Blood Culture Test: an Information for the Patient

The target readers of this blog post are general people (people who do not have 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 to have basic understanding . To address common queries of such people/student this post is written in  simplest possible way.
Infectious disease specialist including Physician, Microbiologist and Technologist are requested to read technical version if they wish to have in-depth understanding of Blood Culture test.

General Introduction

Blood is a connective tissue which circulates all over 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 blood stream and reach to various organs where they can initiate infection.
Most of the times, our immune cells constantly patrol in blood vessels searching 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 are requesting lab professionals (Technician/Microbiologist) to find out if your blood contain 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. Lab will communicate this information to your physician and physician will prescribe one or more of such antibiotics for certain duration to make sure such organisms is killed and you will be fine.

When Doctor requests for Blood Culture Test

Physician may prefer blood culture in number of suspected disease conditions which includes but not limited too;
  1. Enteric Fever (Typhoid/Paratyphoid fever)
  2. Infective Endocarditis (a disease of the heart)
  3. Meningitis (a disease which affects brain)
  4. Pneumonia  (a disease of the lungs)
  5. 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.

Collection of Blood for Culture
As the outer surface of our body contains microorganism (called normal flora), technician should make sure,  he/she do 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 syringe, blood culture tubes or bottles are free from living organism.
They also have to make sure that sample reach the laboratory on time for necessary incubation (so that the organism which are in blood, if any, won’t get killed while in transit). Some organism are delicate, so will die, if the sample processing is delayed thus giving false negative report i.e. you are infected with that pathogen but laboratory won’t be able to find out.

What the Lab People Do


If your blood contains Microorganism, 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 which has controlled environment i.e. Temperature-37°C (our body temperature), aeration, agitation.

BACTEC Blood Culture Bottle
After certain days, (generally 3-5 days) that instrument will give signal about the presence of organism.
Instrument used for  incubation and signalling used to rule out presence of organism differs according to the infrastructure available or used in your laboratory. In resource poor settings  conventional blood culture bottle and general incubators are used but sophisticated laboratory might  have access to Bactec Blood Culture System and some might be using Automated Blood Culture System.
If such instrument signals presence of organism laboratory professional  have  to sub-culture that sample in appropriate media. Generally Blood Agar, Chocolate Agar and MacConkey Agar are used for this purpose.
Just like we human, organisms do have preference over nutrients i.e. some are able to utilize certain compound 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. Organism that cause disease to us and are frequently isolated from blood are:

  1. Salmonella Typhi/ParaTyphi
  2. Staphylococcus aureus 
  3. Streptococcus pneumoniae
  4. Escherichia coli
  5. Acinetobacter spp
  6. Psuedomonas spp
  7. Streptococcus pyogenes 
  8. 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.

Testing to find out which antibiotic will kill the organism
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 organism and find out which antibiotics actually kills that organism. After the completion of the processing, laboratory gives report mentioning which organisms is isolated (if any) and which antibiotics will kill that organism. Your physician choose among these antibiotics and prescribe 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 involvement of slow growing organism sometimes it may take weeks to rule out presence of organism in your blood.