Blood Culture

Bacteremia: Types

Transient: Disruption of mucosal surfaces (dental or surgical procedures)

Intermittent: Associated with abscesses

Continuous: Infective endocarditis

Bacteremia: Pathogens S. Aureus S. Pyogenes S. Pneumoniae H. Influenzae Enterobacteriaceae Bacteroides Pseudomonas Aeruginosa Candida species

Occurrence of False Positive Blood Cultures (Trash)

True )%(

Trash )%(

Maybe )%(

S. aureus8766

Coag negative staph

12826

Enterococcus701614

Diphtheroids2962

C. perfringens2377

C. albicans9010

Blood Cultures: Methods 2 blood cultures for separate

venipuncture sites is adequate 3 sets of blood cultures for I.E. At least 10ml/ venipuncture BLD CX > 5ml blood: 92% yield BLD CX < 5 ml blood: 69% yield Diagnostic yield increased by 3% for

every 1 ml of blood drawn

Blood Cultures: Interpretation Organisms isolated > 72 hours are often

contaminants (+) BLD cultures not compatible with a

clinical syndrome are usually contaminants A single BLD CX with coagulase (-)

staphylococci is often a contaminant A single (+) BLD CX with S. Aureus, gm (-)

bacillie or candida is always a pathogen and requires therapy.

Bacteremia: Contaminants Coagulase (-) Staphylococci Corynebacterium species Bacillus species If multiple isolated from separate sites are

obtained, the organisms could be pathogenic

Viridans Streptococci can be a contaminant

Aim of the test Diagnosis of bacteremia by

aerobic and anaerobic cultivation of the blood, with identification and susceptibility test of the isolated organism (s).

Blood culture should be made for cases with suspected septicemia, endocarditis, and bacteremia secondary to localized infections (pneumonia, intraabdominal abscesses, pyelonephritis, epiglottitis, meningitis).

Aerobic/Anaerobic Blood Culture Bottles

Criteria of specimen rejection Blood collected in tubes or bottles

other than aerobic and anaerobic blood culture bottles.

If the information on the label does not match that of the request form.

Specimens for anaerobic blood culture received in aerobic bottles or vice versa.

Pathogens

Patient preparing

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

Obtaining Blood Culture Locate the vein Prep kit

Alcohol 5 sec. Dry 30-60 sec Tincture of Iodine-center to periphery. Dry 45-60 sec

Remove caps, clean with alcohol Put on gloves Without palpating, draw 20 ml and put 10 in

anaerobic and 10 in aerobic bottle Dispose of syringe in sharps container Label bottles and send to lab

Quantity of specimen

Method

Blood is injected to both aerobic and anaerobic bottles and incubated for up to 10 days at 37 C.

Discard as negative after the 10 days During the incubation period, a gram

stain and subculture onto appropriate media should be done.

Interpretation of Positive Blood Cultures

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.

The most difficult interpretation problem is to determine whether an organism that is usually considered normal skin flora is a true pathogen.

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