REVISITING BLOOD CULTURING - WHAT WE ALL SHOULD KNOW?

Dr.T.V.Rao MD Most common Critical Investigation REQEUSTS we receive in Microbiology department continues to

be Culturing of Blood, Detection of bacteraemia or fungaemia by blood culture is critical in managing

patients with infection, and directs the appropriate selection of antimicrobials. Blood culture

remains one of the most important microbiological tests available to the clinician. Members of the

healthcare team who perform blood cultures must have a sound comprehension of the principles

that underlie this important diagnostic tool Blood culture is a common laboratory investigation

where blood is inoculated into culture medium and incubated. Media used in blood culture bottles

support the growth of most medically important bacteria and fungi, including anaerobes, which

grow adequately in the aerobic blood culture bottle, hence separate anaerobic bottles are

infrequently used. METHODS IN BLOOD COLLECTION -Technique — Careful technique is critical to

avoid contamination of the blood culture media by normal skin flora during the process of collection.

This is important because normal bacterial skin flora can also cause systemic disease, such as

infective endocarditis, and in some circumstances blood culture contamination can make it difficult

to distinguish between false-positive results and true infection. Measures to reduce contamination

include effective disinfection of the venepuncture site and avoiding blood culture collection through

existing intravenous lines. The recommended antiseptic preparations are 70% isopropyl alcohol,

followed by an iodophor or iodine tincture. lodophors require 1.5-2 minutes of contact time for

maximum antiseptic effect. Iodine tincture exerts its antiseptic effect more rapidly than do

iodophors and may lower contamination rates in institutions where these rates are high

AUTOMATION IS EMERGING NEED FOR CULTRUING BLOOD - Many laboratories are adopting the

culturing blood by Automation Most blood culture systems are now automated, with some form of

continuous monitoring to detect growth. The bottles contain broth supplemented with additives.

Different bottles are designed either to be used in different patient groups, or to isolate different

classes of pathogens. A few general principles apply: Standard aerobic bottles are suitable for the

recovery of most common bacterial pathogens, including aerobes and facultative anaerobes. These

are not as suitable for supporting the growth of strict anaerobes, and therefore traditional advice

has been to use a paired set of aerobic and anaerobic bottles when taking blood cultures. The

routine use of anaerobic bottles has been questioned, as the incidence of anaerobic bacteraemia is

decreasing some manufacturers produce aerobic and anaerobic bottles with additives (such as

charcoal or resins) that inactivate antibiotics. These are preferable when taking blood cultures from

patients already on antimicrobials, but are more expensive. There is evidence that the average time

to positivity with these bottles is shorter than with normal aerobic and anaerobic bottles; however,

their cost-effectiveness is unclear Paediatric bottles are often supplemented with growth factors and

usually have additives which will bind antibiotics, allowing improved recovery of bacteria. Together

with the lower concentration of SPS, these features allow smaller volumes of blood to be inoculated

NEW AFFORDABLE TECHNOLOGIES FOR BLOOD CULTRUING The biphasic Septi-Chek system and the

Opticult blood culture systems (Becton Dickinson) and the Oxoid Signal broth displacement blood

culture system Instrumented blood culture systems. Commercially available instrumented blood

culture methods were introduced in the 1970s. Until recently, the BACTEC instrumented systems

were the only products commercially available in the United States; these systems were initially

equipped with radiometric instruments and media, followed in the mid-1980s by the nonradiometric

instruments and media. Both systems (as well as in the newer BACTEC and BacT/Alert continuous-

monitoring devices) are based on the utilization of carbohydrate substrates in the culture media and

subsequent production of CO2 by growing microorganisms.

Limitations of Blood Cultures

Blood cultures, as described herein, currently represent the "gold standard" for diagnosis of

septicaemia. Nonetheless, they have limitations. Positive results require hours to days of incubation.

No one culture medium or system in use has been shown to be best suited to the detection of all

potential bloodstream pathogens. Some microorganisms grow poorly, or not at all, in conventional

blood culture media and systems. Whether culture-based systems will remain the diagnostic

methods of choice into the next century or be replaced by molecular techniques or other methods

remains to be determined.

VITEK® MS is bioMérieux's mass spectrometry-based system for the rapid identification of

bacteria, fungi, and mycobacteria. -

ECONOMY AND NEW TECHNOLOGIES With many new technologies becoming commonplace, maybe

it is time to re-think how we use our budgets and how we roster the microbiology staff. Maybe we

need to divert the money used to work-up bacteria from abscesses and chronic venous ulcers and

routine clinical specimens towards rapid identification of bacteria from positive blood cultures,

where it is going to make a real clinical difference and be potentially life-saving. WHAT IS NEW IN

BLOOD CULTRING - Fluorescent In-Situ Hybridisation (FISH) technology along with Maldi-TOF

sepsityper, now allows us to identify the causative bacterium accurately in the majority of cases

within an hour of the blood culture becoming positive. Pathogen identification is crucial to confirm

bacterial infections and to guide antimicrobial therapy and clinical laboratories need ever more rapid

and reliable methods. Identification of a microorganism to the species level typically requires several

time-consuming steps and definitive results can sometimes take up to 24 hours or more. -It can

provide an identification of a microorganism’s genus and species in just a few minutes. This

technique allows significant time savings with respect to traditional identification methods. MALDI-

TOF mass spectrometry cannot be used for antibiotic susceptibility testing and an additional method

must be used. The technology is particularly well-suited to large laboratories. - This has obvious

positive clinical implications, particularly if the identified bacterium is a beta-haemolytic

streptococcus, a Pseudomonas aeruginosa or a Staphylococcus aureus. Even rapidly identifying

coagulase negative staphylococci may avert the need for unnecessary anti-microbial therapy. These

technologies are NOT cost-prohibitive, and should be within the scope and skill level of most

reasonably sized diagnostic laboratories. And maybe the blood culture analyser needs to be staffed

24 hrs a day, 7 days a week, with blood cultures being processed and bacteria identified as soon as

possible. These sort of changes might cause a few grumbles but this is the sort of direction we need

to be heading in, in order to have a clinical microbiology service that makes a genuine difference to

the patient and saves many valuable lives, supports the clinicians for better utility of Microbiological

advances,

HOWEVER WE NEED MORE TEHCNOLOGICAL ADVANCES TO REDUCE MORTALITY AND MORBIDITY

Ref 1 Guideline for the optimal use of blood cultures Ntobeko Ntusi, Lindsey Aubin, Stephen Oliver,

Andrew Whitelaw, and Marc Mendelson

Ref 2 Current Blood Culture Methods and Systems: Clinical Concepts, Technology, and

Interpretation of Results Melvin P. Weinstein

Dr.T.V.Rao MD Professor of Microbiology Freelance writer