DRAWING A BLOOD CULTURE specimen correctly is important because poor technique could lead to additional discomfort for the patient, additional testing, longer hospital stays, and associated expenses.1 In addition, if the specimen is contaminated, the resulting culture may be falsely positive, usually for organisms commonly found on the skin. The patient may then receive unnecessary antibiotics to treat organisms that aren't causing disease. Unnecessary antibiotics put the patient at greater risk for acquiring a multidrug-resistant organism and acquiring Clostridium difficile.
Rates of blood culture contamination vary widely, with a range 0.6% to 9.1%, in part because standards for defining contamination vary.2,3 Most facilities consider a blood specimen contaminated if the isolates classified as contaminants were coagulase-negative Staphylococci, viridans group and nonhemolytic Streptococci, Corynebacteria, Micrococci, and nonpathogenic Bacillus and Neisseria species.4
Rates of blood specimen contamination vary with the experience of the clinician, including how often the clinician draws blood specimens. How much time the clinician has to accomplish the task is another factor: The less time available, the higher the contamination rate.3
The steps for drawing a blood culture specimen outlined in this article are general guidelines; become familiar with your facility's practices and follow them.
Step 1: Check the timing
Unless otherwise specified by a healthcare provider, blood culture specimens can be collected over a short time frame according to your facility's policy (5 minutes apart, 10 minutes apart). If possible, avoid drawing a culture specimen from a patient taking an antibiotic—draw the specimen before the patient starts it. If drawing the specimen afterward is unavoidable, some automated blood culture lab testing systems provide special vials that neutralize the effects of the antibiotic.
Collect no more than two to three sets of blood specimens over a 24-hour period (more sets may be drawn in special circumstances, such as endocarditis). Never draw single blood specimens from adult patients because doing so would reduce the chances of recovery of a bloodstream infection by limiting the amount of blood cultured.
Collect blood culture specimens separately from specimens drawn for other lab blood work. If this isn't practical due to the patient's condition, additional blood work can be obtained from this site after the blood culture specimens have been collected.
Step 2: Select the site
Perform the initial blood collection by peripheral venipuncture, not through an I.V. catheter, to prevent possible contamination. Draw blood from two different peripheral venipuncture sites if possible and document each site. This increases the likelihood of detecting a bloodstream infection and helps differentiate true bacteremia from a false positive due to skin contamination.
The second specimen may be drawn from a central venous catheter if catheter-related bloodstream infection (CRBSI) is suspected or peripheral access sites are unavailable. If you obtain a blood culture specimen from an intravascular catheter, clearly document it in the patient's medical record. Peripheral sites are preferred. For pediatric patients, the decision to obtain a sample from a second site is at the discretion of the healthcare provider.
When patients have a central venous access device, especially in critical care units, clinicians may be tempted to draw blood culture specimens from the central venous catheter upon insertion. Drawing blood culture specimens upon line insertion can result in more false-positive results and should be performed on a separate venous draw from a different site. In some cases, specimens may be obtained using dialysis vascular accesses during dialysis.
Step 3: Prepare the site
Cleansing the site with alcohol followed by povidone-iodine was once the standard for preparing the site for venipuncture. Studies have shown, however, that the use of chlorhexidine-alcohol is associated with significantly lower blood culture contamination rates in both adult and pediatric patients (but not the youngest neonates).5–7 An advantage of chlorhexidine is its shorter drying time than povidone-iodine, which may increase the likelihood that the skin has been thoroughly decontaminated when the draw is done.8
Apply the chlorhexidine-alcohol and let the area completely dry to allow full bactericidal activity on the skin. Never repalpate after you prep the site; this increases the risk of specimen contamination.
Step 4: Determine the volume
Drawing a sufficient volume of blood is probably the most significant factor in the detection of a bloodstream infection. The maximum volume of blood collected (per vial manufacturer requirements) not only increases the likelihood of recovering an organism but also decreases the time to recovery. Most blood culture systems require 10 mL of blood per vial, but each manufacturer has several vials to choose from and they may differ in volume. There must be a balance between volume of blood collected and the clinical condition of the patient.
If the vials aren't already marked, mark the appropriate volume on the vial before collection. For adults, mark the recommended maximum volume printed on the vial above the fluid level in the vial.
For most common automated systems, which perform automatic readings on the vials and alarm when a possible positive culture has been detected, remove the cap and disinfect the septum of the blood culture vial with an alcohol swab and allow it to dry. Don't use povidone-iodine or alcoholic chlorhexidine because they may damage the septum.9
Step 5: Obtain the specimen
The preferred method of obtaining blood culture specimens in adults is with a butterfly needle with an attached adapter to directly fill the vials.10 Using a syringe isn't recommended because of the increased risk to healthcare providers during the manipulation required to transfer the blood from syringe to the vials. Syringe use has also been linked to such problems as blood culture contamination. For safety reasons, never change the needle before accessing the blood culture vial.
Collect blood culture specimens starting with the aerobic vial, filling it to the maximum recommended volume. Place the remaining blood into the anaerobic vial up to the maximum recommended collection volume.
Collect blood from the second site in the same manner, starting with the aerobic vial followed by the anaerobic vial because most of the organisms isolated from positive blood cultures are aerobic.
After collection, mix the vials thoroughly by gentle inversion. Send the vials to the lab within 2 hours. Delays in the entering of vials into an automated blood culture instrument can delay the results.
Following these steps and complying with your facility's policies and procedures will help you collect a specimen for blood culture properly. Getting it right the first time minimizes the chances of drawing a contaminated specimen that could result in unnecessary complications for your patient.
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4. Centers for Disease Control and Prevention. Device-associated module: central line-associated bloodstream infection (CLABSI) event
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9. BacT/Alert Blood Culture Collection manufacturer recommendations package insert. Biomerieux 2008.
10. Ernst DJ, Ballance JO, Calam RR, et al. Procedures for the Collection of Diagnostic Blood Specimens for Venipuncture; Approved Standard
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