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Blood Culture Test- Why It’s Done And What The Results Mean


The blood culture test is used to detect and identify bacteria, fungi and other microorganisms in your bloodstream. The presence of these foreign invaders in the blood may indicate an infection in the blood, known as Bacteremia. This infection involves the blood that flows in your entire body. Common sources of blood infections are bacteria that grow in your gastrointestinal tract lungs, skin or urine. This infection can spread to your blood and turn into systemic, a most severe condition known as sepsis. The blood culture test is a simple blood test, and a positive blood culture test indicates the presence of bacteria in your bloodstream. 


Purpose Of Blood Culture Test 


A blood culture test is ordered when your doctor suspects that you have a blood infection. It’s extremely important to test for sepsis since it’s a serious life-threatening complication. In this condition, the foreign invaders impede the functioning of your immune system as well as interfere with normal defenses of your body. The toxins produced by foreign invaders may damage your internal organs. With the test results, your doctor can ascertain the type of bacteria which is causing the infection as well as a way to combat the bacteria.


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Why Would I Need One?


You need to  visit the doctor as early as possible if you have any of the blood infection symptoms such as-


  • High or moderate fever
  • Shivering or Shaking chills 
  • Rapid or shallow breathing
  • Fatigue 
  • Fast heart rate
  • Muscle pain
  • Headache


If you never undergo treatment, the infection can turn to sepsis. The symptoms associated with sepsis, besides those mentioned above are-


  • Mottled skin( a condition characterised by a red-purplish appearance on the skin)
  • Mental confusion
  • Dizziness
  • Nausea
  • Low urine output


With the progression of sepsis, you may experience more severe complications such as-


  • Multiple organ failure
  • Inflammation all over the body
  • Blood clots in the smallest blood vessels
  • Dangerous drop in blood pressure 


Risk Factors


If you’re at higher risk of getting a blood infection, then you need to undergo a blood culture test on a regular basis. If you have any of the conditions as listed out below, then you’re at higher risk. 


  • Cancer
  • AIDS or HIV
  • Diabetes
  • An autoimmune disease.


You are at risk if you recently had/undergone-


There is a higher risk of developing a blood infection in older adults as well. Children and a newborn who has a fever, with no symptoms of sepsis need to undergo a blood culture test. 


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Risks Of A Blood Culture Test 


Blood culture tests rarely cause any side effects, which are serious. 

After the blood draw, you may experience 

  • Loss of consciousness
  • Heavy bleeding
  • Infection
  • Bleeding under skin


Blood Culture Procedure


There is no preparation required prior to a blood culture test. Before a blood culture test, you need to inform your doctor regarding the nutritional supplements and medications that you take. Sometimes the doctor asks you to stop taking some medicines since it can impact your test results. 


During the test, your puncture site is cleaned and the diagnostic center staff will tie an elastic band around your arm so that the vein fills with blood and becomes more visible. Different blood samples are taken, from different veins in your arm. This is done so that it’s much easier to detect microorganisms in your blood. 


Soon after the blood draw, the healthcare provider will cover the puncture site to stop further bleeding using a gauze. Your blood sample is thereafter cultured in the microbiology lab.

For blood culture, each sample of blood is added to the broth, a substance that promotes the growth of microorganisms in your blood sample. 


Results Of A Blood Culture Test


A positive blood culture indicates that you have bacteria or yeast in your bloodstream. Negative or normal blood culture results indicate that there is no presence of bacteria or fungus in your bloodstream. With the test results, your doctor can identify the type of bacteria or fungi, which is causing the infection.


Based on the type of microorganism in your bloodstream, your doctor may order a susceptibility or sensitivity test. Your doctor can determine the medication that works well against the microorganism, with this test. This test is usually conducted as a follow-up of the blood culture test, where the results are positive. 


If you’re tested positive for a blood culture test, then you need to undergo a treatment- broad-spectrum antibiotics, a medication that helps fight the microorganisms while you await the results of your sensitivity test. 


Bacteria becomes visible within 2 to 3 days, and certain types of bacteria can take 10 or more days to become visible. Fungus becomes visible in the culture within 30 days. 


You’re at high risk if you have a weak immune system in case sepsis develops and you need to undergo treatment at once. 


Inform your doctor beforehand if you’re experiencing blood infection symptoms or if you’re at risk of developing a blood infection since it can lead to serious health complications in future. Immediately inform your doctor if you have a moderate or high fever for more than 3 days. Equally important is fever in infants below 3 months, who require immediate medical attention. 


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Frequently Asked Questions


Why are blood cultures taken from 2 sites?


For a blood culture test, multiple blood samples are taken, from different veins in your arm to detect microorganisms present in small numbers. With the test, your doctor can confirm that the detected microorganism is responsible for the infection and not contaminants. 


Why do the test results take so long?


The microorganisms should grow to sufficient numbers in the broth before the microorganisms can be identified, and normally takes a few days. But in the case of some microorganisms, it takes much longer. Added to that, certain microorganisms are present in small numbers in our blood and take much time to reproduce and grow to sufficient numbers that can be identified. 


When should blood cultures be taken?


Blood cultures should be taken after a recurrent sharp rise of fever and prior to any antimicrobial therapy. It should also be taken when your doctor suspects Bacteremia.


What happens if the blood culture is positive?


A positive blood culture indicates that you have bacteria or yeast in your bloodstream. If you’re tested positive for more than one blood culture test for the same microorganism, it indicates that it is the type of microorganism which is causing the infection. 


Is fasting required for a blood culture test?


Whether you eat and drink prior to the test won’t affect your results. You can eat and normally drink before the test.


Why is Blood culture done?


The blood culture test is used to detect bacterial infections, fungal infections, type of bacteria which is causing the infection, and diagnose endocarditis. Blood culture is also used to ascertain the cause of unexplained fever, and to determine the best antibiotic that works well against the microorganism. 


Which media is used for blood culture?


Blood is injected into bottles with a medium for aerobic and anaerobic microorganisms. The broth is the common medium used for anaerobic microorganisms. It promotes the growth of microorganisms in your blood sample.