Taking blood cultures

Introduction

Introduction

The taking of blood cultures to detect bacteraemia is an important medical investigation affecting the diagnosis and treatment of our patients. Their value will be maximised by taking the cultures for the appropriate indications and with the correct technique in order to reduce the risk of contamination.

Indications for taking a blood culture

Blood cultures are taken to identify patients with bacteraemia. There are many clinical signs and symptoms which may suggest bacteraemia and clinical judgement is required, but the following indicators should be taken into account when assessing a patient for signs of bacteraemia or sepsis:

• Core temperature out of normal range (there is no absolute temperature below which blood cultures are not required)
• Focal signs of infection
• Signs of sepsis
• Chills or rigors
• Raised or very low peripheral blood white cell count
• New or worsening confusion

NB. Signs of sepsis may be minimal or absent in the very young and the elderly.

Blood cultures should be taken before starting antibiotics. If the patient is already on antibiotics the cultures should be taken immediately before the next dose.

Blood cultures should not routinely be performed to ensure that a previously positive culture is now negative.  Do not take blood cultures in instances where the patient is for palliative care only and where further investigations have been deemed inappropriate.

How many bottles to use

In patients with suspected bacteraemia it is generally recommended that two cultures are taken at separate times from separate sites. The total volume of blood is important and ideally 10-20 mL of blood should be cultured. Take three separate sets over a 24 hour period in suspected endocarditis and PUO. 

Which bottle types?

The Trust uses four types of bottle – aerobic, anaerobic, mycobacterial and paediatric.

The following table lists the available bottles, the colours of their tops/labels, the volume of blood required and the common indications for their use.

Where are the bottles stored?

Aerobic bottles are stored in all wards in addition to A&E and AMU and are regularly topped up by Pathology Stores at RSCH and by the phlebotomists at PRH.

Anaerobic bottles are stored in Pathology Stores at RSCH and with the phlebotomists at PRH and in the following locations: 

Mycobacterial bottles are stored in the Microbiology Laboratory at RSCH. There are usually small stocks on Howard 2 ward and in the Lawson Unit.

Paediatric bottles are stored on the paediatric wards at RAH, L14 at RSCH and SCBU at PRH. These locations order stocks from Pathology Stores at RSCH or are topped up by PRH phlebotomists as required.

Managers of areas with stocks of blood culture bottles and those performing the stocking up are responsible for ensuring that stocks are within their expiry dates.

Contact details

• Pathology Stores at RSCH on ext. 64407 between 8.30am and 4.30pm Mon-Fri
• Microbiology Laboratories - RSCH ext. 64619, PRH ext. 68228.
• PRH phlebotomists ext. 68320 or 68321

When can blood culture bottles be used for fluids other than blood?

Peritoneal dialysis fluids should be inoculated into both an aerobic and an anaerobic bottle, in addition to a plain gold-top bottle for cell counts.

Other body fluids should be sent in a sterile white topped universal tube. Do not put fluids in blood culture bottles; doing so allows contaminants to overgrow and does not allow the laboratory to perform microscopy for cell counts and Gram’s stain.

Can I take blood cultures from lines?

Do not use existing peripheral lines/cannulae or sites immediately above peripheral lines. If central line sepsis is suspected two separate peripheral cultures taken via fresh venous stabs are preferred, although one fresh peripheral venous stab and one central culture are acceptable. Femoral stabs have a high contamination rate and must only be used when there is no alternative.

Procedure for taking a blood culture

For more details see the Trust’s Venepuncture in Adults Policy C063.

Kit preparation

Remove the flip-top of the bottle, disinfect the rubber septum with 2% chlorhexidine gluconate/70% isopropyl alcohol and allow to dry. Do not touch the septum again.

Skin preparation

1. Wash your hands with soap and water then dry.
2. Clean any visibly soiled skin on the patient with soap and water then dry.
3. Apply tourniquet if required and palpate to identify a suitable vein.
4. Clean the patient’s skin with 2% chlorhexidine gluconate/70% isopropyl alcohol and allow to dry.
5. If the culture is being collected from a central venous catheter, disinfect the access port with 2% chlorhexidine gluconate/70% isopropyl alcohol.
6. Do not re-palpate the vein once the patient’s skin has been disinfected, even when wearing gloves.
7. Don non-sterile gloves.

Sample collection

Method A is better and safer than using a syringe and needle (Method B). There is less chance of contaminating the blood and less risk of a needlestick injury.

Method A: winged needle (butterfly)

1. Insert needle into prepared site.
2. Push and hold vacutainer holder over top of the blood collection bottle.
3. Hold bottle upright and use bottle graduation lines to gauge accurately sample volume and collect sample.
4. If blood is being collected for other tests, always collect the blood culture first.
5. Only apply pressure for venestasis after the needle has been removed from the patient.
6. Cover the puncture site with an appropriate dressing.
7. Discard needle into a sharps container.
8. Wash hands after removing gloves.

NB. The vacuum in the bottle exceeds 10ml. Do not overfill.

Method B: needle and syringe

1. Take sample with needle and syringe.
2. Do not change the needle between sample collection and the inoculation of the bottles (risk of needlestick injury outweighs slightly increased risk of contamination).
3. Inoculate the blood culture bottles before inoculating other blood bottles (bottles for haematology and chemistry are not sterile so there is a risk of contaminating the blood culture bottle if done in the wrong order).
4. Discard needle and syringe into a sharps container.
5. Wash hands after removing gloves.

Documentation and labelling

Record the procedure with indication for culture, time, site of venepuncture and any complication in the patient’s notes.

Label each blood culture bottle with appropriate patient information.

Do not remove barcodes from the bottles, these are for laboratory use. Do not cover the barcodes with any additional labels.

Complete a microbiology request form including indication for culture, underlying diagnosis, time and date of culture, site of venepuncture or lines used.

Transportation to the laboratory

Place the bottle in the bag of a microbiology form to a maximum of two bottles. If two bottles are taken from different sites they can be placed in one bag but make sure they are labelled to distinguish them from each other.

Send the bottles to the microbiology laboratory by the next routine transport. Do not call the laboratory to process the samples urgently.

Do not refrigerate or incubate the bottles; leave them at room temperature until they are collected.

Do not use the pod system for blood culture bottles.

Blood culture processing and results

The laboratory incubates all blood cultures for a minimum of 5 days.

A preliminary negative report is issued at 48 hours (36 hours for neonates).

Mycobacterial cultures are incubated for 6 weeks.

The medical microbiologists will contact the relevant clinican in the event of any blood culture growing a significant organism; there is no need to telephone the laboratory to chase results. 

References

Taking blood cultures: A summary of best practice. Saving Lives, Department of Health. 2007

Weinstein, MP.  Current blood culture methods and systems: Clinical concepts, technology, and interpretation of results. Clin Inf Dis 1996;23:40-6.

Mylotte, JM and Tayara, A. Blood cultures: clinical aspects and controversies. Eur J Clin Microbiol Infect Dis.  2000;19:157-63.