PatientPlus articles are written by UK doctors and are based on research evidence, UK and European Guidelines. They are designed for health professionals to use, so you may find the language more technical than the condition leaflets.
Any patient in a hospital setting who develops a fever or evidence of sepsis should have blood cultures sent. Multiple sets may be required and cultures of other sites should also be considered, eg urine, skin and throat.
How to take blood cultures
- Aseptic techniques should be used so that bacteria from the skin do not contaminate the culture bottles.
- Ideally the skin should be washed with soap and rinsed with sterile water. This should be followed by the application of iodine-based solution, which should then be washed off after 30 seconds of drying, with 70% alcohol solution. Practically, this rarely occurs; however, every effort to use aseptic technique must be made, eg Betadine® and/or alcohol spray and gloves. Some studies suggest contamination rates may be dependent on the type antiseptic solution employed.
- The most common contaminants include Staphylococcus epidermidis, Corynebacterium spp., Propionibacterium spp.and Bacillus spp. (but not B. anthracis).
- At least 20 mL of blood obtained by venepuncture is needed (but this may vary with the specific type of blood culture bottles employed). In children, age-dependent smaller volumes are considered adequate (eg 0.5-1 mL in an infant, 3 mL in a young child and ≥6 mL in an older child) although, as with adults, larger sample volumes are associated with better detection of pathogens.
- Standard blood culture bottles consist of two bottles - one containing aerobic and the other anaerobic culture media.
- At least 10-15 mL should be injected into blood culture bottles (but again this depends on bottles used).
- Some centres require a new needle to be placed on the syringe before transferring blood into culture bottles. However, this increases the risk of needlestick injuries and does not reduce contamination rates and thus it is not recommended. Newer vacuum devices allow direct venepuncture with transfer of blood into blood cultures.
- In some hospitals, phlebotomists will take blood cultures. Although the rates of contamination may be lower, there are practical issues relating to availability of phlebotomists.
- Contamination results may be lower if taken by more experienced clinicians than junior staff.
- Multiple cultures from multiple sites increase the yield:
- This is especially so in infective endocarditis.
- One example is to take blood from one antecubital fossa, 30 minutes later from the opposite antecubital fossa and the last one from a third site 30 minutes later.
- Blood cultures can also be taken from lines, eg central venous pressure lines, arterial lines, and may have lower contamination rates than simple venepuncture, but introduce the complication of line colonisation.  The same rules apply, especially regarding attention to asepsis.
- True and false positive rates for blood cultures vary but are commonly each in the range 5-10%. This means that when a positive result is returned, there is only approximately a 30-60% chance that the result is a true bacteraemia. In one study the rates were 7% and 8% respectively. Factors associated with true bacteraemia were temperature over 38.3°C, presence of a fatal disease, rigors, intravenous drug misuse, acute abdomen and the presence of major comorbidities. In another study in a paediatric population, the chances of a positive blood culture predicting a true-pathogen was 0.366 with inexperienced doctors and 0.523 with experienced doctors.
- True positive results also depend on the type of infection. For example, in the presence of pneumonia, blood cultures may not be helpful with a low level of positive results.
Types of culture bottles
- There are a variety of different types of blood culture systems, both manual and automated. (See 'Internet and further reading' section, below, for more detail).
- Other culture bottles are available and their use will depend upon the clinical scenario, eg culture bottles for tuberculosis or fungi.
Blood culture processing
- Once blood cultures are taken they should be labelled and sent to the laboratory without delay.
- In the laboratory the bottles are agitated and incubated at body temperature. If it is out of hours then cultures are usually incubated overnight.
- Basic sets of cultures are incubated for 14 days and blind culture performed after 3, 7 and 14 days or as soon as there are signs of growth (eg turbidity, haemolysis, colonies on agar slope in Castaneda's bottles).
- Other bottles may be incubated for 7 days or up to 3 weeks if SBE/fastidious organisms are suspected.
- Observation of bottles, looking for a positive result, is performed at least twice a day when using manual systems.
- Automatic systems are available and are being increasingly used. One example is the BacT/Alert Blood Culture System® where a positive growth releases CO2 which is detected by a sensor that alerts the laboratory staff (bottles are placed in a special cabinet and linked to the patient by a barcode).
- If growth is detected, the bottles are subcultured and a Gram stain performed. From this, relevant sensitivities are performed. In some cases the organism can be identified within hours of detecting a positive result, using Gram stain and further tests.
- Further tests that may be performed directly on the blood culture to hasten identification include streptococcal grouping, coagulase testing, antigen tests for pneumococcus and meningococcus, etc. Modern methods such as Vitek® have helped to speed up identification of organisms and can be performed directly from culture broths.
- The microbiologist will then review the results and inform the staff involved in the patient's care. Thus, patients will begin provisional therapy and this will be confirmed once sensitivities are known.
- Usually, if there is to be a growth, there is generally only one organism; however, very rarely, there may be more than one organism and the laboratory will perform sensitivities and further tests on all.
Further reading & references
- Longmore M, Wilkinson IB and Rajagopalan SR; Oxford Handbook of Clinical Medicine, 6th ed, 2004
- Koneman's Color Atlas and Textbook of Diagnostic Microbiology; Lippincott Williams & Wilkins, 2006
- Suwanpimolkul G, Pongkumpai M, Suankratay C; A randomized trial of 2% chlorhexidine tincture compared with 10% aqueous J Infect. 2008 May;56(5):354-9. Epub 2008 Apr 14.
- Little JR, Murray PR, Traynor PS, et al; A randomized trial of povidone-iodine compared with iodine tincture for Am J Med. 1999 Aug;107(2):119-25.
- Gonsalves WI, Cornish N, Moore M, et al; Effects of volume and site of blood draw on blood culture results. J Clin Microbiol. 2009 Nov;47(11):3482-5. Epub 2009 Sep 30.
- Connell TG, Rele M, Cowley D, et al; How reliable is a negative blood culture result? Volume of blood submitted for Pediatrics. 2007 May;119(5):891-6.
- Kumar P, Clarke M; Clinical Medicine, 6th Ed, (2005), WB Saunders
- Krumholz HM, Cummings S, York M; Blood culture phlebotomy: switching needles does not prevent contamination. Ann Intern Med. 1990 Aug 15;113(4):290-2.
- Surdulescu S, Utamsingh D, Shekar R; Phlebotomy teams reduce blood-culture contamination rate and save money. Clin Perform Qual Health Care. 1998 Apr-Jun;6(2):60-2.
- Pavlovsky M, Press J, Peled N, et al; Blood culture contamination in pediatric patients: young children and young Pediatr Infect Dis J. 2006 Jul;25(7):611-4.
- Hall KK, Lyman JA; Updated review of blood culture contamination. Clin Microbiol Rev. 2006 Oct;19(4):788-802.
- Bates DW, Cook EF, Goldman L, et al; Predicting bacteremia in hospitalized patients. A prospectively validated model. Ann Intern Med. 1990 Oct 1;113(7):495-500.
- Shah SS, Dugan MH, Bell LM, et al; Blood Cultures in the Emergency Department Evaluation of Childhood Pneumonia. Pediatr Infect Dis J. 2011 Jan 4.
- Erdede M, Denizbasi A, Onur O, et al; Do we really need blood cultures in treating patients with community-acquired Bratisl Lek Listy. 2010;111(5):286-9.
- Afshar N, Tabas J, Afshar K, et al; Blood cultures for community-acquired pneumonia: are they worthy of two quality J Hosp Med. 2009 Feb;4(2):112-23.
- Provan, D; Oxford Handbook of Clinical and Laboratory Investigation, 3rd Ed, 2010, Oxford University Press
Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.
Dr Gurvinder Rull
Dr Gurvinder Rull