Blood Culture, Routine

Three negative sets of blood cultures in the absence of antimicrobial therapy are usually sufficient to exclude the presence of bacteremia. One set is seldom ever sufficient.¹ Prior antibiotic therapy may cause negative blood cultures or delayed growth. Blood cultures from patients suspected of having Brucella must be requested as special cultures. Consultation with the laboratory for special culture procedures for the recovery of these organisms prior to collecting the specimen is recommended. Yeast often are isolated from routine blood cultures; however, if yeast or other fungi are specifically suspected, a separate fungal blood culture should be drawn along with each of the routine blood culture specimens. See separate list for proper collection of Fungus (Mycology) Culture [008482]. Mycobacterium avium complex (MAC) is frequently recovered from blood of immunocompromised patients, particularly those with acquired immunodeficiency syndrome (AIDS). Special procedures are required for the recovery of these organisms; use test Acid-fast (Mycobacteria) Smear andCulture With Reflex to Identification [183753].

Sequential blood cultures in nonendocarditis patients using a 20 mL sample resulted in an 80% positive yield after the first set, a 90% yield after the second set, and a 99% yield after the third set. Volume of blood cultured seems to be more important than the specific culture technique being employed by the laboratory. The isolation of coagulase-negative Staphylococcus poses a critical and difficult clinical dilemma. Although coagulase-negative Staphylococcus is the most commonly isolated organism from blood cultures, only a few (6.3%) of the isolates represent “true” clinically significant bacteremia.² Conversely, coagulase-negative Staphylococcus is well recognized as a cause of infections involving prosthetic devices, cardiac valves, CSF shunts, dialysis catheters, and indwelling vascular catheters.³ Ultimately, the physician is responsible for determining whether an organism is a contaminant or a pathogen. The decision is based on both laboratory and clinical data. Frequently this determination includes patient data (ie, patient history), physical examination, body temperatures, clinical course, and laboratory data (ie, culture results, white blood cell count, and differential). The number of positive cultures as defined by a venipuncture is the most relevant criterion to use in determining whether an isolate is a contaminant. Clinical experience and judgment may play a significant role in resolving this clinical dilemma.⁴

In patients who have received antimicrobial drugs, four to six blood cultures may be necessary. Any organism isolated from the blood is usually tested for susceptibility. It is not recommended to culture blood while antimicrobials are present unless verification of an agent's efficacy is needed. This is confirmed with a single culture.

The diagnosis of bacterial meningitis is accomplished by blood culture, as well as culture and examination of the cerebrospinal fluid.⁵ Most children with bacterial meningitis are initially bacteremic.⁶ See tables.

Blood Culture Collection