TABLE 30-4.

Interpretation of Heartworm Diagnostic Procedures Tests in the Cat

Test	Description	Result	Interpretation	Limitation
Antibody test	Detects antibodies produced in response to heartworm larvae; may detect infection as early as 8 weeks after transmission	Negative	Lowers index of suspicion	Antibodies confirm infection with heartworm larvae but do not confirm disease causality
		Positive	Increases index of suspicion, confirms cat is at risk of disease; 50% or more of cats will have pulmonary arterial disease
Antigen test	Detects antigen produced by adult female heartworms or from >5 dying male or female heartworms	Negative	Lowers index of suspicion	Immature or male-only heartworm infections are rarely detected
		Positive	Confirms presence of heartworms
Thoracic radiography	Detects vascular enlargement, pulmonary parenchymal inflammation, edema	Normal	Lowers index of suspicion	Radiographic signs are subjective, affected by clinical interpretation
		Signs consistent with heartworm disease	Enlarged arteries greatly increases index of suspicion
Echocardiography	Detects echogenic walls of immature or mature heartworms in the lumen of the pulmonary arterial tree	No heartworms seen	Does not change index of suspicion	Experience of ultrasonographer influences accuracy
		Heartworms seen	Confirms presence of heartworms

Note: In the cat no single test will detect all heartworm cases. Although the antigen tests are highly specific for detecting adult heartworm antigen, they will not detect infections with only live male worms. The clinician must use a combination of test results to determine the likelihood of heartworm disease as the etiology of the cat's clinical signs.

Adapted and reprinted with permission from the American Heartworm Society (www.heartwormsociety.org/veterinary-resources/feline-guidelines.html).