How to read your medical test report: Blood culture test

All you need to know about reading your blood culture report.

By: Dr Anitha Anchan | | Updated: April 6, 2016 6:50 pm

The blood circulation is usually germ-free. A bacterial infection in the blood is called bacteremia. The presence of bacteria or other microorganisms in the blood can cause a blood infection. It results from the spread of an infection somewhere else in the body. The body reacts to the infection leading to multiple organ failure. This life-threatening condition is called sepsis. Also Read - Travel ban on immune cells could prevent sepsis

Also Read - Former President Pranab Mukherjee passes away after going into septic shock

Blood culture test what is it? Also Read - New treatment strategy brings hope of genital herpes cure: Know all about this condition

A blood culture is a blood test that detects the presence of microorganisms like bacteria, fungi and virus in the blood. The blood sample drawn is placed in a culture media (solid or gas substance containing nutrients). The sample is then incubated in a controlled environment and observed for signs of microbial growth like color change cloudiness, bubble formation, the formation of distinct colonies, etc. In recent years, the automated continuous-monitoring blood culture systems have helped monitor the blood sample automatically for evidence of microbial growth.

If there is an evidence of bacterial growth, then the blood sample is stained with red and purple stains and examined under a microscope. This procedure helps identify the type of bacteria. An antibiotic susceptibility testing is done to determine what antibiotics are effective against them. Fungi are rapidly and accurately detected by polymerase chain reaction (PCR)-based molecular approaches.

To detect virus in the blood, the infected blood sample is inoculated onto a culture media with cells in which the virus can grow. Changes in the cells observed under a microscope may indicate viral growth.

When is the test recommended?

Blood cultures are commonly prescribed when infection is suspected. They help discover and identify bacteria, fungi or virus. Once the presence of a microorganism is established, additional blood culture tests may be done to figure out the type of the pathogen present. It can help determine the best possible treatment.

Often, other tests like complete blood count (CBC) or blood chemistry panel ordered prior to the blood culture may help indicate a potential infection or identify its source.

Reading your blood culture report

The bacteria or fungi take a couple of days or longer to grow to sufficient numbers that can be detected. Viruses may take several weeks.

A couple of blood cultures are usually done before the result is considered negative or positive. A no growth report means the blood culture is negative, i.e. no pathogens are discovered. The probability of you having a blood infection caused by bacteria and fungi is low. Failure to recover the pathogen present in the blood is termed false negative . A false negative result may occur under these circumstances:

Too little blood sample collected
Poor handling of the blood sample
Unfavorable incubation environment for microbial growth
You are on antibiotics before the blood sample is taken
Slow-growing bacteria

A positive blood culture means the microorganism has been discovered in the blood sample, and you are likely to be infected with it. The specific bacteria or fungi causing the blood infection are identified with further lab tests. A positive blood culture even in the absence of pathogens in the blood is termed false positive . A false positive result may occur due to contamination of the blood sample mostly with the skin bacteria.

If symptoms of blood infection persist even after a blood culture report shows no growth , then additional tests like white blood count, urine, or sputum culture, etc. may be prescribed.

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Published : April 6, 2016 6:45 pm | Updated:April 6, 2016 6:50 pm

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