.95. There was a statistically significant difference with time to positivity and inocula volume (p <0.01), but the difference was not clinically important.

CONCLUSIONS

If one or two viable colony-forming units are in the blood inoculated into culture media, the BacT/Alert system will detect growth rapidly. Because there appears to be a sizable subset of neonates who are at risk of sepsis with a colony count less than 4 CFU/ml, then a 0.5 ml inoculum of blood into the culture media is inadequate for sensitive and timely detection of bacteremia. One to two milliliters of blood should increase microorganism recovery in the face of low-colony-count sepsis.