Last updated on June 7th, 2021
The target readers of this blog post are general people (people who do not have a health science background) who want to get basic information about “blood culture” as they or their relatives have recently been requested for blood culture or students who have just commenced their study of health science and want to have a basic understanding.
To address common queries of such people/students this post is written in the simplest possible way. Infectious disease specialists including physicians, microbiologists, and technologists are requested to read the technical version if they wish to have an in-depth understanding of blood culture test.
Blood is a connective tissue that circulates all over the body via its channel called blood vessels. All the major organs of our body are interconnected by blood vessels. These internal organs (e.g. kidney, liver, brain, heart, lungs) and the blood itself is free from any living organisms (bacteria/ virus).
Organisms gain access to our blood vessels by various means such as brushing teeth, abrasion, trauma, surgery, etc. Such organisms now can travel via the bloodstream and reach various organs where they can initiate infection.
Most of the time, our immune cells constantly patrol in blood vessels searching for any foreign invaders (organisms/pathogens), recognizing, killing, and remembering the encounter. If immune cells fail to find such invaders or kill them, these organisms will be able to colonize in some of the organs thus causing unwanted killing of our body cells or affecting our body, which generally manifests as fever, malaise, inflammation of that part, rigors depending on the types or nature of such organisms.
Prescribing blood culture means your physician is requesting lab professionals (technician/microbiologist) to find out if your blood contains such foreign invaders. If lab people find any organism in your blood, they need to identify which organism is it and which antibiotics (if it is bacteria) will kill it. The lab will communicate this information to your physician and the physician will prescribe one or more of such antibiotics for a certain duration to make sure such organisms are killed and you will be fine.
When Doctor requests for Blood Culture Test
Physician may prefer blood culture in a number of suspected disease conditions which includes but not limited to;
- Enteric fever (typhoid/paratyphoid fever)
- Infective endocarditis (a disease of the heart)
- Meningitis (a disease which affects brain)
- Pneumonia (a disease of the lungs)
- Infection of the kidney and associated organs
Collection of the Blood for Culture:
In any time, generally such foreign invaders (organisms) are generally very less in number in blood and may not present in blood all the time (may be sitting in their home e.g organ where they have colonized). So to increase the chance of finding (isolating) such organism, your physician may request more amount of blood (more than 5 ml depending on the age and nature of suspected organism) or multiple samples.
One of the biggest challenge faced during blood sample collection is making sure an organism is inadvertently introduced in the blood. As organisms are everywhere in the environment, instrument and even our skin, the challenge is immense.
As the outer surface of our body contains microorganisms (called normal flora), a technician should make sure, he/she does not contaminate the blood vial while drawing blood. For this, they use certain chemicals (iodine/alcohol) to kill the normal flora around the expected area of vein puncture. They also have to make sure syringes, blood culture tubes, or bottles are free from living organisms.
They also have to make sure that the sample reaches the laboratory on time for necessary incubation (so that the organism which is in blood, if any, won’t get killed while in transit). Some organisms are delicate, so will die, if the sample processing is delayed thus giving a false negative report i.e. you are infected with that pathogen but the laboratory won’t be able to find out.
What the Lab People Do
If your blood contains microorganisms, they will utilize (eat) the nutrient available in the broth and multiply. To make sure such organisms do not die appropriate media and incubation are used. Inoculated broth are incubated in an special instrument that has a controlled environment i.e. temperature-37°C (our body temperature), aeration, agitation.
After certain days, (generally 3-5 days) that instrument will give a signal about the presence of the organism. An instrument used for incubation and signaling used to rule out the presence of an organism differs according to the infrastructure available or used in your laboratory. In resource-poor settings, conventional blood culture bottles and general incubators are used but sophisticated laboratories might have access to Bactec blood culture system and some might be using an automated blood culture system.
If such an instrument signals the presence of an organism laboratory professionals has to sub-culture that sample in an appropriate media. Generally, blood agar, chocolate agar and MacConkey agar are used for this purpose. Just like we humans, organisms do have preference over nutrients i.e. some are able to utilize certain compounds but others are not. Based on this nature, organisms present in your blood preferentially grow in some or all media.
When these organisms grow in the above-mentioned media, they may make the changes in the media where they grow. Thus, lab professionals track such changes and the appearance of such organisms to identify them. Laboratory professionals use various techniques such as Gram staining, study of morphological characteristics, or biochemical tests to identify them. Thus finally being able to recognize which organism is present in your blood sample.
Organisms that cause disease to us and are frequently isolated from blood are:
- Salmonella Typhi/ParaTyphi
- Staphylococcus aureus
- Streptococcus pneumoniae
- Escherichia coli
- Acinetobacter spp
- Pseudomonas spp
- Streptococcus pyogenes
- Enterococcus spp
Lab also tells which drug to prescribe
No single antibiotics is able to kill all organism. So laboratory has to find out, which antibiotics may kill such bacteria.
As organisms are evolving and acquiring resistance (called drug resistance or antibiotics resistance which is a major global issue) laboratory has to find out which antibiotic will kills that particular isolate.
For this, they test that organism against a panel of antibiotics based on the nature of the organism and find out which antibiotics actually kill that organism. After the completion of the processing, the laboratory gives a report mentioning which organism is isolated (if any) and which antibiotics will kill that organism. Your physician chooses among these antibiotics and prescribes you the medication.
After how many days I will get report of Blood Culture Test?
Generally, it takes 3-7 days to get your blood culture report. If the suspicion is the involvement of a slow-growing organism sometimes it may take weeks to rule out the presence of the organism in your blood.