Preparation of the Site

Because blood culture media have been developed as enrichment broths to encourage the multiplication of as few as a single organism, these media will enhance growth of contaminating organisms, including a normal inhabit ant of human skin. Therefore, careful skin preparation before collecting the blood sample is of paramount importance to reduce the risk of introducing contaminants into blood culture media.

The vein from which the blood is to be drawn must be chosen before the skin is disinfected. If a patient has an existing IV line, the blood should be drawn below the existing line; blood drawn above the line will be diluted with fluid being infused. It is less desirable to draw blood through a vascular shunt or catheter, because these prosthetic devices are difficult to decontaminate completely.

Antisepsis. Once a vein is selected, the skin site is defatted (fat removal) with 70% isopropyl alcohol and an antiseptic is applied to kill surface and subsurface bacteria. Regardless of the antiseptic used, it is critical to follow the manufacturer’s recommendation for the length of time the antiseptic is allowed to remain on the skin. Available data indicate that iodine tincture (iodine in alcohol) and chlorhexidine are equivalent for skin preparation before drawing blood cultures. The steps necessary for drawing blood for culture are given in Procedure 1, which can be found on the Evolve site.

PROCEDURE1.

As part of ongoing quality assurance, laboratories should determine the rate of blood culture contamination by clinically evaluating patients’ conditions in con junction with the organism isolated from culture. Laboratories that recover contaminants at rates greater than 3% should suspect improper phlebotomy techniques and should institute measures to educate the phlebotomists in proper skin preparation methods.

Precautions. Standard precautions require that phlebotomists wear gloves for blood drawing. Because blood for culture must be obtained aseptically, it is important that contaminated surfaces that might come in contact with the disinfected venipuncture site be disinfected. For example, if the site must be touched after preparation, the phlebotomist must disinfect the gloved fingers used for palpation. Also, if the rubber stopper or septum of the container into which blood is to be inoculated (e.g., test tubes or commercial culture bottles) is potentially contaminated, the phlebotomist must disinfect the septum.

Specimen Volume

 Adults. For many years, it has been recognized that most bacteremias in adults have a low number of colony forming units (CFU) per milliliter (mL) of blood. For example, in several studies, fewer than 30 CFU per mL of blood were commonly found in patients with clinically significant bacteremia. Therefore, a sufficient sample volume is critical for the successful detection of bacteremia.

There is a direct relationship between the volume of blood and an increased probability that the laboratory will isolate the infecting the organism. Therefore, collection of two sets of cultures using10 to 20 mL of blood per culture is strongly recommended for adults. To illustrate, Cockerill and colleagues reported that in patients without infective endocarditis, volumes of 20 mL increased the yield, identification of the organism, by 30% compared with 10-mL volumes. Unfortunately, a study confirmed that it is common practice to under inoculate blood culture bottles; findings from this study suggested that the yield increases by 3.2% for each milliliter of blood cultured.

Children. It is not safe to take large samples of blood from children, particularly infants. The optimal volume of blood required for successful identification of organ isms from infants and children has not been clearly delineated. Similar to adults, this patient population has low level (small numbers of organisms) bacteremia. In light of low-level bacteremia in infants and children and based on the premise that it is safe to obtain as much as 4% to 4.5% of a patient’s known total blood volume for culture and the relationship between blood volume and patient weight, Baron and colleagues have determined recommendations for blood volumes for cultures from infants and children (Table 1). For infants and small children, only 1 to 5 mL of blood should be drawn for bacterial culture. Blood culture bottles are available designed specifically for the pediatric patient. Because blood specimens from septic children may yield fewer than 5 CFU/mL of the organism, quantities less than 1 mL may not be adequate to detect pathogens. Nevertheless, smaller volumes should still be cultured because high levels of bacteremia (more than 1000 CFU/mL of blood) are detected in some infants.

Table1. Suggested Blood Volumes for Cultures from Infants and Children

Number of Blood Cultures

 Because periodicity of microorganisms in the blood stream may be characteristic for some diseases, continuous for some and random in others, patterns of bacteremia must be considered in establishing standards for the timing and number of blood cultures. If the volume of blood is adequate, usually two or three blood cultures are sufficient to achieve the optimum blood culture sensitivity. In patients with endocarditis who have not received antibiotics, a single blood culture is positive in 90% to 95% of the cases, whereas a second blood culture establishes the diagnosis in at least 98% of patients, depending on the study. For patients who have received prior antibiotic therapy, three separate blood collections of 16 to 20 mL each, and an additional blood culture or two taken on the second day, if necessary, detects most etiologic agents of endocarditis. This presumes use of a culture system adequate for growth of the organism involved, which often entails extending the incubation period. Similarly, for patients without infective endocarditis, 65.1% are detected in the first culture, 80% by the first two cultures, and 95.7% were detected in the first three blood cultures.

Timing of Collection

The timing of cultures is not as important as other factors in patients with intravascular infections because organ isms are released into the bloodstream at a fairly constant rate. Because the timing of intermittent bacteremia is unpredictable, it is generally accepted that two or three blood cultures be spaced an hour apart. However, a study found no significant difference in the yield between multiple blood cultures obtained simultaneously or those obtained at intervals. The authors concluded that the overall volume of blood cultured was more critical to increasing organism yield than timing.

When a patient’s condition requires therapy to be initiated as rapidly as possible, little time is available to collect multiple blood culture samples over a timed inter val. An acceptable compromise is to collect 40 mL of blood at one time, 20 mL from each of two separate venipuncture sites, using two separate needles and syringes before the patient is given antimicrobial therapy. Regardless, blood should be transported immediately to the laboratory and placed into the incubator or instrument as soon as possible. With blood culture instrumentation, a delay beyond 2 hours can delay the detection of positive cultures.

Miscellaneous Matters

 Anticoagulation. Blood drawn for culture must not be allowed to clot. If bacteria become entrapped within a clot, their presence may go undetected. Thus, blood drawn for culture may be either inoculated directly into the blood culture broth media or into a sterile blood collection tube containing an anticoagulant for transport to the laboratory for subsequent inoculation. Heparin, ethylenediaminetetraacetic acid (EDTA), and citrate inhibit numerous organisms and are not recommended for use. Sodium polyanethol sulfonate (SPS, Liquoid) in concentrations of 0.025% to 0.03% is the best anticoagulant available for blood cultures. As a result, the most commonly used preparation in blood culture media today is 0.025% to 0.05% SPS. In addition to its anticoagulant properties, SPS is also anticomplementary and antiphagocytic, and interferes with the activity of some antimicrobial agents, notably aminoglycosides. SPS, however, may inhibit the growth of a few microorganisms, such as some strains of Neisseria spp., Gardnerella vaginalis, Streptobacillus moniliformis, and all strains of Peptostreptococcus anaerobius. Because of the inhibitory effect of SPS on some organisms in conjunction with the necessity for an additional step to transfer the blood to the ultimate culture bottles that increases the risk of exposure to blood-borne pathogens as well as contamination, using collection tubes instead of direct inoculation into culture bottles may compromise organism recovery. For these reasons, the use of intermediate collection tubes is discouraged. Although the addition of 1.2% gelatin has been shown to counteract this inhibitory action of SPS, the recovery of other organisms decreases.

Dilution. In addition to the volume of blood collected and type of medium chosen, the dilution factor for the blood in the medium must be considered. To conserve space and materials, it is desirable to combine the largest feasible amount of blood from the patient (usually 10 mL) with the smallest amount of medium that will still encourage the growth of bacteria and dilute out or inactivate the antibacterial components of the blood. Traditionally, a 1 : 10 ratio of blood to medium was required for successful bacterial growth; however, several new commercial media containing resins or other additives have demonstrated enhanced recover with as low as a 1 : 5 ratio. For this purpose, a 1 : 5 ratio of blood to unmodified medium has been found to be adequate in conventional blood cultures. All commercial blood culture systems specify the appropriate dilution.

Blood Culture Media. The diversity of bacteria recovered from blood requires an equally diverse and large number of media to enhance the growth of these bacteria. Basic blood culture media contain a nutrient broth and an anticoagulant. Several different broth formulations are commercially available. Most blood culture bottles available commercially contain trypticase soy broth, brain-heart infusion broth, supplemented peptone, or thioglycolate broth. More specialized broth bases include Columbia or Brucella broth.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم الشؤون الفكرية يصدر كتابًا توثيقيًّا عن جرائم الإرهاب بحق زائري الأربعين في العراق

في مقابلة خاصة مع المشرف على قسم الشؤون الطبية: مركز السيد جعفر الحلي يعزز الاستجابة الطبية الطارئة ولا سيّما في الزيارات المليونية

لمنتسبي العتبة العباسية المقدسة.. قسم الشؤون الدينية يجري الاختبارات العملية للمشاركين في الدورة التطويرية الثانية

المجمع العلمي يقيم الدورة الــ 20 في تعليم أحكام التلاوة لطلبة العلوم الدينية

المزيد

الآخبار الصحية

العلاج بالماء البارد.. باحثون يكشفون فوائده الصحية

من دون صالة رياضية.. عادة بسيطة لتحسين الصحة وخسارة الوزن

لصحة القلب وتفادي السرطان.. استهلك هذا النوع من الطعام

دراسة تحذر من خطر شرب القهوة في أكواب البلاستيك

المزيد

الآخبار التكنلوجية

إنجاز طبي تاريخي.. جراحة قلب بلا شق في الصدر

اليابان.. مراحيض توفر فحص للحالة الصحية عبر "تحليل البراز" !

اختراق واسع لمنصة "إنستغرام".. ونشر بيانات على "الدارك ويب"

علماء يحذرون: هكذا سيفنى كوكب الأرض

المزيد

مواضيع ذات صلة

Detection of Bacteremia: Types of Blood Culture Bottle

The Study of the Intestinal Microbiota

Chlamydia psittaci and Psittacosis

Chlamydia Pneumonia and Respiratory Infections

Chlamydia trachomatis Genital Infections and Inclusion Conjunctivitis

Trachoma

Coxiella burnetii

Ehrlichia and Anaplasma

Rickettsia and Orientia

Cultivation of Mycoplasma and Ureaplasma

Laboratory Diagnosis of Mycoplasma and Ureaplasma

Epidemiology and Pathogenesis of Mycoplasma and Ureaplasma