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Blood cultures

Sept 2013 A/Prof John Ferguson

jferguson@hnehealth.nsw.gov.au www.hicsiganz.org (nepal)

Clinical importance

• Diagnostic : – Establishes or confirms the infectious aetiology

– Provides susceptibility test to guide antibiotic rx

• Prognostic: – Indicates a failure of the host to contain an infection

at its source

– If taken 48-96 hrs after treatment commences, indicates a failure of source control- especially relevant for Staph. aureus bloodstream infection

– Specific organism may have prognostic significance

Timing

• No evidence of an effect

• Fever/chills follow 1 hour after influx of bacteria to blood

• Studies show no increased yield related to collection around fever spikes or at other times

Volume of sampling

• Adults – yield of pathogens increases in direct proportion to volume 2 to 30mLs

• Additional yield with higher volumes

• Collection of two sets is best practice – Increases volume of sample

– Allows to distinguish between contamination and significant isolates for organisms such as coagulase negative staph

Logistics

• Point of care availability • Patient identification and sample labeling • No collection via existing iv lines – scope for

contamination of line or of culture • Safety considerations-

– Collector – gloves and eye protection – Don’t change needle between collection and culture

inoculation – sharps bins at the bedside / on trolley

• Transport to lab within 2 hrs; do not refrigerate or allow to remain at room temperature for more than a few hours

Skin disinfection to prevent contamination

• Ethanol or isopropyl alcohol + chlorhexidine or tincture of iodine

– Two wipes at least (first one defats the skin)

– At least 30 seconds application time

– Apply solution also to disinfect the blood culture injection port if there is one

• Avoid chlorhexidine in neonates < 2 months

Specimen rejection criteria

Manual system: components

• Broth options: – Tryptose phosphate – cheapest – Brain heart infusion – Trypticase soy – [Thioglycolate broth for anaerobes (separate bottle)]

• Supplements – Sodium polyanethosulphonate (SPS) 0.025-0.05%

• Anticoagulant; assists isolation of Haemophilus • inhibits some bactericidal activity of blood- suppresses phagocytosis which

persists for 24hrs post collection • Complexes with certain antibiotics • Inhibitory to Neisseria species and some others – counteract with gelatin

– Gelatin 1.2% – Agar 0.1% - creates an upper layer that facilitates aerobic growth

Volume considerations

• Blood to broth ratio 1:5 to 1:10

• Manual systems need 1:7 at least

Incubation and reading

• 35 deg; daily reading, no venting required

• Subculture to blood & chocolate first day and thence 3 and 7 days

• Incubate subs for 48 hrs in enriched CO2 atmosphere (candle jar)

Gratten M 1983

Isolate types

• Contamination: coagulase negative staph, bacillus, corynebacterium, propionobacterium, environmental gram negatives – Rate should not be > 3%

• Significant (unlikely to be contaminants) – Staph. aureus

– Strep. pneumoniae

– Gram negatives

– Candida

Commercial systems

• Complex broth system with antibiotic removal devices; controlled atmosphere above broth

• 2 bottles – aerobic and anaerobic; aerobes grow in either media

• 10mLs each bottle in adult; separate single paediatric bottles for smaller volume

• Continuous incubation and monitoring of broth for growth with agitation (improves isolation of S. pneumoniae)

• BACTEC system – in use in Africa and at Patan Hospital

• Time to positive generally < 12 hrs for significant isolates

• An important study – highly relevant to Nepal Lancet 2011

Nosocomial BSI incidence x 40 higher than community patient incidence

Causes of nosocomial BSI (children and adults)

• IV line-associated sepsis

– Peripheral cannulae

– Central lines

– ICU and non-ICU

• Catheter associated urinary infection

• Surgical wound (post-operative) infections

• Decubitus ulcers

• Pneumonia

MRSA only in one case Gives a useful survey of the presentations and mortality risk factors

Febrile outpatient children only – relatively high pathogen isolation rates

• BHI broth, small 5mL sample for adults, subculture BA and Maconkey agar incubated aerobically

• Nosocomial isolates not distinguished

• Oxacillin used for detection of MRSA – none detected (cefoxitin should have been used)

• No mention of contamination rates

What is done with the individual patient blood culture result?

1. Direct liaison with the clinician at the time of a

positive result (Gram stain, culture or susceptibility result) . Essential for building trust between lab and clinical services

2. Reporting of results – specify when contamination likely; don’t report

susceptibilities – provide direct written report advice on correct antibiotic

choice and dose for common pathogens consistent with local guidelines if available

Isolate pattern Hunter New England Health , Newcastle, Australia 2010

Staph. aureus BSI: from local protocol in Newcastle, Australia

Staph. aureus BSI

Summaries of collected blood culture results

• What proportion of blood cultures are

contaminated for different patient groups? Essential information to provide quality assurance of collection technique- if contamination high then improvements required.

• What are the common blood bacterial isolates for different patient groups and what susceptibilities are seen? This information helps clinicians to chose empiric antibiotic treatment for future septic patients.

Isolate storage (-70deg)

• Significant isolates for later subtyping or study

• Contaminant isolates: for correlation with subsequent

Study tours

• Question guide

• Visit Patan (BACTEC system)

• Model Hospital (manual system)

Questions for study tour 1. Describe the blood culture system – broth components, mode of incubation,

subculture practices- how many & when, how long are subcultures incubated for and in what atmosphere? Ask the scientist from the location about the good and bad points of the system and its cost.

2. What volume of blood sample is accommodated in the broth? What is the effective blood to broth ratio when the recommended blood volume is added to the broth? Does the lab check periodically to see whether a correct volume of blood sample is added to the bottle?

3. On average how quick is the system at detecting a pathogen? Does this differ by the type of pathogen?

4. What contaminants are isolated? What is the rate of contamination? How are contaminant isolates reported back to the clinician – ie is there a comment to explain likely contamination? Are susceptibility results suppressed?

5. By examining the lab registry, summarise the recent (past 6 months) range of isolates detected. If possible, distinguish MRSA from MSSA. There may already be a report prepared for this that you can access.

6. Does the lab capture information to say whether an event is nosocomial or community acquired?

References

• CLSI M47 standard- Principles and Procedures for blood cultures

• ISBAR overview – a useful communication tool for hospitals to ensure clarity of communication

• Patan Hospital blood culture studies

• Kilifi, Kenya hospital paediatric blood culture study

• Kilifi Paediatric SAB study