Clinical Laboratory - Microbiology

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Microbiologic Testing

Factors Affecting Turnaround Time

How quickly can a microbiology result be obtained? A gram stain may be done close to the bedside or in the clinical laboratory, and can be viewed in less than an hour. There are methods for detection of growth in media, such as blood cultures, that may yield a positive result in less than a day. Positive means some organism is present, but may not yet be completely identified.

For example, if an organism were identified in a blood culture this is highly signficant, and initial antibiotic therapy may be empirically based upon the most commonly encountered organisms for the clinical scenario. Laboratory testing may include an instrument designed to indicate quickly if a blood culture is positive.

In order to obtain a pure culture of bacterial organisms for identification and for antibiotic sensitivity testing, at least a day is typically necessary, followed by more testing for the specific identification and sensitivity tessting methods over the next day.

Serologic testing results depends upon the frequency of testing, and how esoteric tests are. These tests are generally done no more often than once per day.

Unusual tests for organisms infrequently encountered are generally not performed on a daily basis in the laboratory, even reference laboratories. This means that a specimen may need to be shipped to another laboratory, and even then the test may only be performed once a week.

Refer to the laboratory manual, or call the laboratory, to determine special requirements for diagnosis of certain infectious agents.

Point-of-care-testing (POCT) in a patient care setting can make a significant contribution to early diagnosis and treatment of infectious diseases. Therefore, simple testing systems may be available for some infectious agents, such as Streptococcus pyogenes (Group A Streptococcus, GAS) and influenza virus and COVID-19. These testing systems typically make use of molecular methods, including PCR. Even small amounts of contaminants can be amplified and lead to false positive results. Therefore, it is imperative to avoid contamination of patient samples, the testing system, and the facility in which testing is performed.

Some microbiologic agents are difficult to identify based upon their biology. Examples include:

  • Fastidious organisms such as Legionella pneumophila are possible to culture, but require special media. A direct fluorescent antigen (DFA) test or polymerase chain reaction (PCR) test has specificity for diagnosis.

  • Neisseria gonorrheae and chlamydial organisms are so fastidious that culture is difficult, generally not attempted, and immunoassays performed instead.

  • Pneumocystis jiroveci cannot be cultured and must be diagnosed by obtaining a tissue or fluid that must be examined by DFA or PCR or by special stains microscopically.

  • Organisms such as Mycobacteria may grow slowly in culture, requiring days to weeks for results.

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