Heartworm antigen test kits are currently the most common method of detecting heartworm infections, in part because of their high sensitivity and specificity. However, the sensitivity of these tests is lower in dogs with all-male heartworm infections and in those infected with low numbers of female heartworms. This study was designed to evaluate three commercially available test kits with serum samples from dogs infected with low numbers (£ 4) of adult female heartworms. The tests evaluated were the VetScan CHAT (Abaxis Inc, Union City, CA), Snap RT (IDEXX Laboratories, Portland, ME), and Solo Step CH (HESKA Corp, Fort Collins, CO). Serum samples from three sources were used. A total of 240 serum samples from dogs naturally infected with heartworms were tested. Although serum testing may not provide the same results as blood testing, serum was used instead because the samples had to be frozen and the Solo Step CH required that blood be tested within 24 hours. The testing was done by six licensed veterinary technicians who were blinded to the infection status.

Sensitivity of all the test kits was relatively good, ranging from 78% to 84%. Sensitivity of the Snap test kit was significantly higher than that of the other two kits. Sensitivity increased as the number of infecting female worms increased. Positive predictive value and specificity was the same for all test kits. Each of the test kits yielded one false-positive. Although the conditions of this test were particularly challenging, the overall performance of these tests confirms their high sensitivity. Variations in performance may be seen depending on infection status, with low worm burdens being the most difficult to detect.

This study was supported by IDEXX Laboratories, Portland, ME.

Comparison of results of three commercial heartworm antigen test kits in dogs with low heartworm burdens. Atkins CE. JAVMA 222:1221-1223, 2003.