Blood Cultures Flashcards
Define bacteremia:
Presence of bacteria in the bloodstream- may be transient
Occult bactermis - usually seen in children under the age of 2 with no signs and symptoms. The organism usually isolated is Strep. pneumonia.
Define septicemia:
Bacteremia plus physical signs and symptoms in the patient
Which variable, timing or volume, has the most significant effect on blood cultures? Why?
Volume - optimal ratio of blood to culture medium is 1:10, dilution is necessary to eliminate bactericidal effects of the patient own serum (20 ml draw)
The age volume protocol is younger than 10 years 1 mL for every year of life. If 1 to 3 ml of blood is collected use 5 ml bottles
How many blood culture sets and how often should they be drawn to ensure the highest recovery rate?
80% of bacterias will be discovered in the 1st set of bottles
over 90% will be discovered in 2nd set of bottles.
Both sets should be drawn within the 1st 2 hours of clinical presentation from two separate venipuncture sites.
Third set does not significantly increase the likelihood of isolation over the first 2 sets.
What do the blood culture bottles contain in each set?
Aerobic -
Anaerobic -
Aerobic - usually contains soybean casein digest broth, tryptic or trypticase soy broth, brain heart infusion, brucella agar or columbia broth base
Anaerobic - same media as above with 0.5% cysteine
What organisms may be inhibited if the anaerobic bottle contains thioglycolate or thiol containing broth?
Pseudomonas and Yeasts
What can be done if the patient is already on antimicrobial agents at the time the blood cultures are drawn?
- b lactam anitmicrobial agents (such as penicillinase may be added)
- ARD bottle (contain resins that absorb antibiotics)
- SPS (sodium polyethanol sulfonate will inactivate certain antibiotics but will inhibit the growth of some microbes)
What antibiotics will SPS inactivate? What bacteria will be inhibited from growing?
Inactivates: streptomycin, kanamycin, gentamicin, polymyxin B
Inhibited: Peptostreptococcus anaerobes, N. gonorrhaeae, N. meningitidis, Gardnerella vaginalis
1.2% gelatin can be added to neutralize the inhibitory effects of the SPS
How do the continuous-monitoring blood culture systems identify microbial growth?
Bactec series: if an org. is beginning to grow it will produce CO₂, which will diffuse into the sensor-generating hydrogen ions. the increase in H⁺ conc. increases the amount of fluorescence, which is measured (noninvasive)
EAP: detects head-space pressure changes and measures consumption or production of gas
Bacti-alert: pH indicator (measured photometrically) non invasive into bottle
What are the five most frequently isolated pathogens from the blood?
E. coli Enterococcus Staph aureus Pseudomonas Strep pneumoniae
What are the most common contaminants in blood cultures?
Coag. neg staph
Hemolytic Strep (alpha or gamma)
Corynebacterium sp.
Propionibacterium acnes
What is the common protocol for the proper collection of blood cultures?
- cleanse skin with betadine (iodine)
- Iodine-based compound scrubbed in concentric fashion around venipuncture site and left on for at least one minute.
- After venipuncture the antiseptic should be removed with an alcohol pad.
Discuss a routine blood culture work-up when automated systems are not available?
- 6-18 hrs subculture and trasnfer all bottles to media
- Prepare direct smear for gram stain or acridine orange, subculture Anaerobic/Aerobic to choc plate
- Anaerobic subculturing should be performed after 2 days.
- Gram staining and subculturing should occur every other day, subcultures should be held 48 hrs. before listed as no growth
- Routine blood cultures should be held for 7 days
What is the most common cause of infectious endocarditis?
Staph. aureus/Strep. viridans
What is the most common organism associated with nosocominal bacteremia?
Coag neg staph
