Blood: culture

• If blood is collected by a sterile technique and bacteria are subsequently cultured from the sample → bacteremia.

• Diagnosis of bacteremia.
• Investigation of fever - to rule out bacteremia.
• Investigation of discospondylitis Diskospondylitis , to try to identify etiological agent.
• Investigation of suspected endocarditis Endocarditis: bacterial.
• In combination with intravenous catheter culture.

Advantages

• Simple technique.
• Relatively non-invasive.

Disadvantages

• Bacteria may not be constantly present in blood but shed into bloodstream on an intermittent basis.
• Administration of antibiotics prior to sampling can result in false negative results.
• May take up to two weeks to get a result on culture and sensitivity testing.
• Positive results may represent contamination.

• Non-sterile collection technique can result in false positive results due to growth of contaminant.

• Cells are lysed and then sample centrifuged.
• Pellet is innoculated onto culture medium.

Preparation

• 10 mins to prepare patient.

Procedure

• Rapid sample collection and processing time.

Criteria for choosing test

• Usually recommended that samples collected during rising phase, (if temperature fluctuating).
• Preferably withdraw antibiotics for 3-5 days before sampling.

Minimum consumables

• 10-20 ml syringe (depending on size of dog).
• Sterile needle.
• Bottle of blood culture medium.

Ideal consumables

• Sterile gloves.

• None needed.

• Ideally 3-4 hour fast before procedure.

• A 10 cm area over the jugular vein is clipped and surgically prepared.

• Minimal restraint as for normal jugular blood sampling Jugular venipuncture.

Step 1 - Collect blood sample

• Routine collection of jugular blood sample Jugular venipuncture , by operator wearing sterile gloves and under aseptic conditions.
• Collect 10-20 ml of whole blood.
  Larger volumes of blood are more likely to produce positive culture results.
• Swab top of collection bottle and change needle used for blood collection for sterile needle.
• Inject volume of blood indicated on bottle (9 volumes of medium to 1 volume of blood), through bottle bung and then cap with sterile top.
  Sampling should be repeated on at least 2 occasions in a 24 hour period, and preferably from different venepuncture sites.

Step 2 - Culturing blood

• The sample bottle is immediately incubated at 35°C for 24 hours, after which time a sample is withdrawn and innoculated onto blood agar media for both anaerobic and aerobic culture.
• This task is repeated daily for at least 1 week.
• Culture plates are also incubated at 35°C and analysed daily for signs of growth.
  Most clinically relevant fungi will also grow under these conditions.
• If bacterial colonies are identified they are subcultured onto plates with antibiotic sensitivity disks.

Antimicrobial therapy

• The most common bacteria isolated from canine blood culture are:
  • Staphylococcus aureus Staphylococcus aureus.
  • Escherichia coli Escherichia coli.
  • Beta hemolytic streptococci Streptococcus spp.
• In the absence of positive blood culture results, antibiotic selection should be based on this information.
  A combination of a cephalosporin Cefalexin , with gentamicin Gentamicin , is likely to be effective in the absence of laboratory results. This combination is expensive and gentamicin is potentially nephrotoxic so the regime may be altered in the light of sensitivity results.

• Positive blood cultures are difficult to obtain:
  • Administration of antibiotics may sufficiently suppress bacterial growth to prevent replication in sub-ideal conditions, ie outside the body but not sufficiently to cure infection.
  • Careless handling of sample, ie chilling may reduce bacterial viability.
  
  
  False positives
• Contamination is likely when collecting a sample from an animal with pyoderma.
  Try to sample through an area of non-infected skin.
• Culture of the same organisms from at least 2 samples taken at different times (and preferably from different sites), in the same animal usually helps to define a positive culture.

Refereed papers

• Recent references from PubMed and VetMedResource.
• Calvert C A & Greene C E (1986) Bacteremia in dogs; diagnosis, treatment and prognosis. Cont Ed Pract Vet 8 (3), 179-186 VetMedResource.
• Hirsch D C, Jang S S & Biberstein E L (1984) Blood culture of the canine patient. JAVMA 184 (2), 175-178 PubMed.
• Washington J A 2nd (1975) Blood cultures, principles and techniques. Mayo Clinical Proceedings 50 (2), 91-98 PubMed.