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

A. Types of bacteremia: Extravascular via 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

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(%)

S. aureus876

Coag negative staph1282

Enterococcus7016

Diphtheroids296

C. perfringens2377

C. albicans90

Blood Cultures: Methods

• Two blood cultures for separate venipuncture sites 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

Blood Cultures: Interpretation

• Organisms isolated > 72 hours are often contaminants.

• A single blood cultures with coagulase (-) staphylococci is often a contaminant.

• A single (+) blood cultures with S. Aureus, gm (-) bacillie or candida is always a pathogen and requires therapy.

• The patient does not have leukocytosis or a left shift

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 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).

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 microbiota.

• So: careful attention to the details of skin preparation and antisepsis prior to collection of the specimen.

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.

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.

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 microbiota, 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 microbiota is a true pathogen.