Blood Culture Final1

8/2/2019 Blood Culture Final1

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Only 5-15% of blood cultures are (+) in febrile

patients

A. Types of bacteremia:

Extravascularvia the lymphatic's

Intravascular: i.e. CVC infections

B. Types of bacteremia:

Transient: Disruption of mucosal surfaces (dental

or surgical procedures)

Intermittent: Associated with abscesses

Continuous: Infective endocarditis


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Bacteremia: Pathogens

S. Aureus

S. Pyogenes

S. Pneumoniae

H. Influenzae

Enterobacteriaceae

Bacteroides

Pseudomonas Aeruginosa

Candida species


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Occurrence of False Positive Blood

Cultures (Trash)

True (%) Trash (%)

S. aureus 87 6

Coag negative staph 12 82Enterococcus 70 16

Diphtheroids 2 96

C. perfringens 23 77C. albicans 90


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Blood Cultures: Methods

Two blood cultures for separate venipuncturesites is adequate

Three sets of blood.

At least 10ml/ venipuncture.

Blood culture > 5ml blood: 92% yield

Blood culture < 5 ml blood: 69% yieldDiagnostic yield increased by 3% for every 1 ml

of blood drawn


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Bacteremia: Contaminants

Coagulase (-) Staphylococci.

Corynebacterium species

Bacillus species

If multiple isolated from separate sites areobtained, the organisms could be pathogenic

Viridans Streptococci can be a contaminant


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Aim of the test

Diagnosis of bacteremia byAerobic and anaerobic cultivation of the blood,

With identification and

Susceptibility test of the isolated organism (s).

Pediatrics: only aerobic.

Blood culture should be made for cases with :

suspected septicemia, endocarditis, and bacteremia

secondary to localized infections (pneumonia,

intraabdominal abscesses, pyelonephritis, epiglottitis,

meningitis).


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Aerobic/Anaerobic Blood Culture

Bottles


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Criteria of specimen rejection

Blood collected in tubes or bottles other thanaerobic and anaerobic blood culture bottles.

If the information on the label does not matchthat of the request form.

Specimens for anaerobic blood culture receivedin aerobic bottles or vice versa.


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Pathogens


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Patient preparing

The major difficulty in interpretation of blood cultures

is potential contamination by skin microbiota.

So: careful attention to the details of skin preparation

and antisepsis prior to collection of the specimen.


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Obtaining Blood Culture

Locate the vein (usually anticubital fossa)

Attention to IV line.

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.


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Set 1 = L. antecubital fossa at 0 minutes

Set 2 = R. antecubital fossa at 30 minutes

Set 3 = L. or R. antecubital fossa at 90 minutes.

Best time for sample collection: during fever spike\chills.

1st sample: 90% detection.


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Quantity of specimen


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Method

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

Discard as negative after the 10 days

During the incubation period, a gram stain andsubculture onto appropriate media should bedone.


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Interpretation of Positive Blood

Cultures

Virtually any organism, including normalmicrobiota, can cause bacteremia.

A negative culture result does not necessarilyrule out bacteremia;

false-negative results occur when pathogens fail togrow.

A positive culture result does not necessarilyindicate bacteremia;

false-positive results occur when contaminants grow.


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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 usuallyconsidered normal skin microbiota is a true

pathogen.


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Blood Culture Final1

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