Detection of Bloodstream infections (BSI)
Peter H Gilligan
Feb 8, 2008
All laboratory tests have three stages:
Preanalytical stage- The physician decides what clinical condition the patient might have based on signs, symptoms, and clinical history. S/he orders laboratory tests to attempt to make/confirm a specific diagnosis. For example, a patient with suspected endocarditis will require blood cultures be done to determine the infectious etiology and determine antimicrobial susceptibility of the infecting organism. The physician must understand what test to order so that the diagnosis of infection can be established.
Sensitivity and specificity of laboratory tests are greater influenced by pre-test probability of a positive so doing six blood cultures in someone with a vague history and limited signs and symptoms will result in laboratory tests with poor positive predictive values and a greater likelihood of infection.
Finally and most importantly, don’t order a test if you don’t know how to use the results that are generated.
Analytical stage- Discussion of this stage of the laboratory test revolves around the technical aspects of performing the specific laboratory test in question in this case, blood cultures. Although we will discuss how blood cultures are done briefly, we will discuss them only in the sense of needing to know how long it will take to generate a positive or negative result.
Post-analytical stage-This is the stage at which the physician is told of the result, either preliminary or final, and will be given assistance in determining the clinical significance of the result for the care of the patient. Two key points about blood cultures:
1. We consider blood cultures a critical value and call these results 24/7. We do use the fellows as a “court of last resort” to take responsibility for the results when we can not find someone who will take “ownership” of these critical values.
2. The most common question we get with blood cultures is, “Is this a contaminant or is this a true positive?” In our discussion today, we will address this issue in some detail.
Overview of the pre-analytical stage of blood cultures
1. We have the capability to use two different blood culture systems: a broth based system (BacTAlert) and a direct plating technique that uses lysis centrifugation (Isolator). Lysis centrifugation tends to have a higher contamination rate with more coagulase negative staphylococci recovered. These systems can be used for the isolation of most commonly encountered bacteria and fungi. Parasites (toxoplasmosis) and viremia (assorted viruses) are detected using PCR techniques and you will learn more about them later.
a. The default blood culture system is the broth system and can be used for most microbes encountered in blood.
i. Please tell us when you are concerned about Francisella because we will need to do special subcultures for it and need special precautions for handling isolates.
b. Lysis centrifugation is reserved for selected organism, usually intracellular, as follows:
i. Dimorphic fungi
ii. Malassezia furfur-must add olive oil to ensure growth; positive patients are typically on TPN
iii. Brucella sp.
iv. Salmonella typhi
v. Bartonella
vi. Legionella (probably never occurs)
c. Special broth medium and incubation conditions are used to isolate mycobacterium. These organisms should only be sought in patients with CD 4 counts of <100 cells/ ul.
d. Our blood culture systems do not have the capability to detect/recover:
i. Chlamydia, Coxiella, Rickettsia, Leptospira; for these organisms we recommend serology or PCR when available
ii. Aspergillus and Fusarium although they may occasionally grow from a blood culture
2. Collection of blood cultures.
a. Volume of blood is critical
i. Adults should have 20 ml of blood drawn from a site for one blood culture-the sensitivity of a blood culture is dependent upon the volume of blood cultured in large part because organism concentrations of <1 cfu/ml of blood are frequent in adults
b. We allow up to 4 blood cultures in a 48 period. In the modern blood culture era, that should detect 99%+ cases of true bacteremia with commonly encountered bacteremia.
c. Contamination of blood culture is a common problem (approximately 2% of our blood cultures grow contaminants). The use of chlorohexidine as a skin disinfectant is important measure to prevent contamination.
3. Things not to do with blood cultures.
a. Avoid blood cultures drawn from lines; in patient with poor access, always try to send one peripheral culture.
b. Culturing different “ports” from a multi-channel catheter is a waste of time and effort.
c. Low volume blood cultures are of limited value; avoid them.
d. Do not send single blood cultures on adult patients; do not send a high volume of blood from a single venipuncture that are then inoculated into multiple bottles and called two or three blood cultures. A blood culture means blood obtained from a unique venipuncture.
e. We lack enthusiasm for test of cure cultures; getting additional cultures in patients who you think is failing therapy is fine but how does one interpret “test of cure” blood cultures?
Analytical phase
Overview- We use continuously monitored blood culture for 95%+ of our blood cultures. These systems work by detecting the evolution of CO2 which occurs when micro-organisms are actively growing. Cultures are routinely held for 5 days. Cultures are not blindly subcultured prior to being reported as negative.
When the machine gives a positive signal, the blood bottle is removed, the bottle is gram stained, and depending upon the organism observed, the bottle is sub-cultured using specific media and incubation conditions.
Important fact about the detection of micro-organisms in blood cultures
1. Most micro-organisms will be detected in the first 48 hours of incubation.
2. All cultures are typically positive in patients with bacterial endocarditis so two or three are all that are needed.
3. HACEK organisms will be detected by five day incubation.
4. Extension of blood cultures incubation may be helpful for the isolation of Francisella and Brucella. Ten days are all that is needed but we will hold them for 21 days if you like (the yield after 10 days has been zero in my experience).
5. Yeast including dimorphics will grow in broth media but mycobacteria with the exception of rapidly growing mycobacteria will not.
6. We use both a aerobic and anaerobic medium but some labs use two aerobic bottles. Why? fungemia increasing while anaerobic bacteremia is declining.
7. Lysis centrifugation can be used for quantitating blood cultures and some labs use this to determine if a patient has line related sepsis. The idea in line related sepsis, the culture from the catheter will have 5 times the number of organisms compared to the number in the peripheral blood.
Post-anlaytical phase
Bacteremia can be:
Transient- Frequently occurs. Organism generally cleared rapidly. May occur after dental procedures, tooth flossing , bowel movements; any event that may disrupt the mucous membranes.
Intermittent- Occurs in patients that may have a sequestered site of infection such as an abscess. Requires multiple blood cultures to detect; Brucella, bacteremia secondary to abscess. osteomyelitis.
Continuous- Seen in patients with endovascular infections such as endocarditis. All blood cultures are positive.
True bacteremia rates at UNC are approximately 7%. Below 5%, you are doing to many blood cultures; above 12%, doing too few.
The major issue in interpreting blood cultures is determining whether an organism is a true isolate or a contaminant.
Organisms always considered clinically significant
Group A and B streptococci
HACEK
Haemophilus influenzae
Enterobactericeae
Streptococcus pneumoniae
Brucella and Francisella
Bacteroides
Bacillus anthracis
Neisseria meningitides and gonorrhoeae
Burkholderia spp
Cryptococcus neoformans
Dimorphic fungi
Organism usually but not always clinically significant (only one positive out of multiple cultures done may indicate contamination)
Staphylococcus aureus
Enterococcus spp.
Pseudomonas aeruginosa
Acinetobacter
Candida spp
Clostridium perfringens
Organisms usually considered contaminants but may cause bactermia (multiple positive cultures may suggest “true” bacteremia- particularly true in endovascular infections.)
Coagulase negative staphylococci
Viridans streptococci
Lactobacillus spp
Bacillus spp not anthracis
Corynebacterium spp
rapidly growing mycobacteria
saprophytic Neisseria spp.