• 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 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. Occurrence of False Positive Blood Cultures (Trash) True (%) Trash (%) S. aureus 87 6 Coag negative staph 12 82 Enterococcus 70 16 Diphtheroids 2 96 C. perfringens 23 77 C. albicans 90 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. 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.
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