Blood Cultures and Immunocompromised Patients

Bloodstream Infections

Bacteremia and sepsis are conditions associated with high mortality and are of great impact to health care operations. Among the top causes of mortality in the United States, these conditions cause over 600 fatalities each day. Empiric, broad-spectrum treatment is a common but often a costly approach that may fail to effectively target the correct microbe, may inadvertently harm patients via antimicrobial toxicity or downstream antimicrobial resistance. To meet the diagnostic challenges of bacteremia and sepsis, laboratories must understand the complexity of diagnosing and treating septic patients, in order to focus on creating algorithms that can help direct a more targeted approach to antimicrobial therapy and synergize with existing clinical practices defined in new Surviving Sepsis Guidelines. Significant advances have been made in improving blood culture media; as yet no molecular or antigen-based method has proven superior for the detection of bacteremia in terms of limit of detection. Several methods for rapid molecular identification of pathogens from blood cultures bottles are available and many more are on the diagnostic horizon. Ultimately, early intervention by molecular detection of bacteria and fungi directly from whole blood could provide the most patient benefit and contribute to tailored antibiotic coverage of the patient early on in the course of the disease. Although blood cultures remain as the best means of diagnosing bacteremia and candidemia, complementary testing with antigen tests, microbiologic investigations from other body sites, and histopathology can often aid in the diagnosis of disseminated disease, and application of emerging nucleic acid test methods and other new technology may greatly impact our ability to bacteremic and septic patients, particularly those who are immunocompromised.

Bacteremia in an immunocompromised patient caused by a commensal Neisseria meningitidis strain harboring the capsule null locus (cnl)

We recently described the capsule null locus (cnl) of constitutively unencapsulated Neisseria meningitidis clonal lineages. cnl meningococci were recovered from healthy carriers at high frequency. We here report on the first case of invasive disease caused by cnl meningococci in a severely immunosuppressed patient with chronic graft-versus-host disease after allogeneic peripheral blood stem cell transplantation. The sequence type 845 strain was extensively typed and, furthermore, shown to be sensitive to serum bactericidal activity.

Blood cultures in the critical care unit: improving utilization and yield

Sepsis is a common cause of morbidity and death in critically ill patients, and blood culture samples are often drawn in an effort to identify a responsible pathogen. Blood culture results are usually negative, however, and even when positive are sometimes difficult to interpret. Distinguishing between true bacteremia and a false-positive blood culture result is important, but complicated by a variety of factors in the ICU. False-positive culture results are costly because they often prompt more diagnostic testing and more antibiotic prescriptions, and increase hospital length of stay. A number of factors influence the yield of blood cultures in critically ill patients, including the use of antibiotics, the volume of blood drawn, the frequency with which culture samples are drawn, and the site from which the culture samples are taken. Skin preparation techniques, handling of the cultures in the microbiology laboratory, and the type of blood culture system employed also influence blood culture yield. Attempts to identify predictors of true bacteremia in critically ill patients have been disappointing. In this review, we discuss factors that influence blood culture yield in critically ill patients, suggest ways to improve yield, and discuss true bacteremia vs false-positive blood culture results. We also discuss the costs and consequences of false-positive blood culture results, and list noninfectious causes of fever in the ICU.

Performance of five agar media for recovery of fungi from isolator blood cultures

We studied the recovery of 1,270 fungal isolates from 176,144 Isolator blood cultures (0.72% positive) on bacterial and fungal media, under routine and differing incubation conditions. Except with Histoplasma capsulatum, chocolate agar incubated for only 3 days proved to be an excellent medium for the recovery of fungi from the Isolator system.

Making the most of blood cultures. Tips for optimal use of this time-honored test

When bacteremia is suspected, the physician's first step is to determine the most likely source by using all available data from the history, physical examination, and laboratory studies. When evaluation for bacteremia is warranted, two or three sets of blood cultures should be carefully obtained using strict procedure guidelines, and empirical antibiotic therapy should be initiated. If the source of bacteremia has not been identified, blood culture results must be correlated with the clinical situation so that further investigations may be pursued and antimicrobial therapy may be initiated or modified. A high index of suspicion must be maintained for unusual or fastidious pathogens in immunocompromised patients.

Detection of bacteraemia in patients with fever and neutropenia using 16S rRNA gene amplification by polymerase chain reaction

Episodes of fever and neutropenia are common complications of treatment for cancer. The use of prophylactic and early empirical antibiotics has reduced mortality but decreases the sensitivity of diagnostic tests based on culture. The aim of this study was to determine the potential of a broad diagnostic approach (eubacterial) based on 16S rRNA gene amplification and sequencing to augment cultural methods of diagnosis of bacteraemia in patients with fever and neutropenia in a regional paediatric oncology centre. One hundred eleven patient-episodes of fever and neutropenia were evaluated during the study period, 17 of which were associated with positive blood cultures, as follows: Staphylococcus epidermidis (n = 6 episodes), Enterococcus faecium (n = 2), Streptococcus sanguis (n = 3), Streptococcus mitis (n = 3), Staphylococcus aureus (n = 1), Micrococcus spp. (n = 1), and Stenotrophomonas maltophilia (n = 1). Eubacterial polymerase chain reaction (PCR) detected bacterial DNA in nine of 11 blood culture-positive episodes for which a sample was available for PCR; the species identified by sequence analysis were identical to those derived from the conventional identification of the cultured isolates. Bacterial DNA was detected in 20 episodes (21 bacterial sequences) associated with negative blood cultures, 18 of which occurred in patients who were receiving antibiotics at the time of sample collection. The species presumptively identified by partial 16S rRNA gene sequencing were as follows: Pseudomonas spp. (n = 6 episodes), Acinetobacter spp. (n =5 ); Escherichia spp. (n = 3); Moraxella spp. (n = 3); Staphylococcus spp. (n = 2); Neisseria spp. (n = 1); and Bacillus spp. (n = 1). The results of this study suggest that molecular techniques can augment cultural methods in the diagnosis of bacteraemia in patients who have been treated with antibiotics.

An autopsy study of systemic fungal infections in patients with hematologic malignancies

The aim of this study was to determine the incidence of fungal infections detected on autopsy in a group of 40 patients with hematologic malignancies treated with intensive chemotherapy or bone marrow transplantation, and to evaluate the risk factors for fungal infections. A control group included 38 patients with nonhematologic diseases and without granulocytopenia but with at least one of the known risk factors for fungal infections. Standard histopathological and microbiological methods were used. A higher incidence of invasive fungal infections was found in patients with hematologic malignancies as compared to the control group (p < 0.01). The predominant causes of fungal infections were Candida albicans and Aspergillus spp. The incidence of fungal infections caused by Aspergillus was higher (p < 0.05) in patients with hematologic malignancies than in the control group. The independent risk factors for fungal infections were fungal colonization, number of antibiotics and duration of antibiotic therapy, duration of fever and skin rash. A higher proportion of fungal infections was diagnosed on autopsy than during the patients' life (p < 0.01).