Blood Culture, Routine

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

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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.

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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

Clinical Disease Suspected

Culture Recommendation

Rational

Sepsis, meningitis osteomyelitis, septic arthritis, bacterial pneumonia

Two sets of cultures—one from each of two prepared sites, the second drawn after a brief time interval, then begin therapy.

Assure sufficient sampling in cases of intermittent or low level bacteremia. Minimize the confusion caused by a positive culture resulting from transient bacteremia or skin contamination.

Fever of unknown origin (eg, occult abscess, empyema, typhoid fever, etc)

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Two sets of cultures—one from each of two prepared sites, the second drawn after a brief time interval (30 minutes). If cultures are negative after 24 to 48 hours obtain two more sets, preferably prior to an anticipated temperature rise.

The yield after four sets of cultures is minimal. A maximum of three sets per patient per day for three consecutive days is recommended.

Endocarditis

Acute

Obtain three blood culture sets within two hours, then begin therapy.

95% to 99% of acute endocarditis patients (untreated) will yield a positive in one of the first three cultures.

Subacute

Obtain three blood culture sets on day one, repeat if negative after 24 hours. If still negative or if the patient had prior antibiotic therapy, repeat again.

Adequate sample volume despite low level bacteremia or previous therapy should result in a positive yield.

Immunocompromised Host (eg, AIDS)

Septicemia, fungemia mycobacteremia

Obtain two sets of cultures from each of two prepared sites; consider lysis concentration technique to enhance recovery for fungi and broth systems for recovery of mycobacteria.

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Low levels of fungemia and mycobacteremia frequently encountered.

Previous Antimicrobial Therapy

Septicemia, bacteremia; monitor effect of antimicrobial therapy

Obtain two sets of cultures from each of two prepared sites; increased volume >10 mL/set.

Recovery of organisms is enhanced by dilution and increased sample volume.

Interpretation of Positive Blood Cultures*

*Adapted from Flournoy DJ, Adkins L. Understanding the blood culture report. Am J Infect Control. 1986 Feb; 14(1):41-46.

Virtually any organism, including normal flora, can cause bacteremia.

A negative culture result does not necessarily rule out bacteremia; false-negative results occur when pathogens fail to grow.

A positive culture result does not necessarily indicate bacteremia; false-positive results occur when contaminants grow.

Gram-negative bacilli, anaerobes, and fungi should be considered pathogens until proven otherwise.