Blood cultures. Sept 2013 A/Prof John Ferguson (nepal)

Transcription

1 Blood cultures Sept 2013 A/Prof John Ferguson (nepal)

2 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 hrs after treatment commences, indicates a failure of source control- especially relevant for Staph. aureus bloodstream infection Specific organism may have prognostic significance

3 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

4 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

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6 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

7 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

8 Specimen rejection criteria

9 Manual system: components Broth options: Tryptose phosphate cheapest Brain heart infusion Trypticase soy [Thioglycolate broth for anaerobes (separate bottle)] Supplements Sodium polyanethosulphonate (SPS) % 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

10 Volume considerations Blood to broth ratio 1:5 to 1:10 Manual systems need 1:7 at least

11 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

12 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

13 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

14 An important study highly relevant to Nepal Lancet 2011

15 Nosocomial BSI incidence x 40 higher than community patient incidence

16 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

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

18 Febrile outpatient children only relatively high pathogen isolation rates

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21 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

22 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

23 Isolate pattern Hunter New England Health, Newcastle, Australia 2010

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

25 Staph. aureus BSI

26 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.

27 Isolate storage (-70deg) Significant isolates for later subtyping or study Contaminant isolates: for correlation with subsequent

28 Study tours Question guide Visit Patan (BACTEC system) Model Hospital (manual system)

29 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?

30 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