New ANCA cut-off value may aid accuracy of AAV diagnosis

A number of diseases have similarities to AAV, creating 'diagnostic dilemma'

Patricia Inacio, PhD avatar

by Patricia Inacio, PhD |

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Researchers have proposed a new cut-off value for blood levels of anti-neutrophil cytoplasmic antibodies (ANCAs) to improve the accuracy of ANCA-associated vasculitis (AAV) diagnosis.

Using this cut-off value, researchers could accurately distinguish AAV from other diseases also characterized by inflammation and damage to small blood vessels.

“This study provides a pragmatic approach to the diagnostic dilemma associated with ANCA positivity in cases that cannot rely on [tissue-based] evidence of systemic vasculitides,” the researchers wrote. Systemic vasculitides are a group of conditions, including AAV, that are characterized by blood vessel inflammation and damage.

“Adding a threshold approach to the diagnostic workup may assist clinicians in reassessing concerns for [diseases with similarities with AAV],” they added.

The study, “Diagnostic significance of antineutrophil cytoplasmic antibody (ANCA) titres: a retrospective case-control study,” was published in the journal RMD Open.

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ANCAs in blood central to diagnosing AAV

AAV is a group of autoimmune diseases in which certain autoantibodies, called ANCAs, target certain proteins in a type of immune cell, leading them to erroneously cause inflammation and damage in small blood vessels. In most cases, ANCAs target two proteins: proteinase 3 (PR3) and/or myeloperoxidase (MPO).

Testing for the presence of these ANCAs in the blood has long been a central component of AAV diagnosis. However, ANCAs can be detected in situations other than AAV such as infections, inflammatory bowel disease, and connective tissue disease. Also, ANCA levels do  not always correlate with disease activity and treatment response.

Still, “in the absence of [tissue-based] evidence of AAV, clinicians must rely on ANCA status and are required to eliminate differential diagnoses of vasculitis mimickers,” or conditions that resemble those marked by blood vessel inflammation, the researchers wrote.

Now, a team led by researchers at the University Hospital of Nice, France, set out to define new cut-off values for blood ANCAs levels that could help distinguish AAV from vasculitis mimickers, and improve the accuracy of AAV diagnosis.

They retrospectively analyzed clinical and lab data from 288 adults positive for ANCAs who were seen at their center over an eight-year period (January 2010 to December 2018).

AAV diagnosis were based on the 2022 European Alliance of Associations for Rheumatology (EULAR)/American College of Rheumatology (ACR) classification criteria for AAV.

Alternative diagnoses were categorized either as non-AAV autoimmune disorders affecting all sizes of blood vessels (ANCA-AI) or disorders without autoimmune features (ANCA-O).

A total of 49 patients (mean age 65.4 years; 57% women) were classified as having AAV, 99 patients (mean age 53.4 years; 61% women) comprised the ANCA-AI group, and 140 patients (mean age 57.7 years; 35% women) comprised the ANCA-O group.

In the AAV group, 33 patients were diagnosed with microscopic polyangiitis and 16 with granulomatosis with polyangiitis, the two most common types of AAV.

Besides significant group differences regarding age and sex, patients across the three groups also showed significant differences in ANCA levels and selected organ involvement.

A significantly greater proportion of AAV patients showed impairments in the lungs (63% vs. 21%) and kidneys (60% vs. 10%-13%) compared with the ANCA-AI and ANCA-O groups. AAV patients also had significantly higher levels of protein in the urine, a sign of kidney damage.

In turn, the ANCA-AI group showed a significantly greater proportion of patients with joint, skin, and gastrointestinal involvement when compared with the AAV group.

Anti-PR3 ANCAs were the most common type of ANCAs, and were significantly more frequent in the ANCA-AI (53%) and ANCA-O (67%) groups than in the AAV group (31%).

However, median ANCA levels were significantly higher in the AAV group (186 units per milliliter, U/mL) compared with those in the ANCA-AI (37 U/mL) and ANCA-O (36 U/mL) groups. This was true also for both anti-PR3 ANCAs and anti-MPO ANCAs.

No significant differences in terms of ANCA levels were observed between the non-AAV groups.

Threshold value of ANCA levels identified

Additional analyses indicated the best threshold value of ANCA levels was 65 U/mL or higher, allowing the discrimination between AAV patients and those in the other two groups with a sensitivity of 94% and a specificity of 73%.

A test’s sensitivity is its ability to correctly identify those with a given disease, while its specificity refers to the percentage of unaffected cases being correctly ruled out.

Also, this cut-off value was associated with a negative predictive value of 98%, meaning that 98% of cases ruled out through this test truly did not have AAV, irrespective of the type of ANCAs.

Using levels of anti-PR3 ANCAs allowed the researchers to more accurately distinguish AAV from other diseases characterized by inflammation of small blood vessels than using anti-MPO ANCA or total ANCA levels (86% vs. 76% of accuracy).

A cut-off value of 106 U/mL for anti-PR3 ANCAs allowed the accurate discrimination between AAV patients and those with mimickers, with a negative predictive value of 98%.

Further statistical analyses showed an ANCA threshold of 65 U/mL or higher was an independent diagnostic biomarker for distinguishing AAV from mimickers, being associated with a 34 times higher chance of having AAV.

Overall, these findings support that the ANCA cut-off value, regardless of PR3 or MPO targets, and along with the 2022 EULAR/ACR classification criteria “can be used to distinguish AAV from alternative autoimmune or non-autoimmune diseases,” the researchers wrote.

Future studies validating an approach based on this new cut-off value “might help clinicians to better discriminate between AAV and alternative diagnoses,” the team concluded.