Control line failure in Angiostrongylus vasorum point‐of‐care serology test in dogs with angiostrongylosis due to suspected hook effect

Objectives Angiostrongylosis is a significant differential for a diverse range of clinical signs in dogs, many of whom present acutely and sometimes with fatal consequences. Point‐of‐care diagnostic assays include a commercially available Angiostrongylus vasorum qualitative direct lateral flow assay. Materials and Methods Case records from one referral centre from dogs with an invalid A. vasorum lateral flow assay, comprising an absent control line alongside a visible test line, were reviewed. As control line failure was hypothesised to be due to antigen excess; where available the A. vasorum lateral flow assay was repeated using dilutions of the original serum. Results Six dogs had an invalid A. vasorum lateral flow assay result. Five dogs had presented with acute‐onset, severe clinical disease consistent with angiostrongylosis, and one dog was a clinically healthy in‐contact. Clinical suspicion of angiostrongylosis was confirmed using alternative diagnostic testing and/or response to treatment. Repetition of the A. vasorum lateral flow assay, in four cases, using diluted plasma (10% to 12.5% v/v) resulted in the appearance of a control line alongside the visible test line. Clinical Significance A heavy burden of A. vasorum infection resulting in angiostrongylosis should be suspected in dogs with compatible clinical signs and an invalid A. vasorum lateral flow assay result due to control failure alongside a visible test line. Repetition of the test with a diluted serum may be considered to account for the hook effect, also known as the postzone phenomenon, as a possible cause.