Overview #
Determining the appropriate course of action upon learning of a positive blood culture result is a common, yet often challenging JMO cover job. It is important to assess the clinical significance of the result and ensure the patient is on appropriate antimicrobials to cover the likely organism if significant. Have a low threshold to discuss with infectious diseases if any concerns.
Triage #
Attend within 30 minutes
Causes #
Most positive blood cultures represent a true infection, however contamination should be considered in cases of atypical organisms isolated in only a single blood culture bottle in a clinically well patient with no other evidence of infection.
Consider following factors in assessing clinical significance of positive blood culture [1]
- Clinical likelihood of bacteraemia
- Indication for taking blood culture initially
- {{ Each indication has varying likelihood of bacteraemia [1]
- High risk (>50%)
- Discitis, vertebral osteomyelitis or epidural abscess
- Septic shock
- Native septic joint
- Catheter related bloodstream infections
- Moderate risk (20-50%)
- Pyelonephritis
- Cholangitis
- Severe sepsis
- Shaking chills in febrile patients
- Low risk (<20%)
- Cellulitis
- Community acquired pneumonia
- Lower urinary tract infection
- Isolated fever
- Fever within first 48hrs after surgery }}
- High risk (>50%)
- {{ Each indication has varying likelihood of bacteraemia [1]
- Evidence of haemodynamic instability
- Elevated CRP or WCC
- Indication for taking blood culture initially
- Organism isolated {{call out box to table 1 – see below}}
- Number of positive blood cultures
- If both anaerobic and aerobic bottles positive OR multiple cultures (taken from different sites) are positive with same organism, more likely to be clinically significant [2]
- Time to positivity
- If becomes positive >72hrs after collection, consider contamination
{{Table 1
Significance | Organisms [1] |
Always true infection | Staphylococcus aureusStreptococcus pneumoniaeGroup A streptococcusStrep pyogenesEnterobacteriaceaeKlebsiellaEnterobacterEscherichia coliSalmonellaCitrobacterShigellaProteusSerratiaHaemophilus influenzaePseudomonas aeruginosaBacteroidaceaeCandida species |
May be true infection | Enterococci (70% true pathogen)E faecalisE faeciumViridans streptococci (38% true pathogen)S mutansS anginosus |
Usually contaminants | Coagulase negative staphylococciS epidermidisCorynebacterium speciesPropionibacterium acnesBacillus speciesMicrococcus species }} |
Clinical Features #
Thorough clinical assessment covering the following:
- Original source of bacteraemia
- Refer to the table here {{Call out box to table 2 – see below}} to determine if the organism isolated in blood culture is typical of the likely infective source gained from Hx and Ex
- Evidence of spread of infection (i.e. infective endocarditis)
- Indwelling lines and other foreign material
- Allergies, especially to antibiotics
- Previous microbiological data, including organisms and resistance patterns
- Assess haemodynamic stability and for signs of sepsis
Table 2
Source of infection | Common organisms [3] |
Respiratory | Strep pneumoniaeH influenzaeStaph aureusEnterobacteriaceae (E Coli, Klebsiella) |
Skin: cellulitis, abscess | Strep pyogenesStaph aureus |
Urinary | E ColiKlebsiellaStaph saprophyticusProteusEnterococcus |
Intra-abdominal | Often polymicrobialE ColiKlebsiellaEnterobacterEnterococcusAnaerobes (bacteroides, clostridium) |
Classification #
It is not uncommon to be phoned with preliminary results of positive blood cultures which only include information of gram stain and morphology (cocci/bacilli). Refer to the table below {{call out box to table 3 – see below}} to help determine possible organisms that can be attributed to each category.
Table 3
Gram positive [4]
Cocci in clusters: staphylococcus | Coagulase positiveStaph aureusCoagulase negativeStaph epidermisStaph saprophyticus |
Cocci in chains: streptococcus or enterococcus | Alpha-haemolyticStrep pneumoniaeViridans streptococciBeta-haemolyticStrep pyogenesStrep agalactiaeNo haemolysisEnterococci |
Bacilli | ClostridioidesBacillusCorynebacteriumPropionibacteria |
Gram negative [4]
Cocci | N meningitidisN gonorrhoeae |
Coccoid | H influenzaePasteurellaBrucellaPertussis |
Bacilli | KlebsiellaE ColiEnterobacterCitrobacterSerratiaPseudomonasShigellaSalmonellaProteus |
Investigations #
Initial investigations
Ensure recent
- Baseline bloods and infective markers (i.e. FBE, UEC, CRP)
- Septic screen to confirm source of infection (i.e. CXR, cultures of urine/faeces/sputum/wound as appropriate)
Further investigations
Investigations | Indication |
Repeat blood cultures [1,5] | Repeat within 24-48hrs of commencing anti-infectives if:Organism isolatedStaph aureusStrep lugdenensisMulti-drug resistant organismCandidaInfection sourceEndovascular infectionInfective endocarditisPacemaker/defibrillator infectionIntravascular catheter infectionSite with limited antibiotic penetration (i.e. abscess, joint)Central nervous systemIntra-abdominalUnknown sourceRisk factorsProsthetic valve, intravascular graft, pacemakerIntravascular catheter Repeat if ongoing signs of infection >72hrs after commencing anti-infectives |
Echocardiogram [5] | Staph aureus bacteraemiaCandidaemiaProsthetic valve, intra-cardiac or intra-vascular devicesClinical concern re: infective endocarditis (Duke’s criteria) |
Catheter tip culture | If suspicion of catheter infection as source of bacteraemia (note: results must be interpreted in conjunction with BC results) |
Referrals
Infectious Diseases | Best to refer to ID in most casesAlways refer if:Staph aureus or candida bacteraemiaMultidrug resistant organismAny other concerns |
Ophthalmology | If candidaemia or K. pneumoniae bacteraemia, should perform a dilated fundoscopic examination to examine for endophthalmitis (ideally within a week of diagnosis) [6] |
Management – Overview #
As a JMO, do not alter an antibiotic regimen from blood culture results without discussing with the unit registrar or infectious diseases.
Key Steps
- Assess haemodynamic stability
- If evidence of sepsis, treat empirically as per septic pathways and likely source
- Notify unit registrar urgently
- If stable, determine if current antibiotics cover the likely organism grown in blood cultures
- Refer to table here {link to:https://drug.wellingtonicu.com/Appendices/5/ OR include image 1 below} or similar
- If likely requires change in antibiotic therapy, discuss with unit registrar or infectious diseases before implementing any changes
- Await susceptibilities
- If on a cover rotation, ensure this is handed over to treating team
- Ensure unit registrar is aware of the positive result
Antibiotic coverage chart:: https://drug.wellingtonicu.com/Appendices/5/
Management – Staph Aureus [6] #
- Always discuss with ID as evidence this improves patient outcomes
- Echocardiogram to rule out infective endocarditis (most sensitive at day 5-7)
- Repeat blood cultures until several negative to ensure resolution of bacteraemia
Suspected S aureus (gram positive cocci in clusters) AND awaiting susceptibility
Flucloxacillin 2g IV 6hrlyAND Vancomycin 20-30mg/kg IV loading doseIf non-severe penicillin allergy: Cefazolin 2g IV 8hrly instead of flucloxacillin |
MSSA
Flucloxacillin 2g IV 6hrlyIf non-severe penicillin allergy: Cefazolin 2g IV |
MRSA
Vancomycin IV {link to vancomycin dosing guideline} |
Duration of therapy
- Uncomplicated: 14 days IV therapy
- Complicated: 4-6 weeks IV therapy
- Evidence of metastatic infection
- Infective endocarditis
- Intravascular prosthetic infection (PPM, valve)
- Unknown source
- Fever >72hrs post-antibiotic commencement
- Positive blood cultures >48hrs post-antibiotic commencement
Management – Strep Pneumoniae [6] #
If NOT associated with meningitis
Benzylpenicillin 1.8g IV 4hrlyIf non-severe penicillin allergy: Ceftriaxone 2g IV dailyIf severe penicillin allergy: Moxifloxacin 400mg IV daily |
Management – Strep Pyogenes [6] #
If evidence of sepsis, pneumonia, necrotising fasciitis, meningitis or toxic shock syndrome
Benzylpenicillin 1.8g or 2.4g IV 4hrly AND Clindamycin 600mg IV 8hrlyIf non-severe penicillin allergy: Cefazolin 2g IV 8hrlyIf severe penicillin allergy: Vancomycin IV {link to vanc dosing guideline } |
If NO evidence of sepsis, pneumonia, necrotising fasciitis, meningitis or toxic shock syndrome
Benzylpenicillin 1.8g IV 4hrlyIf non-severe penicillin allergy: Cefazolin 2g IV 8hrlyIf severe penicillin allergy: Vancomycin IV {link to vanc dosing} |
Swap to amoxicillin 1g 8hrly PO once well
Management – Pseudomonas Aeruginosa [6] #
- Discuss with ID
If awaiting susceptibilities
Gentamicin 4-5mg/kg IV loading doseAND Ceftazidime 2g 8hrly IV OR Piperacillin + tazobactam 4.5g 6hrly IVIf severe penicillin allergy:Gentamicin 4-5mg/kg IV loading dose AND Ciprofloxacin 400mg IV 8hrly |
Management – Candida [6] #
- Always discuss with ID
- Echocardiogram to rule out infective endocarditis (most sensitive at day 5-7)
- Dilated fundoscopic exam by ophthalmologist within a week of diagnosis to examine for endophthalmitis
- Repeat blood cultures until several negative to ensure resolution of fungaemia
If haemodynamically unstable
Anidulafungin 200mg IV stat, then 100mg IV daily OR Caspofungin 70mg IV stat, then 50mg IV daily (if <80kg) or 70mg IV daily (if >80kg)OR Micafungin 100mg IV daily |
If haemodynamically stable
Fluconazole 800mg IV/PO stat, then 400mg IV/PO daily |
If susceptible to fluconazole and haemodynamically stable
Swap to Fluconazole 400mg PO daily |
Duration of therapy
- 14 days after last positive blood culture
- 4-6 weeks if evidence of metastatic infection
Management – Enterobacteriaceae [6] #
- Discuss with ID
- Treat as per susceptibility results
- While awaiting susceptibility, treat as per likely infection site (i.e. biliary/urinary tract)
References #
- Doern G. Detection of bacteraemia: Blood cultures and other diagnostic tests [Internet]. UpToDate. 2021 [cited 21 September 2021]. Available from: https://www.uptodate.com/contents/detection-of-bacteremia-blood-cultures-and-other-diagnostic-tests
- Tufariello J, Lowy F. Infection due to coagulase-negative staphylococci: Treatment [Internet]. UpToDate. 2021 [cited 21 September 2021]. Available from: https://www.uptodate.com/contents/infection-due-to-coagulase-negative-staphylococci-treatment
- Anti-infectives – Tables [Internet]. AMH (Australian Medicines Handbook). 2021 [cited 21 September 2021]. Available from: https://amhonline.amh.net.au.acs.hcn.com.au/chapters/anti-infectives#anti-infectives-tables
- Freeman J, Roberts S. Approach to Gram stain and culture results in the microbiology laboratory [Internet]. UpToDate. 2019 [cited 21 September 2021]. Available from: https://www.uptodate.com/contents/approach-to-gram-stain-and-culture-results-in-the-microbiology-laboratory
- Chela HK, Vasudevan A, Rojas-Moreno C, Naqvi SH. Approach to Positive Blood Cultures in the Hospitalized Patient: A Review. Mo Med. 2019;116(4):313-317.
- Directed therapy for bloodstream infections, including sepsis and septic shock [Internet]. eTG. 2019 [cited 21 September 2021]. Available from: https://tgldcdp.tg.org.au.acs.hcn.com.au/viewTopic?topicfile=bloodstream-infections-septic-shock-directed-therapy#toc_d1e47
- Antibiotic sensitivity overview [Internet]. 2021 [cited 21 September 2021]. Available from: https://drug.wellingtonicu.com/Appendices/5/
Contributors
Reviewing Consultant/Senior Registrar
Dr Kate Drummond
Dr Alice Liu