Blood cultures and immunocompromised patients.

The diagnosis of blood-borne infections in immunocompromised patients is a major challenge for the clinical microbiology laboratory. Isolation of blood-borne pathogens in these patients has profound clinical implications, yet is fraught with technical problems. Contamination of blood cultures by skin flora is particularly problematic in the immunosuppressed host as these low-virulence microorganisms can be the cause of infection. Careful skin preparation is necessary to minimize such contamination. Drawing at least two blood samples for culture improves the specificity of this test on an individual patient. If possible, blood for culture should not be obtained through indwelling vascular devices. The microbiology laboratory must be aware of the wide range of blood-borne pathogens in immunocompromised patients and have available suitable techniques for the isolation of prevalent microorganisms. For most hospitals, this now must include techniques for the isolation of mycobacteria. Newer blood culture techniques have made the isolation of unusual pathogens easier. These techniques include the use of fluorescent stains, special media, antigen assays, and DNA probes. Each laboratory must evaluate current techniques in terms of suitability for use in its individual setting. Each must also be aware of new technologic developments. When clinical infection is suspected despite negative blood cultures, staff in the microbiology laboratory should be aware of special blood culture techniques or adjunctive procedures such as bone marrow and liver biopsies. These techniques may assist the clinician in making a diagnosis. Due to the practical limitations of blood culture technology as well as to economic constraints, the microbiology laboratory cannot routinely offer blood cultures suitable for the isolation of all blood-borne pathogens. Therefore, the microbiology laboratory must act in consultation with the clinician to determine the optimal approach for blood cultures in the immunocompromised patient. Such a consultative role with special blood culture techniques made available for selected types of immunosuppressed hosts allows the cost-effective use of the latest technology.