Transcript Occurrence of False Positive Blood Cultures

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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: Contaminants
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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.
Occurrence of False Positive Blood
Cultures (Trash)
True (%) Trash (%)
S. aureus
Coag negative staph
Enterococcus
Diphtheroids
C. perfringens
C. albicans
87
12
70
2
23
90
6
82
16
96
77
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% yield
Note: Diagnostic yield increased by 3% for every
1 ml of blood drawn
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 (false-positive blood cultures).
Aim of the test
• Diagnosis of bacteremia by
 Aerobic and anaerobic cultivation of the blood,
 With identification&
 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.
Common pathogens
Streptococcus spp
Bacteroides fragilis and other anaerobic
bacteria
Staphylococcus aureus
Coagulase negative staphylococci
Listeria monocytogenes
Enteric gram negative bacilli
Corynebacterium jeikeium
Neisseria meningitides
Haemophilus influenza
Non fermenter gram negative bacilli
Salmonella typhi
Pseudomonas aeruginosa
Parasitic infection
Parasite can be found as transiently in the blood stream for example tachyzoites of
Toxoplasma gondii
Viruses
Epstein barr virus
HIV virus
Cytomegalovirus
Other human Retroviruses
Fungi
Candida albicans
Cryptococcus neoformans
Other candida spp
immitis Coccidoides
Histoplasma capsulatum
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.
Sodium polyanethol sulphonate (SPS)
• The anticoagulant in blood culture medium must not
harm the bacteria and must prevent clotting of the
blood, which entrap bacteria and prevent their
detection .
• The most commonly used preparation in blood media
is 0.025% to 0.05% SPS.
• In addition to it’s anticoagulant properities, SPS is
also anticomplementary, antiphagocytic, and
interferes with the activity of some antimicrobial
agents.
Post specimen processing
• Interfering factors
 Patient on antibiotic therapy
• Result reporting
 Any isolated organism will be reported. Antibiotic sensitivity
will also be included with the report.
• Turn around time
Initial blood culture results will be reported as soon as
it shows growth.
Final results with sensitivity will be issued after 2448 hours of the initial report.
Negative results will be issued after 10 days of culture
submission.
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.
Limitations
• 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.
• Prior antibiotic therapy may cause negative blood
cultures or delayed growth.
• Blood cultures from patients suspected of having
Brucella or Leptospira 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.
• Observed that performance of biphasic system to be superior
in recovering Brusella spp .
• The bi phasic system is feasible and practical method , it has
the advantage of repeated exposure of agar medium to
actively proliferating organisms in the liquid broth during sub
culturing, which is simply by tilting the bottle..