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The presence of a D. immitis infection can be identified by testing for antigens derived from female heartworms. The drawback to this type of testing is that only adult, female worms can be tested for. If the infection is a young one, or solely male, the pathogen will not be identified (Germinder, 2007).
Antibodies produced by the dog against the pathogen can also be tested for (Nash, 2007). The positive attribute in this type of test is that an infection of any size or gender biased can be detected. False positive results, however, are common (Nash, 2007).
A cheaper, but less accurate, test for D. immitis infection is simply looking at a blood sample under a microscope. Juvenile larvae will be visibile upon inspection if the infection is significant, consists of males and female worms, and has inhabited the dog for a full lifecycle of D. immitis. The "Knotts test" consists of a similar inspection for juveniles in the blood: blood is centrifuged to concentrate juvenile worms for inspection. This test, however, has the same aforementioned drawbacks (Nash, 2007).
Germinder and Associates, Inc. 2007. Canine heartworm disease. American Heartworm Society <http://www.heartwormsociety.org/CanineHeartwormInfo.htm>Accessed 2007 April 20.
Nash, H. 2007. "Heartworms." <http://www.peteducation.com/article.cfm?cls=2&cat=1621&articleid=743>. (C) Accessed 2007 April 20.