How to Read Microbiology Lab Test Results

Updated April 17, 2017

The microbiology laboratory performs a series of moderate and complex tests on samples to determine if they contain bacteria, what kind of bacteria those are, whether or not they are capable of causing disease and, finally, what antibiotics work best against those bacteria. The reports sent out by the lab contain all the information a health care provider needs to diagnose and treat a patient with a suspected infection. Reading the results contained in the report requires some understanding of the tests being performed.

Double-check the specimen source data to make sure it corresponds to the patient or site from which the sample was taken. One of the most common mistakes in the clinical laboratory is mishandling or mislabelling of specimens.

Check to make sure the sample was taken at the right time, from the right site and delivered to the laboratory in a timely manner.

Check to make sure the specimen type is correct. For example, make sure the sample is urine if the suspected infection is a urinary tract infection. The report must include all of this information to comply with the Clinical Laboratory Improvement Act.

Read the results for the initial Gram stain of the specimen, making sure to read whether or not bacteria were seen in the stain and what their Gram stain reaction was (positive or negative) and their morphology (rods versus spheres).

Read the results of the culture colony morphology. Different bacteria grow in culture with a different size, shape, colour and appearance to their colonies.

Read the results of any carbohydrate utilisation tests performed on the bacteria in order to identify them. These tests may be manual or automated, depending on the size and capacity of the lab. Different bacteria use carbohydrates (sugars) differently, so knowing which bacteria use several carbohydrates aids in their identification.

Double-check the laboratory interpretation of the bacterial identification by making sure that the Gram stain, colony morphology and carbohydrate utilisation tests noted in the report agree with the interpretation. Remember, some of these tests are performed by humans, and errors are an inherent, albeit relatively rare, factor of laboratory testing.

Read the results of any antibiotic susceptibility testing and their interpretation.

Check to make sure all relevant antibiotics to your practice are included in the panel. If any are missing, you may call the lab and see if they were included but not reported.

If susceptibility results are reported numerically instead of categorically (resistant vs. susceptible), make sure the reference, or normal, range is included in the report, comparing it to the results on the sample.

Things You'll Need

  • Lab report
  • Microbiology reference text
  • Antibiotic drug schedule
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About the Author

Rene Najera has been writing about health-related issues for over five years through different media. He holds a Master of Public Health degree from the George Washington University and conducts infectious disease surveillance at a state health department. He has also been a lab professional for over 14 years.