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Introduction to Laboratory Diagnosis of Infectious Disease

By

Maria T. Vazquez-Pertejo

, MD, FACP, Wellington Regional Medical Center

Reviewed/Revised Oct 2022
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Topic Resources

Laboratory tests may identify organisms directly (eg, visually, using a microscope, growing the organism in culture) or indirectly (eg, identifying antibodies to the organism). General types of tests include

Culture is normally the gold standard for identification of organisms, but results may not be available for days or weeks, and not all pathogens can be cultured, making alternative tests useful. When a pathogen is cultured and identified, the laboratory can also assess its susceptibility Susceptibility Testing Susceptibility tests determine a microbe’s vulnerability to antimicrobial drugs by exposing a standardized concentration of organism to specific concentrations of antimicrobial drugs. Susceptibility... read more to antimicrobial drugs. Sometimes molecular methods can be used to detect specific resistance genes.

Some tests (eg, Gram stain, routine aerobic culture) can detect a large variety of pathogens and are commonly done for many suspected infectious illnesses. However, because some pathogens are missed on these tests, clinicians must be aware of the limitations of each test for each suspected pathogen. In such cases, clinicians should request tests specific for the suspected pathogen (eg, special stains or culture media) or advise the laboratory of the suspected organism(s) so that it may select more specific tests.

Table

Diagnostic Tests for Some Bacterial Pathogens

Microorganism

Microscopy

Culture

NAAT

Antigen Detection

Antibody Detection

✓*

✓*

✓*

✓*

✓*

Rhodococcus equus

Rickettsial species

Adapted from Bennett JE, Dolin R, Blaser MJ: Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, 9th Edition. Elsevier, 2020.

* Test not widely available in clinical practice.

NAAT=Nucleic Acid Amplification Test

Table

Diagnostic Tests for Mycobacterial Pathogens

Microorganism

Microscopy

Culture

NAAT

Antigen Detection

Antibody Detection

✓*

Mycobacterium leprae

Adapted from Bennett JE, Dolin R, Blaser MJ: Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, 9th Edition. Elsevier, 2020.

* Test not widely available in clinical practice.

NAAT=Nucleic Acid Amplification Test

Table

Diagnostic Tests for Some Fungal Pathogens

Microorganism

Microscopy

Culture

NAAT

Antigen Detection

Antibody Detection

Adapted from Bennett JE, Dolin R, Blaser MJ: Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, 9th Edition. Elsevier, 2020.

NAAT=Nucleic Acid Amplification Test

Table

Diagnostic Tests for Some Viral Pathogens

Microorganism

Microscopy

Culture

NAAT

Antigen Detection

Antibody Detection

✓*

BK virus

LaCrosse virus

St Louis encephalitis virus

Tickborne encephalitis virus

Adapted from Bennett JE, Dolin R, Blaser MJ: Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, 9th Edition. Elsevier, 2020.

* Test not widely available in clinical practice.

NAAT=Nucleic Acid Amplification Test

Table

Diagnostic Tests for Some Protozoal and Parasitic Pathogens

Microorganism

Microscopy

Culture

NAAT

Antigen Detection

Antibody Detection

Intestinal nematodes

Adapted from Bennett JE, Dolin R, Blaser MJ: Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, 9th Edition. Elsevier, 2020.

NAAT=Nucleic Acid Amplification Test

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