Table 6-1.

Categorization of Antigen Detection Methods

TECHNIQUE	TARGET	ADVANTAGES	DISADVANTAGES	EXAMPLE	NEW ADVANCES	COMMENTS
Cytology/biopsy	Whole organism	Fast, inexpensive, highly specific	Organism may not be in sample collected; training required to evaluate sample; labor-intensive	Histoplasma capsulatum in lung aspirate	Immunostaining, flow cytometry	Negative test does not rule out infection.
Microbiology	Whole organism	24–48hr; can provide antimicrobial susceptibility; possible to perform indirect measurement of growth (gas production)	Limited by the growth traits of the organism (i.e., intracellular, fastidious, slow growth)	Salmonella, Mycobacterium spp.	Automation	Testing is limited to readily culturable organisms; cannot be used for viruses.
ELISA	Surface antigen, peptide	Easy to perform; rapid results; inexpensive; highly sensitive; patient-side testing available		FeLV, Dirofilaria, Giardia spp.	Molecular techniques for development of highly specific monoclonal antibodies	Substrate (blood, feces) differs depending on the organism.
Latex agglutination	Surface antigen	Easy to perform; rapid results; inexpensive		Cryptococcus neoformans		Negative test does not rule out infection.
PCR	Nucleic acid sequence	Sensitive and specific; can detect minute amounts of DNA and RNA	Can detect residual organisms that are dead after successful treatment	Ehrlichia canis, FIV	Assay formats and equipment that minimize contamination and provide substantiating information (amplicon melting temperatures)	False negatives because of inhibitors; false positives because of contamination can occur; no established quality control protocols.