Diagnostic outcome and indications for testing in patients with positive ANCA at a Canadian tertiary care centre.

Introduction/objective: With widespread availability of anti-neutrophil cytoplasmic antibody (ANCA) testing, interpreting positive results has become increasingly challenging. Here, we conducted a retrospective study to evaluate indications for testing and diagnosis of patients with positive ANCA.

Methods: Positive ANCA tests (immunofluorescence or immunoassay) performed between April 2014 and March 2015 were identified using the Ottawa Hospital (TOH) laboratory information system. TOH electronic records of subjects with positive ANCA were reviewed.

Results: 96 patients had first-time positive ANCA in the study year. The indications for testing were suspicion for: AAV in 22 patients (23%), unspecified vasculitis in 24(25%), an inflammatory condition in 46(48%), and unknown in 4(4%). Twenty-eight patients (29% of first-time positives) were diagnosed with AAV, corresponding to 16(72%), 8(33%), 4(9%), and 0 patients tested for these indications, respectively; 49(51%) of patients had other inflammatory or infectious etiologies, and non-inflammatory diagnoses accounted for the remaining 19(20%). One hundred and forty-four repeat ANCAs were performed with life-time mean of 4.4 re-tests per patient (range 0-44). Routine monitoring accounted for 86(72%) of all repeat tests. Management was changed following 34% of all re-tests performed for changed clinical status and 1% of re-tests conducted routinely.

Conclusion: Few patients who start with low clinical suspicion for AAV and have positive ANCA are subsequently diagnosed with AAV. Serial ANCA testing is common but is not supported by clear evidence, and rarely leads to change in management. Clarification of guidelines on effective ANCA ordering may reduce hospital laboratory costs.