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Blood Culture, Routine

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Category:

Infectious Disease: Screening

Turnaround Time: 5 - 6 days

CPT Code:

87040

Test Type: Adults: 16 to 20 mL. Pediatrics: up to 4 mL

Overview:

Isolate and identify potentially pathogenic organisms causing bacteremia; establish the diagnosis of endocarditis.

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

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)	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.	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*

Collection Details:

Patient Preparation:

The major difficulty in interpretation of blood cultures is potential contamination by skin flora. This difficulty can be markedly reduced by careful attention to the details of skin preparation and antisepsis prior to collection of the specimen.

Skin preparation: First cleanse the venipuncture site with isopropanol. Then use an antiseptic swabstick to disinfect the site, using progressively larger concentric circles. This prepping agent should remain in contact with the skin for 30 seconds and be allowed to dry to ensure adequate disinfection. The venipuncture site must not be palpated after preparation. Blood is then safely drawn.

Collection Instructions:

The test request form must state clinical diagnosis and time of collection. List current antibiotic therapy, clinical diagnosis, and any special organisms suspected or to rule out. Must indicate if culture is for Brucella or Francisella. Do not use expired blood culture media.

One aerobic and one anaerobic blood culture bottle for adults or one pediatric bottle. Do not vent.

Use adult or pediatric blood culture collection kits provided by LabCorp. See the Procedural Chart for Blood Culture Collection provided in each collection kit for detailed information regarding bottle preparation, venipuncture, and bottle inoculation. Blood cultures should be drawn prior to initiation of antimicrobial therapy. The time of collection must be indicated. Strict aseptic technique is essential. If more than one culture is ordered, the specimens should be drawn separately at no less than 30 minutes apart to rule out the possibility of transient bacteremia due to self-manipulation by the patient of mucous membranes in the mouth caused by brushing teeth, etc, or by local irritations caused by scratching of the skin.

• Suspected sepsis, meningitis, osteomyelitis, arthritis, listeriosis, or acute untreated bacterial pneumonia: Obtain two blood cultures from two different sites, such as the left and right arms.

• Fever of unknown origin such as that caused by an occult abscess: Obtain two blood cultures initially. If those are negative, obtain two more 24 to 36 hours later. The yield beyond three or four cultures is virtually nil in this condition.

• Suspected early typhoid fever and brucellosis: Obtain four blood cultures during 24 to 36 hours due to low-grade bacteremia involved in these rarely seen diseases.

• Endocarditis (acute infective endocarditis): Obtain three blood cultures from three separate venipuncture sites during the first one to two hours and begin therapy.

• Subacute infective endocarditis: Obtain three blood cultures within the first 24 hours, ideally within no less than hourly intervals. If all are negative at 24 hours, obtain two more. The yield beyond five blood cultures in subacute and endocarditis is virtually nil.

Maintain specimen at room temperature. Do not refrigerate.

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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.