Overview of Blood Culture Lab Test Results

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If your doctor suspects you have a bacterial infection, often the first step in treatment is getting a gram stain with culture and sensitivity test. The aim of this to determine whether bacteria are present, what kind they are, and the best way to kill them.

Each of the three parts of the test contribute to that process:

  • Gram stain: A sample is looked at under a microscope after having a stain applied.
  • Culture: Bacteria from the sample are grown and examined to help determine what organism is causing the infection.
  • Sensitivity: This determines which drug is best for treating the infection.
Antibiotic drugs being tested to see how resistant they are to bacteria
Andrew Brookes / Getty Images

For these tests to be performed, you'll have to provide a sample to the lab. Many types of samples can be tested, including:

  • Pus or other fluids from a wound
  • Sputum (a thick mucus from the lungs)
  • Throat swab
  • Blood
  • Stool
  • Urine

The lab can typically use the same sample for all three tests.

How a Gram Stain Is Performed

A gram stain is a "quick look" at a sample to determine if bacteria are present, and what kind. Many types of samples can be examined in this way.

The material is stained, meaning a special agent is applied to make the sample easier to see under a microscope. The sample is then examined to determine the shape and color of the bacteria causing the infection.

The sample may not show enough bacteria to determine what is causing the infection or may indicate that infection is unlikely.

To better determine what is present in the sample, and in order to obtain a larger specimen of the bacteria that may be present, the lab can perform a culture.

How a Blood Culture Is Performed

For a blood culture, the nurse or phlebotomist cleans your skin to make sure contaminants don't get into the sample. They then collect the blood into bottles that contain a culture medium (a substance that promotes the growth of bacteria).

The culture medium feeds any bacteria that may be present and allows it to grow. The bottle is placed in a warm, moist area to encourage rapid growth.

The bacteria are typically allowed to grow for at least five days, but an earlier result may be possible if enough bacteria have grown to be examined under a microscope.

How a Sensitivity Test Is Performed

Once the culture contains a large enough colony of bacteria, the sensitivity test can be performed as well.

In this test, tiny samples of usually about ten different antibiotics are placed on the growth plate. The plate is then examined to determine which antibiotics most effectively treat the bacteria present.

For example, say the antibiotics penicillin, ciprofloxacin, and tetracycline are put on the plate. If the bacteria stops growing around the penicillin, grows slightly around the ciprofloxacin, and grows unchecked around the tetracycline, the report for this sensitivity would show:

  • Penicillin as "sensitive"
  • Ciprofloxacin as "intermediate"
  • Tetracycline as "resistant"

Culture and Sensitivity Results

A culture and sensitivity can take nearly a week to perform, but when you go to the doctor with an infection, you'll most likely start treatment right away because waiting a week can be harmful.

The sensitivity results can help optimize treatment, making it possible to choose the best antibiotic for you. Because of this, your doctor might change your medication once the sensitivity results come through so you can take the drug most likely to help you get better.

Testing Before Treating

The fluids for culture and sensitivity tests should be drawn before you start taking antibiotics. Starting the drugs first can skew the results, making it difficult or impossible to identify the bacteria responsible for the infection.

In some cases, you may be sent to the lab for new samples if you develop a new fever or other new symptoms of infection after starting treatment with antibiotics.

A Word From Verywell

Simply put, culture and sensitivity tests are a way to determine the best possible antibiotic treatment for an infection. Many types of infection-causing bacteria exist and they respond to different antibiotics in different ways.

What treats one infection may not treat another infection, which is why this culture and sensitivity is so important. The tests allow your healthcare provider to determine the best antibiotic for the infection in a laboratory rather than by guessing which antibiotic will work the best.

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  1. National Institutes of Health, U.S. National Library of Medicine: MedlinePlus. Gram stain. Update August 13, 2020.

  2. National Institutes of Health, U.S. National Library of Medicine: MedlinePlus. Blood culture. Updated December 3, 2020.

  3. National Institutes of Health, U.S. National Library of Medicine: MedlinePlus. Sensitivity analysis. Updated December 3, 2020.