Preventing False Positive Blood Cultures

1
Preventing False Positive Blood Cultures

• Lisa L. Steed, Ph.D., D(ABMM)
• Diagnostic Microbiology
• Wanda Beardsley, BSN, RN, ICP
• June 2008

2
Objectives

• After viewing the presentation the viewer will be
  able to
• Define false positive blood culture
• State the consequences of false positive blood
  cultures
• List proper techniques to prevent a false
  positive blood culture
• Describe importance of timing and amount in
  obtaining blood culture specimens
• Select the appropriate blood culture bottles

3
Blood culture definitions

• Blood culture or Blood culture set volume of
  blood obtained under aseptic conditions that is
  inoculated into one or more bottles of broth
  culture medium
• Positive culture one or more bottles of a blood
  culture set demonstrate(s) growth

4
What is the Problem?

• The problem is numbers of false positive blood
  cultures obtained at MUHA
• A false positive means that a specimen contains
  an organism that is from a source other than the
  patients blood i.e. skin organisms
• When this occurs a patient may be treated
  unnecessarily for the skin organism when there is
  NO bloodstream infection.

5
Occurrence of False Positive Blood Cultures
(Trash)
6
Question 1

• What is considered a false positive blood
  culture?
• a. Aerobic growth in specimen when anaerobic is
  expected
• b. The specimen is contaminated with an
  organism that is from a source other than the
  patients blood.
• c. Specimen positive for MRSA
• d. Specimen positive for VRE

7
Why should we care about properly collected blood
cultures?

• 1. Inaccurate reimbursement
• 2. Questionable contaminants require obtaining
  more blood cultures to determine true infection.
  This means more work for nursing/ phlebotomy.
• 3. Inappropriate antimicrobial therapy
• Unnecessary drug levels, drug administration
• May lead to C difficile associated diarrhea
  (isolation)

8
Timing is Crucial in Drawing Blood Culture Sets

• Blood culture sets must be drawn within two hours
  of each other.
• Two or more positive cultures growing an organism
  usually considered a contaminant will not be
  considered a blood stream infection if the
  cultures were collected more than 2 hrs apart
• Sensitivities will NOT be done unless otherwise
  specified by the physician.

9
Blood culture collection issues

• Volume
• THE MOST IMPORTANT variable in detecting
  bacteremia and fungemia
• Collect MAXIMUM volume for pt weight bottle
  type
• Timing (within 2 hours of the first set)
• Antiseptic site prep
• Site of collection
• Venipuncture (one set must be peripheral site)
• Line draws (avoid if possible)
• Site must be documented on lab requisition

10
How much blood to draw per set?
(ml)
11
Getting blood from a turnip

• If less than the recommended volume of blood is
  collected for culture, the blood should be
  inoculated into the AEROBIC bottle first,
  maximizing the volume, then into the anaerobic
  bottle
• Most bacteremias are caused by aerobic bacteria
• Yeast are aerobes and grow almost exclusively in
  aerobic bottles

12
Question 2

• To prevent false positives, sets of blood
  cultures must be drawn
• Within 3 hours of each other
• Within 4 hours of each other
• Within 5 hours of each other
• Within 2 hours of each other

13
Question 3

• The most important variable in detecting
• bacteremia and fungemia is the volume of blood
• collected
• T
• F

14
Question 4

• Which of the following are consequences of
• false positive blood cultures?
• a. Inaccurate reimbursement
• b. Repeat blood cultures to determine true
  infection.
• c. Inappropriate antimicrobial therapy
• d. All of the above

15
Antipseptic Preparation

• Venipuncture blood culture
• Disinfect the tops of the blood culture bottles
  by wiping with an alcohol pad prior to collection
  (do not use iodine/iodophor)
• Disinfect the site with ChloraPrep using
  friction for 30 seconds dry for 30 seconds
• Do not re-palpate the site to relocate vessel

Infection Control policy 3-002 Collection of
Blood Samples
16
Antiseptic Preparation

• Intravascular line blood culture
• Use SmartSite valve closest to insertion site
• ChloraPrep SmartSite valve using friction for
  30 seconds
• Allow to air dry for 30 seconds
• Collect blood
• Do not discard initial blood draw. This specimen
  should be used for culture.

17
Antiseptic Preparation

• Tops of blood culture bottles do not come sterile
  from the factory!
• Disinfect the tops of the blood culture bottles
  by wiping with an alcohol pad for 1-2 seconds
• Allow to air dry for one minute prior to
  collection
• Do not use iodine/iodophor or ChloraPrep as
  this may kill organisms from the patients blood

18
Why Let ChloraPrep and Alcohol Dry?

• Drying Time is Dying Time!
• Microorganisms are killed only when the
  antiseptic has dried.
• Hurrying is not an option!

19
Blood Culture Reminders

• Higher contamination from ports/lines
• Disinfect ports/lines with ChloraPrep
• ALWAYS SEND A VENIPUNCTURE blood culture when a
  line blood culture is sent
• 2 line draws is not equivalent to a
  venipuncture
• Why can we use the initial blood draw for blood
  cultures?
• Bacteria may be dwelling in the catheter
• Maximize the volume of blood in the bottles

20
Question 5

• Proper techniques for preventing false positive
  blood cultures when drawing from a venipuncture
• Prep skin with ChloraPrep for 30 seconds
• Allow ChloraPrep to dry for 30 seconds
• Prep top of culture bottle for 1-2 seconds with
  alcohol and allow to dry for 1 minute
• All of the above

21
Question 6

• Proper techniques for preventing false positive
  blood cultures when drawing from an IV line
  include
• Use SmartSite valve closest to insertion site
• ChloraPrep blue portion of SmartSite valve
  using friction for 30 seconds allow to air dry
  for 30 seconds
• Do not discard initial blood draw. This specimen
  should be used for culture.
• All of the above

22
Selecting Blood Culture Bottles

• Blue Standard Aerobic
• Purple Standard Anaerobic

23
Selecting Blood Culture Bottles

• FAN (Activated charcoal) bottles only to be used
  for special situations and must be ordered by
  practitioner
• Orange
• Green
• Yellow (pediatric)

24
Question 7

• A physician order is required to use a FAN
  culture bottle
• T
• F